Cara menggunakan ARRAYDEFINITION pada JavaScript

Pertanyaan JavaScript

Saya post pertanyaan pilihan ganda ke Instagram stories saya, yang saya post juga di sini ! update terakhir: December 24th

Mulai tingkat dasar ke mahir: tes seberapa paham kamu tentang javascript, segarkan sedikit pengetahuan kamu, atau bersiap-siap untuk coding interview kamu! 💪🚀 Saya update repo ini secara berkala dengan pertanyaan baru. Saya masukkan jawaban dibagian yang tersembunyi di bawah pertanyaan, cukup klik pada bagian itu untuk menampilkannya. Pertanyaan ini hanya untuk bersenang-senang, Semoga berhasil ❤️

Jangan sungkan untuk terhubung dengan saya! 😊
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function sayHi[] {
  console.log[name];
  console.log[age];
  var name = 'Lydia';
  let age = 21;
}

sayHi[];

• A: Lydia dan undefined
• B: Lydia dan ReferenceError
• C: ReferenceError dan 21
• D: undefined dan ReferenceError

Jawaban: D

Di dalam function, kita membuat sebuah variabel name dan variabel tersebut di deklarasikan menggunakan var. Artinya variable tersebut di hoisting [dalam fase pembuatan ini menggunakan memory penyimpanan] dengan isi standar-nya undefined, saat javascript mengeksekusi baris code pembuatan variabel-nya. variabel name isinya masih undefined, jadi isi dari variabel tersebut undefined

Mendeklarasikan varibel menggunakan let [dan const] juga terkena hoisting, tidak seperti var, variabel declaration let dan const tidak ditentukan isi standar-nya. let dan const tidak bisa diakses sebelum di tentukan dulu isi-nya. Kejadian ini disebut "temporal dead zone". Saat kita mencoba memanggil variabel yang belum ditentukan isi-nya, Javascript mengeluarkan error ReferenceError.

for [var i = 0; i  console.log[i], 1];
}

for [let i = 0; i  console.log[i], 1];
}

• A: 0 1 2 and 0 1 2
• B: 0 1 2 and 3 3 3
• C: 3 3 3 and 0 1 2

Jawaban: C

Karena antrean peristiwa di JavaScript, fungsi callback setTimeout disebut after loop telah dijalankan. Karena variabel i di loop pertama dideklarasikan menggunakan kata kunci var, nilai ini adalah global. Selama perulangan, kita menambah nilai i sebesar 1 setiap kali, menggunakan operator unary++. Pada saat fungsi callback setTimeout dipanggil, i sama dengan 3 di contoh pertama.

Pada perulangan kedua, variabel i dideklarasikan menggunakan kata kunci let: variabel yang dideklarasikan dengan kata kunci let [dan const] memiliki cakupan blok [blok adalah apa saja di antara {}]. Selama setiap iterasi, i akan memiliki nilai baru, dan setiap nilai dicakup di dalam loop.

const shape = {
  radius: 10,
  diameter[] {
    return this.radius * 2;
  },
  perimeter: [] => 2 * Math.PI * this.radius,
};

console.log[shape.diameter[]];
console.log[shape.perimeter[]];

• A: 20 dan 62.83185307179586
• B: 20 dan NaN
• C: 20 dan 63
• D: NaN dan 63

Jawaban: B

Perhatikan pada nilai 'diameter' adalah fungsi biasa, sedangkan nilai 'perimeter' yaitu fungsi panah.

Dengan fungsi panah, kata kunci 'this' merujuk ke cakupan sekitarnya saat ini, tidak seperti fungsi biasa. Ini berarti bahwa ketika kita memanggil 'perimeter' itu tidak mengacu pada objek bentuk, tetapi pada lingkup sekitarnya.

Tidak ada nilai 'radius' pada objek itu, yang mengembalikan 'tidak ditentukan'.

• A: 1 dan false
• B: false dan NaN
• C: false dan false

Jawaban: A

Tia unary plus mencoba mengonversi operan menjadi angka. true adalah 1, dan false adalah 0.

String "'Lydia'adalah nilai yang benar. Apa yang sebenarnya kami tanyakan adalah "apakah nilai kebenaran ini salah?". Ini mengembalikansalah`.

const bird = {
  size: 'small',
};

const mouse = {
  name: 'Mickey',
  small: true,
};

• A: mouse.bird.size tidak benar
• B: mouse[bird.size] tidak benar
• C: mouse[bird["size"]] tidak benar
• D: Semua jawaban benar

Jawaban: A

Pada JavaScript, semua kunci objek adalah string [kecuali jika itu berupa Simbol]. Meskipun kita mungkin tidak mengetiknya sebagai string, tetap saja mereka selalu berubah menjadi string didalamnya.

JavaScript menginterpretasikan [atau membuka] pernyataan-pernyataan. Saat kita menggunakan notasi kurung siku, ia melihat kurung buka pertama [ dan terus berjalan sampai menemukan kurung tutup ]. Baru setelah itu akan mengevaluasi pernyataannya. mouse[bird.size]: Pertama, ini mengevaluasi bird.size, yang mana "small". mouse["small"] mengembalikan nilai true.

Namun, dengan notasi dot [.], hal ini tidak terjadi. mouse tidak memiliki kunci dengan nama bird, yang menyebabkan mouse.bird bernilai undefined. Kemudian, kita meminta size untuk menggunakan notasi dot [.]: mouse.bird.size. Kita mengetahui bahwa mouse.bird bernilai undefined, yang sebenarnya kita minta adalah undefined.size. Yang mana hal ini tidak valid, dan akan memunculkan kesalahan yang mirip dengan Cannot read property "size" of undefined.

let c = { greeting: 'Hey!' };
let d;

d = c;
c.greeting = 'Hello';
console.log[d.greeting];

• A: Hello
• B: Hey!
• C: undefined
• D: ReferenceError
• E: TypeError

Jawaban: A

Dalam JavaScript, semua objek berinteraksi dengan reference saat menyetelnya agar sama satu sama lain.

Pertama, variabel c menyimpan nilai ke sebuah objek. Nanti, kita menetapkan d dengan referensi yang sama yang dimiliki c ke objek.

Saat Anda mengubah satu objek, Anda mengubah semuanya.

let a = 3;
let b = new Number[3];
let c = 3;

console.log[a == b];
console.log[a === b];
console.log[b === c];

• A: true false true
• B: false false true
• C: true false false
• D: false true true

Jawaban: C

new Number[] adalah konstruktor fungsi bawaan pada JavaScript. Meskipun hasilnya terlihat seperti integer, namun sebenarnya itu bukan integer: aslinya memiliki banyak fitur tambahan dan merupakan sebuah objek.

Saat kita menggunakan operator ==, hal ini hanya akan memeriksa bahwa keduanya memiliki nilai yang sama. Pada kasus ini kedua variabel tersebut memiliki nilai yang sama, yaitu 3, maka akan mengembalikan nilai true.

Namun, saat kita menggunakan operator ===, operator ini memeriksa bahwa kedua variabel memiliki nilai dan tipe yang sama. Bagaimanapun: new Number[] bukanlah sebuah integer, ini adalah sebuah object. Keduanya akan mengembalikan nilai false.

class Chameleon {
  static colorChange[newColor] {
    this.newColor = newColor;
    return this.newColor;
  }

  constructor[{ newColor = 'green' } = {}] {
    this.newColor = newColor;
  }
}

const freddie = new Chameleon[{ newColor: 'purple' }];
console.log[freddie.colorChange['orange']];

• A: orange
• B: purple
• C: green
• D: TypeError

Jawaban: D

Fungsi colorChange adalah statis. Metode statis dirancang hanya dapat aktif pada kontruktor dimana fungsi itu dibuat, dan tidak bisa dibawa ke-turunannya. Kita tahu bahwa freddie adalah sebuah turunan, maka fungsi itu tidak bisa turun, dan tidak tersedia pada instance freddie: sebuah pesan TypeError akan dikembalikan

let greeting;
greetign = {}; // Typo!
console.log[greetign];

• A: {}
• B: ReferenceError: greetign is not defined
• C: undefined

Jawaban: A

Ini mencatat objek, karena kita baru saja membuat objek kosong di objek global! Saat kita salah mengetik greeting sebagai greetign, interpreter JS sebenarnya melihat ini sebagai global.greetign = {} [atau window.greetign = {} di browser].

