Membuat fungsi di javascript

Fungsi pada JavaScript menggunakan keyword function. Terdapat dua jenis fungsi, pertama adalah fungsi yang dideklarasikan (function declaration) dan kedua adalah fungsi sebagai perhitungan (fungcion expression).

Tutorial sebelumnya : JavaScript Break

Function Declaration

Fungsi ini sudah pernah dibahas pada tutorial sebelumnya. Fungsi jenis ini harus dideklarasikan terlebih dahulu dan kemudian dipanggil oleh baris coding lain.

Bahan Belajar JavaScript Fungsi Lain

function functionName(parameters) {
  // code to be executed
}

Berikut contoh function declaration.

function myFunction(a, b) {
  return a * b;
}

Function Expression

Fungsi juga dapat dibuat dengan expression (perhitungan beberapa variable atau nilai). Fungsi jenis ini dapat disimpan sebagai nilai variable. Kemudian variable ini bisa dipanggil pada baris coding lainnya.

Pada contoh di bawah ini, variable x bernilai fungsi dengan dua parameter (a dan b), fungsi ini mengembalikan nilai a dikali b.

var x = function (a, b) {return a * b};

Variable x dapat dipanggil sebagai fungsi (function expression).

var x = function (a, b) {return a * b};
var z = x(4, 3);

Fungsi Sebagai Nilai

Fungsi dapat digunakan untuk memberikan nilai pada suatu variable.

function myFunction(a, b) {
  return a * b;
}

var x = myFunction(4, 3);

Contoh di atas memperlihatkan fungsi myFunction memberikan nilai kepada variable x.

Fungsi juga dapat dimasukkan dalam suatu perhitungan.

function myFunction(a, b) {
  return a * b;
}

var x = myFunction(4, 3) * 2;

Contoh di atas memperlihatkan fungsi myFunction dikalikan dengan 2 dan hasil perkalian menjadi nilai variable x.

Arrow Function

Arrow Function adalah cara penulisan singkat untuk function expression. Kita tidak perlu menuliskan {} serta keyword function dan return.

// ES5
var x = function(x, y) {
  return x * y;
}

// ES6
const x = (x, y) => x * y;

Keyword return dan {} tetap diperlukan apabila fungsi ini berisi lebih dari satu baris coding. Sebaiknya dibiasakan tetap menuliskan keyword return dan {} pada arrow function.

const x = (x, y) => { return x * y };

Tutorial selanjutnya : Parameter Pada Fungsi
Daftar Tutorial : Tutorial Belajar JavaScript

A JavaScript function is a block of code designed to perform a particular task.

A JavaScript function is executed when "something" invokes it (calls it).

Example

// Function to compute the product of p1 and p2
function myFunction(p1, p2) {
  return p1 * p2;
}

Try it Yourself »

JavaScript Function Syntax

A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses ().

Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).

The parentheses may include parameter names separated by commas:
(parameter1, parameter2, ...)

The code to be executed, by the function, is placed inside curly brackets: {}

function name(parameter1, parameter2, parameter3) {
  // code to be executed
}

Function parameters are listed inside the parentheses () in the function definition.

Function arguments are the values received by the function when it is invoked.

Inside the function, the arguments (the parameters) behave as local variables.

Function Invocation

The code inside the function will execute when "something" invokes (calls) the function:

• When an event occurs (when a user clicks a button)
• When it is invoked (called) from JavaScript code
• Automatically (self invoked)

You will learn a lot more about function invocation later in this tutorial.

Function Return

When JavaScript reaches a return statement, the function will stop executing.

If the function was invoked from a statement, JavaScript will "return" to execute the code after the invoking statement.

Functions often compute a return value. The return value is "returned" back to the "caller":

Example

Calculate the product of two numbers, and return the result:

let x = myFunction(4, 3);   // Function is called, return value will end up in x

function myFunction(a, b) {
  return a * b;             // Function returns the product of a and b
}

The result in x will be:

12

Try it Yourself »

Why Functions?

You can reuse code: Define the code once, and use it many times.

You can use the same code many times with different arguments, to produce different results.

Example

Convert Fahrenheit to Celsius:

function toCelsius(fahrenheit) {
  return (5/9) * (fahrenheit-32);
}
document.getElementById("demo").innerHTML = toCelsius(77);

Try it Yourself »

The () Operator Invokes the Function

Using the example above, toCelsius refers to the function object, and toCelsius() refers to the function result.

Accessing a function without () will return the function object instead of the function result.

Example

function toCelsius(fahrenheit) {
  return (5/9) * (fahrenheit-32);
}
document.getElementById("demo").innerHTML = toCelsius;

Try it Yourself »

Functions Used as Variable Values

Functions can be used the same way as you use variables, in all types of formulas, assignments, and calculations.

Example

Instead of using a variable to store the return value of a function:

let x = toCelsius(77);
let text = "The temperature is " + x + " Celsius";

You can use the function directly, as a variable value:

let text = "The temperature is " + toCelsius(77) + " Celsius";

Try it Yourself »

You will learn a lot more about functions later in this tutorial.

Local Variables

Variables declared within a JavaScript function, become LOCAL to the function.

Local variables can only be accessed from within the function.

Example

// code here can NOT use carName

function myFunction() {
  let carName = "Volvo";
  // code here CAN use carName
}

// code here can NOT use carName

Try it Yourself »

Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Local variables are created when a function starts, and deleted when the function is completed.

Test Yourself With