Untuk menghindari hal ini, kita bisa menggunakan " use strict ". Ini memastikan bahwa Anda telah mendeklarasikan variabel sebelum menetapkannya dengan apa pun.

function bark[] {
  console.log['Woof!'];
}

bark.animal = 'dog';

• A: Nothing, this is totally fine!
• B: SyntaxError. You cannot add properties to a function this way.
• C: "Woof" gets logged.
• D: ReferenceError

Jawaban: A

Ini dimungkinkan dalam JavaScript, karena fungsi adalah objek! [Segala sesuatu selain tipe primitif adalah objek]

Fungsi adalah jenis objek khusus. Kode yang Anda tulis sendiri bukanlah fungsi sebenarnya. Fungsinya adalah objek dengan properti. Properti ini tidak dapat dipanggil.

function Person[firstName, lastName] {
  this.firstName = firstName;
  this.lastName = lastName;
}

const member = new Person['Lydia', 'Hallie'];
Person.getFullName = function[] {
  return `${this.firstName} ${this.lastName}`;
};

console.log[member.getFullName[]];

• A: TypeError
• B: SyntaxError
• C: Lydia Hallie
• D: undefined undefined

Jawaban: A

Anda tidak dapat menambahkan properti ke constructor seperti yang Anda lakukan dengan objek biasa. Jika Anda ingin menambahkan fitur ke semua objek sekaligus, Anda harus menggunakan prototipe sebagai gantinya. Jadi dalam kasus ini:

Person.prototype.getFullName = function[] {
  return `${this.firstName} ${this.lastName}`;
};

Akan membuat member.getFullName[] berfungsi. Mengapa ini bermanfaat? Katakanlah kita menambahkan metode ini ke konstruktor itu sendiri. Mungkin tidak setiap instance Person membutuhkan metode ini. Ini akan membuang banyak ruang memori, karena mereka masih memiliki properti itu, yang mengambil ruang memori untuk setiap instance. Sebaliknya, jika kita hanya menambahkannya ke prototipe, kita hanya memilikinya di satu tempat di memori, namun mereka semua memiliki akses ke sana!

function Person[firstName, lastName] {
  this.firstName = firstName;
  this.lastName = lastName;
}

const lydia = new Person['Lydia', 'Hallie'];
const sarah = Person['Sarah', 'Smith'];

console.log[lydia];
console.log[sarah];

• A: Person {firstName: "Lydia", lastName: "Hallie"} dan undefined
• B: Person {firstName: "Lydia", lastName: "Hallie"} dan Person {firstName: "Sarah", lastName: "Smith"}
• C: Person {firstName: "Lydia", lastName: "Hallie"} dan {}
• D:Person {firstName: "Lydia", lastName: "Hallie"} dan ReferenceError

Jawaban: A

Pada sarah, kita tidak menggunakan kata kunci new. Saat menggunakan new, Ini mengacu pada object kosong yang kita buat. Namun, jika Anda tidak menambahkan new ini merujuk pada global object!

Kita tahu bahwa this.firstName setara dengan "Sarah" dan this.lastName sama dengan "Smith". Apa yang sebenarnya kami lakukan adalah mendefinisikan global.firstName = 'Sarah' dan global.lastName = 'Smith'. sarah sendiri dibiarkan undefined, karena kita tidak mengembalikan nilai dari fungsi Person.

• A: Target > Capturing > Bubbling
• B: Bubbling > Target > Capturing
• C: Target > Bubbling > Capturing
• D: Capturing > Target > Bubbling

Jawaban: D

Selama fase capturing, event melewati elemen ancestor hingga ke elemen target. Kemudian mencapai element target, dan bubbling dimulai.

• A: true
• B: false

Jawaban: B

Semua objek memiliki prototypes, kecuali objek dasar. Objek dasar adalah objek yang dibuat oleh pengguna, atau objek yang dibuat dengan menggunakan kata kunci baru. Objek dasar memiliki akses ke beberapa metode dan properti, seperti .toString. Inilah alasan mengapa Anda dapat menggunakan metode JavaScript bawaan! Semua metode tersebut tersedia di prototipe. Meskipun JavaScript tidak dapat menemukannya secara langsung di objek Anda, JavaScript berada di rantai prototipe dan menemukannya di sana, yang membuatnya dapat diakses untuk Anda.

function sum[a, b] {
  return a + b;
}

sum[1, '2'];

• A: NaN
• B: TypeError
• C: "12"
• D: 3

Jawaban: C

JavaScript adalah Bahasa yang bersifat dinamis: yang tidak menentukan jenis variabel tertentu. Values dapat secara otomatis diubah menjadi jenis lain tanpa Anda sadari, yang disebut implicit type coercion. Coercion adalah mengubah dari satu jenis ke jenis lainnya.

Pada contoh ini, JavaScript mengubah number 1 menjadi sebuah string, agar fungsi tersebut masuk akal dan mengembalikan nilai. Selama penambahan tipe numerik [1] dan tipe string ['2'], angka tersebut diperlakukan sebagai string. Kita bisa menggabungkan string seperti "Hello" + "World", jadi yang terjadi di sini adalah "1" + "2" yang mengembalikan "12".

let number = 0;
console.log[number++];
console.log[++number];
console.log[number];

• A: 1 1 2
• B: 1 2 2
• C: 0 2 2
• D: 0 1 2

Jawaban: C

Akhiran operator unary ++:

1. Mengembalikan nilai [ini mengembalikan 0]
2. Menambahkan nilai [angkanya sekarang 1]

Awalan operator unary ++:

1. Menambah nilai [angkanya sekarang 2]
2. Mengembalikan nilai [ini mengembalikan 2]

Ini mengembalikan 0 2 2.

function getPersonInfo[one, two, three] {
  console.log[one];
  console.log[two];
  console.log[three];
}

const person = 'Lydia';
const age = 21;

getPersonInfo`${person} is ${age} years old`;

• A: "Lydia" 21 ["", " is ", " years old"]
• B: ["", " is ", " years old"] "Lydia" 21
• C: "Lydia" ["", " is ", " years old"] 21

Jawaban: B

Jika Anda menggunakan literal template yang diberi tag, nilai argumen pertama selalu berupa array bernilai string. Argumen yang tersisa mendapatkan nilai dari ekspresi yang diteruskan!

function checkAge[data] {
  if [data === { age: 18 }] {
    console.log['You are an adult!'];
  } else if [data == { age: 18 }] {
    console.log['You are still an adult.'];
  } else {
    console.log[`Hmm.. You don't have an age I guess`];
  }
}

checkAge[{ age: 18 }];

• A: You are an adult!
• B: You are still an adult.
• C: Hmm.. You don't have an age I guess

Jawaban: C

Saat menguji persamaan, primitif dibandingkan dengan nilainya, sedangkan objek dibandingkan dengan referensinya. JavaScript memeriksa apakah objek memiliki referensi ke lokasi yang sama di memori.

Dua objek yang kita bandingkan tidak memiliki itu: objek yang kita lewati sebagai parameter merujuk ke lokasi yang berbeda dalam memori dari objek yang kita gunakan untuk memeriksa persamaan.

Inilah mengapa {age: 18} === {age: 18} dan {age: 18} == {age: 18} mengembalikan nilai false.

function getAge[...args] {
  console.log[typeof args];
}

getAge[21];

• A: "number"
• B: "array"
• C: "object"
• D: "NaN"

Jawaban: C

Parameter sisanya [... args.] Memungkinkan kita "mengumpulkan" semua argumen yang tersisa ke dalam sebuah array. Array adalah sebuah objek, jadi typeof args mengembalikan "objek"

function getAge[] {
  'use strict';
  age = 21;
  console.log[age];
}

getAge[];

• A: 21
• B: undefined
• C: ReferenceError
• D: TypeError

Jawaban: C

Dengan " use strict ", Anda dapat memastikan bahwa Anda tidak mendeklarasikan variabel global secara tidak sengaja. Kita tidak pernah mendeklarasikan variabel age, dan karena kita menggunakan " use strict ", ini akan memunculkan kesalahan referensi. Jika kita tidak menggunakan " use strict ", ini akan berhasil, karena properti age akan ditambahkan ke objek global.

const sum = eval['10*10+5'];

• A: 105
• B: "105"
• C: TypeError
• D: "10*10+5"

Jawaban: A

eval mengevaluasi kode yang berlalu sebagai string. Jika itu adalah ekspresi, seperti dalam kasus ini, itu mengevaluasi ekspresi. Ungkapannya adalah 10 * 10 + 5. Ini kembali nomor 105.

sessionStorage.setItem['cool_secret', 123];

• A: Selamanya, data tersebut tidak akan hilang.
• B: Saat pengguna menutup tab browser.
• C: Saat pengguna menutup seluruh browser, tidak hanya tab.
• D: Saat pengguna mematikan komputernya.

Jawaban: B

Data yang disimpan di sessionStorage akan dihapus setelah menutup tab.

Jika anda menggunakan localStorage, data tersebut akan tersimpan selamanya, kecuali misalnya method localStorage.clear[] dipanggil.

var num = 8;
var num = 10;

console.log[num];

• A: 8
• B: 10
• C: SyntaxError
• D: ReferenceError

Jawaban: B

Dengan kata kunci var, anda dapat menyatakan beberapa variabel dengan nama yang sama. Variabelnya akan memegang nilai terakhir.

Anda tidak dapat melakukan ini dengan let atau const karena mereka block-scoped.

const obj = { 1: 'a', 2: 'b', 3: 'c' };
const set = new Set[[1, 2, 3, 4, 5]];

obj.hasOwnProperty['1'];
obj.hasOwnProperty[1];
set.has['1'];
set.has[1];

• A: false true false true
• B: false true true true
• C: true true false true
• D: true true true true

Jawaban: C

All object keys [excluding Symbols] are strings under the hood, even if you don't type it yourself as a string. This is why obj.hasOwnProperty['1'] also returns true.

It doesn't work that way for a set. There is no '1' in our set: set.has['1'] returns false. It has the numeric type 1, set.has[1] returns true.

const obj = { a: 'one', b: 'two', a: 'three' };
console.log[obj];

• A: { a: "one", b: "two" }
• B: { b: "two", a: "three" }
• C: { a: "three", b: "two" }
• D: SyntaxError

Jawaban: C

Jika anda memiliki dua kunci dengan nama yang sama, kunci akan diganti. Ini masih dalam posisi pertama, tetapi dengan nilai terakhir yang ditentukan.

• A: true
• B: false
• C: it depends

Jawaban: A

The base execution context is the global execution context: it's what's accessible everywhere in your code.

for [let i = 1; i  {
  return 'Just give Lydia pizza already!';
};

const name = 'Lydia';

console.log[name.giveLydiaPizza[]]

• A: "Just give Lydia pizza already!"
• B: TypeError: not a function
• C: SyntaxError
• D: undefined

Jawaban: A

String adalah konstruksi dibangun, yang dapat kita tambahkan properti ke. Aku hanya menambahkan metode ke prototipe. String primitif string secara otomatis dikonversi menjadi objek string, dihasilkan oleh fungsi prototipe string. Jadi, semua string [objek string] memiliki akses ke metode itu!

const a = {};
const b = { key: 'b' };
const c = { key: 'c' };

a[b] = 123;
a[c] = 456;

console.log[a[b]];

• A: 123
• B: 456
• C: undefined
• D: ReferenceError

Jawaban: B

Object keys are automatically converted into strings. We are trying to set an object as a key to object a, with the value of 123.

However, when we stringify an object, it becomes "[object Object]". So what we are saying here, is that a["object Object"] = 123. Then, we can try to do the same again. c is another object that we are implicitly stringifying. So then, a["object Object"] = 456.

Then, we log a[b], which is actually a["object Object"]. We just set that to 456, so it returns 456.

const foo = [] => console.log['First'];
const bar = [] => setTimeout[[] => console.log['Second']];
const baz = [] => console.log['Third'];

bar[];
foo[];
baz[];

• A: First Second Third
• B: First Third Second
• C: Second First Third
• D: Second Third First

Jawaban: B

Kami memiliki fungsi setTimeout dan dimohonkan terlebih dahulu. Namun, itu login terakhir.

Hal ini karena di browsers, kita tidak hanya memiliki mesin waktu runtime, kita juga memiliki sesuatu yang disebut WebAPI. WebAPI memberi kita fungsi setTimeout, dan misalnya DOM.

Setelah callback [panggilan balik] didorong ke WebAPI, fungsi setTimeout itu sendiri [tetapi tidak panggilan balik] muncul dari tumpukan.

Sekarang, foo mendapat hambatan, dan "First" yang login.

foo yang muncul dari tumpukan, dan baz mendapat perantara. "Third" akan login.

WebAPI tidak bisa hanya menambahkan barang-barang ke tumpukan setiap kali siap. Sebaliknya, ia mendorong fungsi panggilan balik ke sesuatu yang disebut queue [antrian].

Di sinilah serangkaian acara mulai bekerja. Sebuah event loop [putaran kejadian/peristiwa] melihat tumpukan dan antrian tugas. Jika tumpukan kosong, itu mengambil hal pertama pada antrian dan mendorong ke tumpukan.

bar bisa dipanggil, "Second" akan login, dan itu muncul dari tumpukan.

  
    
      Click!
    
  

• A: Outer div
• B: Inner div
• C: button
• D: An array of all nested elements.

Jawaban: C

The deepest nested element that caused the event is the target of the event. You can stop bubbling by event.stopPropagation

  
    Click here!
  

• A: p div
• B: div p
• C: p
• D: div

Jawaban: A

If we click p, we see two logs: p and div. During event propagation, there are 3 phases: capturing, target, and bubbling. By default, event handlers are executed in the bubbling phase [unless you set useCapture to true]. It goes from the deepest nested element outwards.

const person = { name: 'Lydia' };

function sayHi[age] {
  return `${this.name} is ${age}`;
}

console.log[sayHi.call[person, 21]];
console.log[sayHi.bind[person, 21]];

• A: undefined is 21 Lydia is 21
• B: function function
• C: Lydia is 21 Lydia is 21
• D: Lydia is 21 function

Jawaban: D

With both, we can pass the object to which we want the this keyword to refer to. However, .call is also executed immediately!

.bind. returns a copy of the function, but with a bound context! It is not executed immediately.

function sayHi[] {
  return [[] => 0][];
}

console.log[typeof sayHi[]];

• A: "object"
• B: "number"
• C: "function"
• D: "undefined"

Jawaban: B

The sayHi function returns the returned value of the immediately invoked function [IIFE]. This function returned 0, which is type "number".

FYI: there are only 7 built-in types: null, undefined, boolean, number, string, object, symbol, and bigint. "function" is not a type, since functions are objects, it's of type "object".

0;
new Number[0];
[''];
[' '];
new Boolean[false];
undefined;

• A: 0, '', undefined
• B: 0, new Number[0], '', new Boolean[false], undefined
• C: 0, '', new Boolean[false], undefined
• D: All of them are falsy

Jawaban: A

Hanya ada enam nilai yang salah:

• undefined
• null
• NaN
• 0
• '' [string kosong]
• false

Konstruktor fungsi, seperti Number baru dan Boolean baru, benar.

console.log[typeof typeof 1];

• A: "number"
• B: "string"
• C: "object"
• D: "undefined"

Jawaban: B

typeof 1 returns "number". typeof "number" returns "string"

const numbers = [1, 2, 3];
numbers[10] = 11;
console.log[numbers];

• A: [1, 2, 3, 7 x null, 11]
• B: [1, 2, 3, 11]
• C: [1, 2, 3, 7 x empty, 11]
• D: SyntaxError

Jawaban: C

Saat Anda menyetel nilai ke elemen dalam larik yang melebihi panjang larik, JavaScript membuat sesuatu yang disebut "slot kosong". Ini sebenarnya memiliki nilai tidak terdefinisi, tetapi Anda akan melihat sesuatu seperti:

[1, 2, 3, 7 x empty, 11]

tergantung di mana Anda menjalankannya [berbeda untuk setiap browser, node, dll.]

[[] => {
  let x, y;
  try {
    throw new Error[];
  } catch [x] {
    [x = 1], [y = 2];
    console.log[x];
  }
  console.log[x];
  console.log[y];
}][];

• A: 1 undefined 2
• B: undefined undefined undefined
• C: 1 1 2
• D: 1 undefined undefined

Jawaban: A

The catch block receives the argument x. This is not the same x as the variable when we pass arguments. This variable x is block-scoped.

Later, we set this block-scoped variable equal to 1, and set the value of the variable y. Now, we log the block-scoped variable x, which is equal to 1.

Outside of the catch block, x is still undefined, and y is 2. When we want to console.log[x] outside of the catch block, it returns undefined, and y returns 2.

• A: primitive or object
• B: function or object
• C: trick question! only objects
• D: number or object

Jawaban: A

JavaScript only has primitive types and objects.

Primitive types are boolean, null, undefined, bigint, number, string, and symbol.

What differentiates a primitive from an object is that primitives do not have any properties or methods; however, you'll note that 'foo'.toUpperCase[] evaluates to 'FOO' and does not result in a TypeError. This is because when you try to access a property or method on a primitive like a string, JavaScript will implicitly wrap the object using one of the wrapper classes, i.e. String, and then immediately discard the wrapper after the expression evaluates. All primitives except for null and undefined exhibit this behaviour.

[[0, 1], [2, 3]].reduce[
  [acc, cur] => {
    return acc.concat[cur];
  },
  [1, 2],
];

• A: [0, 1, 2, 3, 1, 2]
• B: [6, 1, 2]
• C: [1, 2, 0, 1, 2, 3]
• D: [1, 2, 6]

Jawaban: C

[1, 2] is our initial value. This is the value we start with, and the value of the very first acc. During the first round, acc is [1,2], and cur is [0, 1]. We concatenate them, which results in [1, 2, 0, 1].

Then, [1, 2, 0, 1] is acc and [2, 3] is cur. We concatenate them, and get [1, 2, 0, 1, 2, 3]

• A: false true false
• B: false false true
• C: false true true
• D: true true false

Jawaban: B

null is falsy. !null returns true. !true returns false.

"" is falsy. !"" returns true. !true returns false.

1 is truthy. !1 returns false. !false returns true.

setInterval[[] => console.log['Hi'], 1000];

• A: a unique id
• B: the amount of milliseconds specified
• C: the passed function
• D: undefined

Jawaban: A

Itu adalah mengembalikan sebuah id unik. id unik dapat digunakan untuk menghapus interval dengan menggunakan fungsi clearInterval[]

• A: ["L", "y", "d", "i", "a"]
• B: ["Lydia"]
• C: [[], "Lydia"]
• D: [["L", "y", "d", "i", "a"]]

Jawaban: A

Sebuah string adalah iterable. Operator memetakan setiap karakter dari sebuah iterable ke dalam satu elemen.

function* generator[i] {
  yield i;
  yield i * 2;
}

const gen = generator[10];

console.log[gen.next[].value];
console.log[gen.next[].value];

• A: [0, 10], [10, 20]
• B: 20, 20
• C: 10, 20
• D: 0, 10 dan 10, 20

Jawaban: C

Regular functions cannot be stopped mid-way after invocation. However, a generator function can be "stopped" midway, and later continue from where it stopped. Every time a generator function encounters a yield keyword, the function yields the value specified after it. Note that the generator function in that case doesn’t return the value, it yields the value.

First, we initialize the generator function with i equal to 10. We invoke the generator function using the next[] method. The first time we invoke the generator function, i is equal to 10. It encounters the first yield keyword: it yields the value of i. The generator is now "paused", and 10 gets logged.

Then, we invoke the function again with the next[] method. It starts to continue where it stopped previously, still with i equal to 10. Now, it encounters the next yield keyword, and yields i * 2. i is equal to 10, so it returns 10 * 2, which is 20. This results in 10, 20.

const firstPromise = new Promise[[res, rej] => {
  setTimeout[res, 500, 'one'];
}];

const secondPromise = new Promise[[res, rej] => {
  setTimeout[res, 100, 'two'];
}];

Promise.race[[firstPromise, secondPromise]].then[res => console.log[res]];

• A: "one"
• B: "two"
• C: "two" "one"
• D: "one" "two"

Jawaban: B

When we pass multiple promises to the Promise.race method, it resolves/rejects the first promise that resolves/rejects. To the setTimeout method, we pass a timer: 500ms for the first promise [firstPromise], and 100ms for the second promise [secondPromise]. This means that the secondPromise resolves first with the value of 'two'. res now holds the value of 'two', which gets logged.

let person = { name: 'Lydia' };
const members = [person];
person = null;

console.log[members];

• A: null
• B: [null]
• C: [{}]
• D: [{ name: "Lydia" }]

Jawaban: D

First, we declare a variable person with the value of an object that has a name property.

Then, we declare a variable called members. We set the first element of that array equal to the value of the person variable. Objects interact by reference when setting them equal to each other. When you assign a reference from one variable to another, you make a copy of that reference. [note that they don't have the same reference!]

Then, we set the variable person equal to null.

We are only modifying the value of the person variable, and not the first element in the array, since that element has a different [copied] reference to the object. The first element in members still holds its reference to the original object. When we log the members array, the first element still holds the value of the object, which gets logged.

const person = {
  name: 'Lydia',
  age: 21,
};

for [const item in person] {
  console.log[item];
}

• A: { name: "Lydia" }, { age: 21 }
• B: "name", "age"
• C: "Lydia", 21
• D: ["name", "Lydia"], ["age", 21]

Jawaban: B

With a for-in loop, we can iterate through object keys, in this case name and age. Under the hood, object keys are strings [if they're not a Symbol]. On every loop, we set the value of item equal to the current key it’s iterating over. First, item is equal to name, and gets logged. Then, item is equal to age, which gets logged.

console.log[3 + 4 + '5'];

• A: "345"
• B: "75"
• C: 12
• D: "12"

Jawaban: B

Operator associativity is the order in which the compiler evaluates the expressions, either left-to-right or right-to-left. This only happens if all operators have the same precedence. We only have one type of operator: +. For addition, the associativity is left-to-right.

3 + 4 gets evaluated first. This results in the number 7.

7 + '5' results in "75" because of coercion. JavaScript converts the number 7 into a string, see question 15. We can concatenate two strings using the +operator. "7" + "5" results in "75".

const num = parseInt['7*6', 10];

• A: 42
• B: "42"
• C: 7
• D: NaN

Jawaban: C

Only the first numbers in the string is returned. Based on the radix [the second argument in order to specify what type of number we want to parse it to: base 10, hexadecimal, octal, binary, etc.], the parseInt checks whether the characters in the string are valid. Once it encounters a character that isn't a valid number in the radix, it stops parsing and ignores the following characters.

* is not a valid number. It only parses "7" into the decimal 7. num now holds the value of 7.

[1, 2, 3].map[num => {
  if [typeof num === 'number'] return;
  return num * 2;
}];

• A: []
• B: [null, null, null]
• C: [undefined, undefined, undefined]
• D: [ 3 x empty ]

Jawaban: C

When mapping over the array, the value of num is equal to the element it’s currently looping over. In this case, the elements are numbers, so the condition of the if statement typeof num === "number" returns true. The map function creates a new array and inserts the values returned from the function.

However, we don’t return a value. When we don’t return a value from the function, the function returns undefined. For every element in the array, the function block gets called, so for each element we return undefined.

function getInfo[member, year] {
  member.name = 'Lydia';
  year = '1998';
}

const person = { name: 'Sarah' };
const birthYear = '1997';

getInfo[person, birthYear];

console.log[person, birthYear];

• A: { name: "Lydia" }, "1997"
• B: { name: "Sarah" }, "1998"
• C: { name: "Lydia" }, "1998"
• D: { name: "Sarah" }, "1997"

Jawaban: A

Arguments are passed by value, unless their value is an object, then they're passed by reference. birthYear is passed by value, since it's a string, not an object. When we pass arguments by value, a copy of that value is created [see question 46].

The variable birthYear has a reference to the value "1997". The argument year also has a reference to the value "1997", but it's not the same value as birthYear has a reference to. When we update the value of year by setting year equal to "1998", we are only updating the value of year. birthYear is still equal to "1997".

The value of person is an object. The argument member has a [copied] reference to the same object. When we modify a property of the object member has a reference to, the value of person will also be modified, since they both have a reference to the same object. person's name property is now equal to the value "Lydia"

function greeting[] {
  throw 'Hello world!';
}

function sayHi[] {
  try {
    const data = greeting[];
    console.log['It worked!', data];
  } catch [e] {
    console.log['Oh no an error:', e];
  }
}

sayHi[];

• A: It worked! Hello world!
• B: Oh no an error: undefined
• C: SyntaxError: can only throw Error objects
• D: Oh no an error: Hello world!

Jawaban: D

With the throw statement, we can create custom errors. With this statement, you can throw exceptions. An exception can be a string, a number, a boolean or an object. In this case, our exception is the string 'Hello world'.

With the catch statement, we can specify what to do if an exception is thrown in the try block. An exception is thrown: the string 'Hello world'. e is now equal to that string, which we log. This results in 'Oh an error: Hello world'.

function Car[] {
  this.make = 'Lamborghini';
  return { make: 'Maserati' };
}

const myCar = new Car[];
console.log[myCar.make];

• A: "Lamborghini"
• B: "Maserati"
• C: ReferenceError
• D: TypeError

Jawaban: B

When you return a property, the value of the property is equal to the returned value, not the value set in the constructor function. We return the string "Maserati", so myCar.make is equal to "Maserati".

[[] => {
  let x = [y = 10];
}][];

console.log[typeof x];
console.log[typeof y];

• A: "undefined", "number"
• B: "number", "number"
• C: "object", "number"
• D: "number", "undefined"

Jawaban: A

let x = y = 10; is actually shorthand for:

When we set y equal to 10, we actually add a property y to the global object [window in browser, global in Node]. In a browser, window.y is now equal to 10.

Then, we declare a variable x with the value of y, which is 10. Variables declared with the let keyword are block scoped, they are only defined within the block they're declared in; the immediately-invoked function [IIFE] in this case. When we use the typeof operator, the operand x is not defined: we are trying to access x outside of the block it's declared in. This means that x is not defined. Values who haven't been assigned a value or declared are of type "undefined". console.log[typeof x] returns "undefined".

However, we created a global variable y when setting y equal to 10. This value is accessible anywhere in our code. y is defined, and holds a value of type "number". console.log[typeof y] returns "number".

class Dog {
  constructor[name] {
    this.name = name;
  }
}

Dog.prototype.bark = function[] {
  console.log[`Woof I am ${this.name}`];
};

const pet = new Dog['Mara'];

pet.bark[];

delete Dog.prototype.bark;

pet.bark[];

• A: "Woof I am Mara", TypeError
• B: "Woof I am Mara", "Woof I am Mara"
• C: "Woof I am Mara", undefined
• D: TypeError, TypeError

Jawaban: A

We can delete properties from objects using the delete keyword, also on the prototype. By deleting a property on the prototype, it is not available anymore in the prototype chain. In this case, the bark function is not available anymore on the prototype after delete Dog.prototype.bark, yet we still try to access it.

When we try to invoke something that is not a function, a TypeError is thrown. In this case TypeError: pet.bark is not a function, since pet.bark is undefined.

const set = new Set[[1, 1, 2, 3, 4]];

console.log[set];

• A: [1, 1, 2, 3, 4]
• B: [1, 2, 3, 4]
• C: {1, 1, 2, 3, 4}
• D: {1, 2, 3, 4}

Jawaban: D

The Set object is a collection of unique values: a value can only occur once in a set.

We passed the iterable [1, 1, 2, 3, 4] with a duplicate value 1. Since we cannot have two of the same values in a set, one of them is removed. This results in {1, 2, 3, 4}.

// counter.js
let counter = 10;
export default counter;

// index.js
import myCounter from './counter';

myCounter += 1;

console.log[myCounter];

• A: 10
• B: 11
• C: Error
• D: NaN

Jawaban: C

Modul yang diimpor adalah read-only: Anda tidak dapat mengubah modul yang diimpor. Hanya modul yang mengekspornya yang dapat mengubah nilainya.

Ketika kita mencoba untuk menambah nilai myCounter, itu melemparkan kesalahan: myCounter adalah baca-saja dan tidak dapat dimodifikasi.

const name = 'Lydia';
age = 21;

console.log[delete name];
console.log[delete age];

• A: false, true
• B: "Lydia", 21
• C: true, true
• D: undefined, undefined

Jawaban: A

The delete operator returns a boolean value: true on a successful deletion, else it'll return false. However, variables declared with the var, const or let keyword cannot be deleted using the delete operator.

The name variable was declared with a const keyword, so its deletion is not successful: false is returned. When we set age equal to 21, we actually added a property called age to the global object. You can successfully delete properties from objects this way, also the global object, so delete age returns true.

const numbers = [1, 2, 3, 4, 5];
const [y] = numbers;

console.log[y];

• A: [[1, 2, 3, 4, 5]]
• B: [1, 2, 3, 4, 5]
• C: 1
• D: [1]

Jawaban: C

We can unpack values from arrays or properties from objects through destructuring. For example:

The value of a is now 1, and the value of b is now 2. What we actually did in the question, is:

This means that the value of y is equal to the first value in the array, which is the number 1. When we log y, 1 is returned.

const user = { name: 'Lydia', age: 21 };
const admin = { admin: true, ...user };

console.log[admin];

• A: { admin: true, user: { name: "Lydia", age: 21 } }
• B: { admin: true, name: "Lydia", age: 21 }
• C: { admin: true, user: ["Lydia", 21] }
• D: { admin: true }

Jawaban: B

It's possible to combine objects using the spread operator .... It lets you create copies of the key/value pairs of one object, and add them to another object. In this case, we create copies of the user object, and add them to the admin object. The admin object now contains the copied key/value pairs, which results in { admin: true, name: "Lydia", age: 21 }.

const person = { name: 'Lydia' };

Object.defineProperty[person, 'age', { value: 21 }];

console.log[person];
console.log[Object.keys[person]];

• A: { name: "Lydia", age: 21 }, ["name", "age"]
• B: { name: "Lydia", age: 21 }, ["name"]
• C: { name: "Lydia"}, ["name", "age"]
• D: { name: "Lydia"}, ["age"]

Jawaban: B

With the defineProperty method, we can add new properties to an object, or modify existing ones. When we add a property to an object using the defineProperty method, they are by default not enumerable. The Object.keys method returns all enumerable property names from an object, in this case only "name".

Properties added using the defineProperty method are immutable by default. You can override this behavior using the writable, configurable and enumerable properties. This way, the defineProperty method gives you a lot more control over the properties you're adding to an object.

const settings = {
  username: 'lydiahallie',
  level: 19,
  health: 90,
};

const data = JSON.stringify[settings, ['level', 'health']];
console.log[data];

• A: "{"level":19, "health":90}"
• B: "{"username": "lydiahallie"}"
• C: "["level", "health"]"
• D: "{"username": "lydiahallie", "level":19, "health":90}"

Jawaban: A

The second argument of JSON.stringify is the replacer. The replacer can either be a function or an array, and lets you control what and how the values should be stringified.

If the replacer is an array, only the property names included in the array will be added to the JSON string. In this case, only the properties with the names "level" and "health" are included, "username" is excluded. data is now equal to "{"level":19, "health":90}".

If the replacer is a function, this function gets called on every property in the object you're stringifying. The value returned from this function will be the value of the property when it's added to the JSON string. If the value is undefined, this property is excluded from the JSON string.

let num = 10;

const increaseNumber = [] => num++;
const increasePassedNumber = number => number++;

const num1 = increaseNumber[];
const num2 = increasePassedNumber[num1];

console.log[num1];
console.log[num2];

• A: 10, 10
• B: 10, 11
• C: 11, 11
• D: 11, 12

Jawaban: A

The unary operator ++ first returns the value of the operand, then increments the value of the operand. The value of num1 is 10, since the increaseNumber function first returns the value of num, which is 10, and only increments the value of num afterwards.

num2 is 10, since we passed num1 to the increasePassedNumber. number is equal to 10[the value of num1. Again, the unary operator ++ first returns the value of the operand, then increments the value of the operand. The value of number is 10, so num2 is equal to 10.

const value = { number: 10 };

const multiply = [x = { ...value }] => {
  console.log[[x.number *= 2]];
};

multiply[];
multiply[];
multiply[value];
multiply[value];

• A: 20, 40, 80, 160
• B: 20, 40, 20, 40
• C: 20, 20, 20, 40
• D: NaN, NaN, 20, 40

Jawaban: C

In ES6, we can initialize parameters with a default value. The value of the parameter will be the default value, if no other value has been passed to the function, or if the value of the parameter is "undefined". In this case, we spread the properties of the value object into a new object, so x has the default value of { number: 10 }.

The default argument is evaluated at call time! Every time we call the function, a new object is created. We invoke the multiply function the first two times without passing a value: x has the default value of { number: 10 }. We then log the multiplied value of that number, which is 20.

The third time we invoke multiply, we do pass an argument: the object called value. The *= operator is actually shorthand for x.number = x.number * 2: we modify the value of x.number, and log the multiplied value 20.

The fourth time, we pass the value object again. x.number was previously modified to 20, so x.number *= 2 logs 40.

[1, 2, 3, 4].reduce[[x, y] => console.log[x, y]];

• A: 1 2 and 3 3 and 6 4
• B: 1 2 and 2 3 and 3 4
• C: 1 undefined and 2 undefined and 3 undefined and 4 undefined
• D: 1 2 and undefined 3 and undefined 4

Jawaban: D

The first argument that the reduce method receives is the accumulator, x in this case. The second argument is the current value, y. With the reduce method, we execute a callback function on every element in the array, which could ultimately result in one single value.

In this example, we are not returning any values, we are simply logging the values of the accumulator and the current value.

The value of the accumulator is equal to the previously returned value of the callback function. If you don't pass the optional initialValue argument to the reduce method, the accumulator is equal to the first element on the first call.

On the first call, the accumulator [x] is 1, and the current value [y] is 2. We don't return from the callback function, we log the accumulator and current value: 1 and 2 get logged.

If you don't return a value from a function, it returns undefined. On the next call, the accumulator is undefined, and the current value is 3. undefined and 3 get logged.

On the fourth call, we again don't return from the callback function. The accumulator is again undefined, and the current value is 4. undefined and 4 get logged.

class Dog {
  constructor[name] {
    this.name = name;
  }
};

class Labrador extends Dog {
  // 1
  constructor[name, size] {
    this.size = size;
  }
  // 2
  constructor[name, size] {
    super[name];
    this.size = size;
  }
  // 3
  constructor[size] {
    super[name];
    this.size = size;
  }
  // 4
  constructor[name, size] {
    this.name = name;
    this.size = size;
  }

};

• A: 1
• B: 2
• C: 3
• D: 4

Jawaban: B

In a derived class, you cannot access the this keyword before calling super. If you try to do that, it will throw a ReferenceError: 1 and 4 would throw a reference error.

With the super keyword, we call that parent class's constructor with the given arguments. The parent's constructor receives the name argument, so we need to pass name to super.

The Labrador class receives two arguments, name since it extends Dog, and size as an extra property on the Labrador class. They both need to be passed to the constructor function on Labrador, which is done correctly using constructor 2.

// index.js
console.log['running index.js'];
import { sum } from './sum.js';
console.log[sum[1, 2]];

// sum.js
console.log['running sum.js'];
export const sum = [a, b] => a + b;

• A: running index.js, running sum.js, 3
• B: running sum.js, running index.js, 3
• C: running sum.js, 3, running index.js
• D: running index.js, undefined, running sum.js

Jawaban: B

With the import keyword, all imported modules are pre-parsed. This means that the imported modules get run first, the code in the file which imports the module gets executed after.

This is a difference between require[] in CommonJS and import! With require[], you can load dependencies on demand while the code is being run. If we would have used require instead of import, running index.js, running sum.js, 3 would have been logged to the console.

console.log[Number[2] === Number[2]];
console.log[Boolean[false] === Boolean[false]];
console.log[Symbol['foo'] === Symbol['foo']];

• A: true, true, false
• B: false, true, false
• C: true, false, true
• D: true, true, true

Jawaban: A

Every Symbol is entirely unique. The purpose of the argument passed to the Symbol is to give the Symbol a description. The value of the Symbol is not dependent on the passed argument. As we test equality, we are creating two entirely new symbols: the first Symbol['foo'], and the second Symbol['foo']. These two values are unique and not equal to each other, Symbol['foo'] === Symbol['foo'] returns false.

const name = 'Lydia Hallie';
console.log[name.padStart[13]];
console.log[name.padStart[2]];

• A: "Lydia Hallie", "Lydia Hallie"
• B: " Lydia Hallie", " Lydia Hallie" ["[13x whitespace]Lydia Hallie", "[2x whitespace]Lydia Hallie"]
• C: " Lydia Hallie", "Lydia Hallie" ["[1x whitespace]Lydia Hallie", "Lydia Hallie"]
• D: "Lydia Hallie", "Lyd",

Jawaban: C

With the padStart method, we can add padding to the beginning of a string. The value passed to this method is the total length of the string together with the padding. The string "Lydia Hallie" has a length of 12. name.padStart[13] inserts 1 space at the start of the string, because 12 + 1 is 13.

If the argument passed to the padStart method is smaller than the length of the array, no padding will be added.

console.log['🥑' + '💻'];

• A: "🥑💻"
• B: 257548
• C: A string containing their code points
• D: Error

Jawaban: A

With the + operator, you can concatenate strings. In this case, we are concatenating the string "🥑" with the string "💻", resulting in "🥑💻".

function* startGame[] {
  const answer = yield 'Do you love JavaScript?';
  if [answer !== 'Yes'] {
    return "Oh wow... Guess we're gone here";
  }
  return 'JavaScript loves you back ❤️';
}

const game = startGame[];
console.log[/* 1 */]; // Do you love JavaScript?
console.log[/* 2 */]; // JavaScript loves you back ❤️

• A: game.next["Yes"].value dan game.next[].value
• B: game.next.value["Yes"] dan game.next.value[]
• C: game.next[].value dan game.next["Yes"].value
• D: game.next.value[] dan game.next.value["Yes"]

Jawaban: C

A generator function "pauses" its execution when it sees the yield keyword. First, we have to let the function yield the string "Do you love JavaScript?", which can be done by calling game.next[].value.

Every line is executed, until it finds the first yield keyword. There is a yield keyword on the first line within the function: the execution stops with the first yield! This means that the variable answer is not defined yet!

When we call game.next["Yes"].value, the previous yield is replaced with the value of the parameters passed to the next[] function, "Yes" in this case. The value of the variable answer is now equal to "Yes". The condition of the if-statement returns false, and JavaScript loves you back ❤️ gets logged.

console.log[String.raw`Hello\nworld`];

• A: Hello world!
• B: Hello
     world
• C: Hello\nworld
• D: Hello\n
     world

Jawaban: C

String.raw returns a string where the escapes [\n, \v, \t etc.] are ignored! Backslashes can be an issue since you could end up with something like:

const path = `C:\Documents\Projects\table.html`

Which would result in:

"C:DocumentsProjects able.html"

With String.raw, it would simply ignore the escape and print:

C:\Documents\Projects\table.html

In this case, the string is Hello\nworld, which gets logged.

async function getData[] {
  return await Promise.resolve['I made it!'];
}

const data = getData[];
console.log[data];

• A: "I made it!"
• B: Promise {: "I made it!"}
• C: Promise {}
• D: undefined

Jawaban: C

An async function always returns a promise. The await still has to wait for the promise to resolve: a pending promise gets returned when we call getData[] in order to set data equal to it.

If we wanted to get access to the resolved value "I made it", we could have used the .then[] method on data:

data.then[res => console.log[res]]

This would've logged "I made it!"

function addToList[item, list] {
  return list.push[item];
}

const result = addToList['apple', ['banana']];
console.log[result];

• A: ['apple', 'banana']
• B: 2
• C: true
• D: undefined

Jawaban: B

The .push[] method returns the length of the new array! Previously, the array contained one element [the string "banana"] and had a length of 1. After adding the string "apple" to the array, the array contains two elements, and has a length of 2. This gets returned from the addToList function.

The push method modifies the original array. If you wanted to return the array from the function rather than the length of the array, you should have returned list after pushing item to it.

const box = { x: 10, y: 20 };

Object.freeze[box];

const shape = box;
shape.x = 100;

console.log[shape];

• A: { x: 100, y: 20 }
• B: { x: 10, y: 20 }
• C: { x: 100 }
• D: ReferenceError

Jawaban: B

Object.freeze makes it impossible to add, remove, or modify properties of an object [unless the property's value is another object].

When we create the variable shape and set it equal to the frozen object box, shape also refers to a frozen object. You can check whether an object is frozen by using Object.isFrozen. In this case, Object.isFrozen[shape] returns true, since the variable shape has a reference to a frozen object.

Since shape is frozen, and since the value of x is not an object, we cannot modify the property x. x is still equal to 10, and { x: 10, y: 20 } gets logged.

const { name: myName } = { name: 'Lydia' };

console.log[name];

• A: "Lydia"
• B: "myName"
• C: undefined
• D: ReferenceError

Jawaban: D

When we unpack the property name from the object on the right-hand side, we assign its value "Lydia" to a variable with the name myName.

With { name: myName }, we tell JavaScript that we want to create a new variable called myName with the value of the name property on the right-hand side.

Since we try to log name, a variable that is not defined, a ReferenceError gets thrown.

function sum[a, b] {
  return a + b;
}

• A: Yes
• B: No

Jawaban: A

A pure function is a function that always returns the same result, if the same arguments are passed.

The sum function always returns the same result. If we pass 1 and 2, it will always return 3 without side effects. If we pass 5 and 10, it will always return 15, and so on. This is the definition of a pure function.

const add = [] => {
  const cache = {};
  return num => {
    if [num in cache] {
      return `From cache! ${cache[num]}`;
    } else {
      const result = num + 10;
      cache[num] = result;
      return `Calculated! ${result}`;
    }
  };
};

const addFunction = add[];
console.log[addFunction[10]];
console.log[addFunction[10]];
console.log[addFunction[5 * 2]];

• A: Calculated! 20 Calculated! 20 Calculated! 20
• B: Calculated! 20 From cache! 20 Calculated! 20
• C: Calculated! 20 From cache! 20 From cache! 20
• D: Calculated! 20 From cache! 20 Error

Jawaban: C

The add function is a memoized function. With memoization, we can cache the results of a function in order to speed up its execution. In this case, we create a cache object that stores the previously returned values.

If we call the addFunction function again with the same argument, it first checks whether it has already gotten that value in its cache. If that's the case, the caches value will be returned, which saves on execution time. Else, if it's not cached, it will calculate the value and store it afterwards.

We call the addFunction function three times with the same value: on the first invocation, the value of the function when num is equal to 10 isn't cached yet. The condition of the if-statement num in cache returns false, and the else block gets executed: Calculated! 20 gets logged, and the value of the result gets added to the cache object. cache now looks like { 10: 20 }.

The second time, the cache object contains the value that gets returned for 10. The condition of the if-statement num in cache returns true, and 'From cache! 20' gets logged.

The third time, we pass 5 * 2 to the function which gets evaluated to 10. The cache object contains the value that gets returned for 10. The condition of the if-statement num in cache returns true, and 'From cache! 20' gets logged.

const myLifeSummedUp = ['☕', '💻', '🍷', '🍫'];

for [let item in myLifeSummedUp] {
  console.log[item];
}

for [let item of myLifeSummedUp] {
  console.log[item];
}

• A: 0 1 2 3 and "☕" "💻" "🍷" "🍫"
• B: "☕" "💻" "🍷" "🍫" and "☕" "💻" "🍷" "🍫"
• C: "☕" "💻" "🍷" "🍫" and 0 1 2 3
• D: 0 1 2 3 and {0: "☕", 1: "💻", 2: "🍷", 3: "🍫"}

Jawaban: A

With a for-in loop, we can iterate over enumerable properties. In an array, the enumerable properties are the "keys" of array elements, which are actually their indexes. You could see an array as:

{0: "☕", 1: "💻", 2: "🍷", 3: "🍫"}

Where the keys are the enumerable properties. 0 1 2 3 get logged.

With a for-of loop, we can iterate over iterables. An array is an iterable. When we iterate over the array, the variable "item" is equal to the element it's currently iterating over, "☕" "💻" "🍷" "🍫" get logged.

const list = [1 + 2, 1 * 2, 1 / 2];
console.log[list];

• A: ["1 + 2", "1 * 2", "1 / 2"]
• B: ["12", 2, 0.5]
• C: [3, 2, 0.5]
• D: [1, 1, 1]

Jawaban: C

Elemen array dapat berisi beberapa nilai. angka, string, objek, array lain, null, nilai boolean, undefined, dan lainnya seperti tanggal, fungsi, dan kalkulasi.

elemen akan sama dengan nilai hasilnya. 1 + 2 menghasilkan 3, 1 * 2 menghasilkan 2, dan 1 / 2 menghasilkan 0.5.

function sayHi[name] {
  return `Hi there, ${name}`;
}

console.log[sayHi[]];

• A: Hi there,
• B: Hi there, undefined
• C: Hi there, null
• D: ReferenceError

Jawaban: B

Secara default, arguments memiliki nilai undefined, kecuali nilai telah diisi ke fungsi. Pada kasus ini, kita tidak mengisi nilai untuk argument name. name sama dengan undefined yang mana mendapat catatan.

Di ES6, kita dapat menulis ulang nilai default undefined dengan parameter default. Sebagai contoh:

function sayHi[name = "Lydia"] { ... }

Pada kasus ini, juka kita tidak mengisi nilai atau mengisi undefined, name akan selalu sama dengan string Lydia

var status = '😎';

setTimeout[[] => {
  const status = '😍';

  const data = {
    status: '🥑',
    getStatus[] {
      return this.status;
    },
  };

  console.log[data.getStatus[]];
  console.log[data.getStatus.call[this]];
}, 0];

• A: "🥑" and "😍"
• B: "🥑" and "😎"
• C: "😍" and "😎"
• D: "😎" and "😎"

Jawaban: B

The value of the this keyword is dependent on where you use it. In a method, like the getStatus method, the this keyword refers to the object that the method belongs to. The method belongs to the data object, so this refers to the data object. When we log this.status, the status property on the data object gets logged, which is "🥑".

With the call method, we can change the object to which the this keyword refers. In functions, the this keyword refers to the the object that the function belongs to. We declared the setTimeout function on the global object, so within the setTimeout function, the this keyword refers to the global object. On the global object, there is a variable called status with the value of "😎". When logging this.status, "😎" gets logged.

const person = {
  name: 'Lydia',
  age: 21,
};

let city = person.city;
city = 'Amsterdam';

console.log[person];

• A: { name: "Lydia", age: 21 }
• B: { name: "Lydia", age: 21, city: "Amsterdam" }
• C: { name: "Lydia", age: 21, city: undefined }
• D: "Amsterdam"

Jawaban: A

We set the variable city equal to the value of the property called city on the person object. There is no property on this object called city, so the variable city has the value of undefined.

Note that we are not referencing the person object itself! We simply set the variable city equal to the current value of the city property on the person object.

Then, we set city equal to the string "Amsterdam". This doesn't change the person object: there is no reference to that object.

When logging the person object, the unmodified object gets returned.

function checkAge[age] {
  if [age  'greeting';

myMap.set[myFunc, 'Hello world!'];

//1
myMap.get['greeting'];
//2
myMap.get[myFunc];
//3
myMap.get[[] => 'greeting'];

• A: 1
• B: 2
• C: 2 and 3
• D: All of them

Jawaban: B

When adding a key/value pair using the set method, the key will be the value of the first argument passed to the set function, and the value will be the second argument passed to the set function. The key is the function [] => 'greeting' in this case, and the value 'Hello world'. myMap is now { [] => 'greeting' => 'Hello world!' }.

1 is wrong, since the key is not 'greeting' but [] => 'greeting'. 3 is wrong, since we're creating a new function by passing it as a parameter to the get method. Object interact by reference. Functions are objects, which is why two functions are never strictly equal, even if they are identical: they have a reference to a different spot in memory.

const person = {
  name: 'Lydia',
  age: 21,
};

const changeAge = [x = { ...person }] => [x.age += 1];
const changeAgeAndName = [x = { ...person }] => {
  x.age += 1;
  x.name = 'Sarah';
};

changeAge[person];
changeAgeAndName[];

console.log[person];

• A: {name: "Sarah", age: 22}
• B: {name: "Sarah", age: 23}
• C: {name: "Lydia", age: 22}
• D: {name: "Lydia", age: 23}

Jawaban: C

Both the changeAge and changeAgeAndName functions have a default parameter, namely a newly created object { ...person }. This object has copies of all the key/values in the person object.

First, we invoke the changeAge function and pass the person object as its argument. This function increases the value of the age property by 1. person is now { name: "Lydia", age: 22 }.

Then, we invoke the changeAgeAndName function, however we don't pass a parameter. Instead, the value of x is equal to a new object: { ...person }. Since it's a new object, it doesn't affect the values of the properties on the person object. person is still equal to { name: "Lydia", age: 22 }.

function sumValues[x, y, z] {
  return x + y + z;
}

• A: sumValues[[...1, 2, 3]]
• B: sumValues[[...[1, 2, 3]]]
• C: sumValues[...[1, 2, 3]]
• D: sumValues[[1, 2, 3]]

Jawaban: C

With the spread operator ..., we can spread iterables to individual elements. The sumValues function receives three arguments: x, y and z. ...[1, 2, 3] will result in 1, 2, 3, which we pass to the sumValues function.

let num = 1;
const list = ['🥳', '🤠', '🥰', '🤪'];

console.log[list[[num += 1]]];

• A: 🤠
• B: 🥰
• C: SyntaxError
• D: ReferenceError

Jawaban: B

With the += operand, we're incrementing the value of num by 1. num had the initial value 1, so 1 + 1 is 2. The item on the second index in the list array is 🥰, console.log[list[2]] prints 🥰.

const person = {
  firstName: 'Lydia',
  lastName: 'Hallie',
  pet: {
    name: 'Mara',
    breed: 'Dutch Tulip Hound',
  },
  getFullName[] {
    return `${this.firstName} ${this.lastName}`;
  },
};

console.log[person.pet?.name];
console.log[person.pet?.family?.name];
console.log[person.getFullName?.[]];
console.log[member.getLastName?.[]];

• A: undefined undefined undefined undefined
• B: Mara undefined Lydia Hallie undefined
• C: Mara null Lydia Hallie null
• D: null ReferenceError null ReferenceError

Jawaban: B

With the optional chaining operator ?., we no longer have to explicitly check whether the deeper nested values are valid or not. If we're trying to access a property on an undefined or null value [nullish], the expression short-circuits and returns undefined.

person.pet?.name: person has a property named pet: person.pet is not nullish. It has a property called name, and returns Mara. person.pet?.family?.name: person has a property named pet: person.pet is not nullish. pet does not have a property called family, person.pet.family is nullish. The expression returns undefined. person.getFullName?.[]: person has a property named getFullName: person.getFullName[] is not nullish and can get invoked, which returns Lydia Hallie. member.getLastName?.[]: member is not defined: member.getLastName[] is nullish. The expression returns undefined.

const groceries = ['banana', 'apple', 'peanuts'];

if [groceries.indexOf['banana']] {
  console.log['We have to buy bananas!'];
} else {
  console.log[`We don't have to buy bananas!`];
}

• A: We have to buy bananas!
• B: We don't have to buy bananas
• C: undefined
• D: 1

Jawaban: B

We passed the condition groceries.indexOf["banana"] to the if-statement. groceries.indexOf["banana"] returns 0, which is a falsy value. Since the condition in the if-statement is falsy, the code in the else block runs, and We don't have to buy bananas! gets logged.

const config = {
  languages: [],
  set language[lang] {
    return this.languages.push[lang];
  },
};

console.log[config.language];

• A: function language[lang] { this.languages.push[lang }
• B: 0
• C: []
• D: undefined

Jawaban: D

The language method is a setter. Setters don't hold an actual value, their purpose is to modify properties. When calling a setter method, undefined gets returned.

const name = 'Lydia Hallie';

console.log[!typeof name === 'object'];
console.log[!typeof name === 'string'];

• A: false true
• B: true false
• C: false false
• D: true true

Jawaban: C

typeof name returns "string". The string "string" is a truthy value, so !typeof name returns the boolean value false. false === "object" and false === "string" both returnfalse.

[If we wanted to check whether the type was [un]equal to a certain type, we should've written !== instead of !typeof]

const add = x => y => z => {
  console.log[x, y, z];
  return x + y + z;
};

add[4][5][6];

• A: 4 5 6
• B: 6 5 4
• C: 4 function function
• D: undefined undefined 6

Jawaban: A

The add function returns an arrow function, which returns an arrow function, which returns an arrow function [still with me?]. The first function receives an argument x with the value of 4. We invoke the second function, which receives an argument y with the value 5. Then we invoke the third function, which receives an argument z with the value 6. When we're trying to access the value x, y and z within the last arrow function, the JS engine goes up the scope chain in order to find the values for x and y accordingly. This returns 4 5 6.

async function* range[start, end] {
  for [let i = start; i  {
  const gen = range[1, 3];
  for await [const item of gen] {
    console.log[item];
  }
}][];

• A: Promise {1} Promise {2} Promise {3}
• B: Promise {} Promise {} Promise {}
• C: 1 2 3
• D: undefined undefined undefined

Jawaban: C

The generator function range returns an async object with promises for each item in the range we pass: Promise{1}, Promise{2}, Promise{3}. We set the variable gen equal to the async object, after which we loop over it using a for await ... of loop. We set the variable item equal to the returned Promise values: first Promise{1}, then Promise{2}, then Promise{3}. Since we're awaiting the value of item, the resolved promsie, the resolved values of the promises get returned: 1, 2, then 3.

const myFunc = [{ x, y, z }] => {
  console.log[x, y, z];
};

myFunc[1, 2, 3];

• A: 1 2 3
• B: {1: 1} {2: 2} {3: 3}
• C: { 1: undefined } undefined undefined
• D: undefined undefined undefined

Jawaban: D

myFunc expects an object with properties x, y and z as its argument. Since we're only passing three separate numeric values [1, 2, 3] instead of one object with properties x, y and z [{x: 1, y: 2, z: 3}], x, y and z have their default value of undefined.

function getFine[speed, amount] {
  const formattedSpeed = new Intl.NumberFormat[{
    'en-US',
    { style: 'unit', unit: 'mile-per-hour' }
  }].format[speed]

  const formattedAmount = new Intl.NumberFormat[{
    'en-US',
    { style: 'currency', currency: 'USD' }
  }].format[amount]

  return `The driver drove ${formattedSpeed} and has to pay ${formattedAmount}`
}

console.log[getFine[130, 300]]

• A: The driver drove 130 and has to pay 300
• B: The driver drove 130 mph and has to pay $300.00
• C: The driver drove undefined and has to pay undefined
• D: The driver drove 130.00 and has to pay 300.00

Jawaban: B

With the Intl.NumberFormat method, we can format numeric values to any locale. We format the numeric value 130 to the en-US locale as a unit in mile-per-hour, which results in 130 mph. The numeric value 300 to the en-US locale as a currentcy in USD results in $300.00.

const spookyItems = ['👻', '🎃', '🕸'];
[{ item: spookyItems[3] } = { item: '💀' }];

console.log[spookyItems];

• A: ["👻", "🎃", "🕸"]
• B: ["👻", "🎃", "🕸", "💀"]
• C: ["👻", "🎃", "🕸", { item: "💀" }]
• D: ["👻", "🎃", "🕸", "[object Object]"]

Jawaban: B

By destructuring objects, we can unpack values from the right-hand object, and assign the unpacked value to the value of the same property name on the left-hand object. In this case, we're assigning the value "💀" to spookyItems[3]. This means that we're modifying the spookyItems array, we're adding the "💀" to it. When logging spookyItems, ["👻", "🎃", "🕸", "💀"] gets logged.

const name = 'Lydia Hallie';
const age = 21;

console.log[Number.isNaN[name]];
console.log[Number.isNaN[age]];

console.log[isNaN[name]];
console.log[isNaN[age]];

• A: true false true false
• B: true false false false
• C: false false true false
• D: false true false true

Jawaban: C

With the Number.isNaN method, you can check if the value you pass is a numeric value and equal to NaN. name is not a numeric value, so Number.isNaN[name] returns false. age is a numeric value, but is not equal to NaN, so Number.isNaN[age] returns false.

With the isNaN method, you can check if the value you pass is not a number. name is not a number, so isNaN[name] returns true. age is a number, so isNaN[age] returns false.

const randomValue = 21;

function getInfo[] {
  console.log[typeof randomValue];
  const randomValue = 'Lydia Hallie';
}

getInfo[];

• A: "number"
• B: "string"
• C: undefined
• D: ReferenceError

Jawaban: D

Variables declared with the const keyword are not referencable before their initialization: this is called the temporal dead zone. In the getInfo function, the variable randomValue is scoped in the functional scope of getInfo. On the line where we want to log the value of typeof randomValue, the variable randomValue isn't initialized yet: a ReferenceError gets thrown! The engine didn't go down the scope chain since we declared the variable randomValue in the getInfo function.

const myPromise = Promise.resolve['Woah some cool data'];

[async [] => {
  try {
    console.log[await myPromise];
  } catch {
    throw new Error[`Oops didn't work`];
  } finally {
    console.log['Oh finally!'];
  }
}][];

• A: Woah some cool data
• B: Oh finally!
• C: Woah some cool data Oh finally!
• D: Oops didn't work Oh finally!

Jawaban: C

In the try block, we're logging the awaited value of the myPromise variable: "Woah some cool data". Since no errors were thrown in the try block, the code in the catch block doesn't run. The code in the finally block always runs, "Oh finally!" gets logged.

const emojis = ['🥑', ['✨', '✨', ['🍕', '🍕']]];

console.log[emojis.flat[1]];

• A: ['🥑', ['✨', '✨', ['🍕', '🍕']]]
• B: ['🥑', '✨', '✨', ['🍕', '🍕']]
• C: ['🥑', ['✨', '✨', '🍕', '🍕']]
• D: ['🥑', '✨', '✨', '🍕', '🍕']

Jawaban: B

With the flat method, we can create a new, flattened array. The depth of the flattened array depends on the value that we pass. In this case, we passed the value 1 [which we didn't have to, that's the default value], meaning that only the arrays on the first depth will be concatenated. ['🥑'] and ['✨', '✨', ['🍕', '🍕']] in this case. Concatenating these two arrays results in ['🥑', '✨', '✨', ['🍕', '🍕']].

class Counter {
  constructor[] {
    this.count = 0;
  }

  increment[] {
    this.count++;
  }
}

const counterOne = new Counter[];
counterOne.increment[];
counterOne.increment[];

const counterTwo = counterOne;
counterTwo.increment[];

console.log[counterOne.count];

• A: 0
• B: 1
• C: 2
• D: 3

Jawaban: D

counterOne is an instance of the Counter class. The counter class contains a count property on its constructor, and an increment method. First, we invoked the increment method twice by calling counterOne.increment[]. Currently, counterOne.count is 2.

Then, we create a new variable counterTwo, and set it equal to counterOne. Since objects interact by reference, we're just creating a new reference to the same spot in memory that counterOne points to. Since it has the same spot in memory, any changes made to the object that counterTwo has a reference to, also apply to counterOne. Currently, counterTwo.count is 2.

We invoke the counterTwo.increment[], which sets the count to 3. Then, we log the count on counterOne, which logs 3.

const myPromise = Promise.resolve[Promise.resolve['Promise!']];

function funcOne[] {
  myPromise.then[res => res].then[res => console.log[res]];
  setTimeout[[] => console.log['Timeout!', 0]];
  console.log['Last line!'];
}

async function funcTwo[] {
  const res = await myPromise;
  console.log[await res];
  setTimeout[[] => console.log['Timeout!', 0]];
  console.log['Last line!'];
}

funcOne[];
funcTwo[];

• A: Promise! Last line! Promise! Last line! Last line! Promise!
• B: Last line! Timeout! Promise! Last line! Timeout! Promise!
• C: Promise! Last line! Last line! Promise! Timeout! Timeout!
• D: Last line! Promise! Promise! Last line! Timeout! Timeout!

Jawaban: D

First, we invoke funcOne. On the first line of funcOne, we call the myPromise promise, which is an asynchronous operation. While the engine is busy completing the promise, it keeps on running the function funcOne. The next line is the asynchronous setTimeout function, from which the callback is sent to the Web API. [see my article on the event loop here.]

Both the promise and the timeout are asynchronous operations, the function keeps on running while it's busy completing the promise and handling the setTimeout callback. This means that Last line! gets logged first, since this is not an asynchonous operation. This is the last line of funcOne, the promise resolved, and Promise! gets logged. However, since we're invoking funcTwo[], the call stack isn't empty, and the callback of the setTimeout function cannot get added to the callstack yet.

In funcTwo we're, first awaiting the myPromise promise. With the await keyword, we pause the execution of the function until the promise has resolved [or rejected]. Then, we log the awaited value of res [since the promise itself returns a promise]. This logs Promise!.

The next line is the asynchronous setTimeout function, from which the callback is sent to the Web API.

We get to the last line of funcTwo, which logs Last line! to the console. Now, since funcTwo popped off the call stack, the call stack is empty. The callbacks waiting in the queue [[] => console.log["Timeout!"] from funcOne, and [] => console.log["Timeout!"] from funcTwo] get added to the call stack one by one. The first callback logs Timeout!, and gets popped off the stack. Then, the second callback logs Timeout!, and gets popped off the stack. This logs Last line! Promise! Promise! Last line! Timeout! Timeout!

// sum.js
export default function sum[x] {
  return x + x;
}

// index.js
import * as sum from './sum';

• A: sum[4]
• B: sum.sum[4]
• C: sum.default[4]
• D: Default aren't imported with *, only named exports

Jawaban: C

With the asterisk *, we import all exported values from that file, both default and named. If we had the following file:

// info.js
export const name = 'Lydia';
export const age = 21;
export default 'I love JavaScript';

// index.js
import * as info from './info';
console.log[info];

The following would get logged:

{
  default: "I love JavaScript",
  name: "Lydia",
  age: 21
}

For the sum example, it means that the imported value sum looks like this:

{ default: function sum[x] { return x + x } }

We can invoke this function, by calling sum.default

const handler = {
  set: [] => console.log['Added a new property!'],
  get: [] => console.log['Accessed a property!'],
};

const person = new Proxy[{}, handler];

person.name = 'Lydia';
person.name;

• A: Added a new property!
• B: Accessed a property!
• C: Added a new property! Accessed a property!
• D: Nothing gets logged

Jawaban: C

With a Proxy object, we can add custom behavior to an object that we pass to it as the second argument. In tis case, we pass the handler object which contained to properties: set and get. set gets invoked whenever we set property values, get gets invoked whenever we get [access] property values.

The first argument is an empty object {}, which is the value of person. To this object, the custom behavior specified in the handler object gets added. If we add a property to the person object, set will get invoked. If we access a property on the person object, get gets invoked.

First, we added a new property name to the proxy object [person.name = "Lydia"]. set gets invoked, and logs "Added a new property!".

Then, we access a property value on the proxy object, the get property on the handler object got invoked. "Accessed a property!" gets logged.

const person = { name: 'Lydia Hallie' };

Object.seal[person];

• A: person.name = "Evan Bacon"
• B: person.age = 21
• C: delete person.name
• D: Object.assign[person, { age: 21 }]

Jawaban: A

With Object.seal we can prevent new properies from being added, or existing properties to be removed.

However, you can still modify the value of existing properties.

const person = {
  name: 'Lydia Hallie',
  address: {
    street: '100 Main St',
  },
};

Object.freeze[person];

• A: person.name = "Evan Bacon"
• B: delete person.address
• C: person.address.street = "101 Main St"
• D: person.pet = { name: "Mara" }

Jawaban: C

The Object.freeze method freezes an object. No properties can be added, modified, or removed.

However, it only shallowly freezes the object, meaning that only direct properties on the object are frozen. If the property is another object, like address in this case, the properties on that object aren't frozen, and can be modified.

const person = {
  name: 'Lydia Hallie',
  address: {
    street: '100 Main St',
  },
};

Object.freeze[person];

• A: person.name = "Evan Bacon"
• B: delete person.address
• C: person.address.street = "101 Main St"
• D: person.pet = { name: "Mara" }

Jawaban: C

The Object.freeze method freezes an object. No properties can be added, modified, or removed.

However, it only shallowly freezes the object, meaning that only direct properties on the object are frozen. If the property is another object, like address in this case, the properties on that object aren't frozen, and can be modified.

const add = x => x + x;

function myFunc[num = 2, value = add[num]] {
  console.log[num, value];
}

myFunc[];
myFunc[3];

• A: 2 4 dan 3 6
• B: 2 NaN dan 3 NaN
• C: 2 Error dan 3 6
• D: 2 4 dan 3 Error

Jawaban: A

First, we invoked myFunc[] without passing any arguments. Since we didn't pass arguments, num and value got their default values: num is 2, and value the returned value of the function add. To the add function, we pass num as an argument, which had the value of 2. add returns 4, which is the value of value.

Then, we invoked myFunc[3] and passed the value 3 as the value for the argument num. We didn't pass an argument for value. Since we didn't pass a value for the value argument, it got the default value: the returned value of the add function. To add, we pass num, which has the value of 3. add returns 6, which is the value of value.

class Counter {
  #number = 10

  increment[] {
    this.#number++
  }

  getNum[] {
    return this.#number
  }
}

const counter = new Counter[]
counter.increment[]

console.log[counter.#number]

• A: 10
• B: 11
• C: undefined
• D: SyntaxError

Jawaban: D

In ES2020, we can add private variables in classes by using the #. We cannot access these variables outside of the class. When we try to log counter.#number, a SyntaxError gets thrown: we cannot acccess it outside the Counter class!

const teams = [
  { name: 'Team 1', members: ['Paul', 'Lisa'] },
  { name: 'Team 2', members: ['Laura', 'Tim'] },
];

function* getMembers[members] {
  for [let i = 0; i  Hello World!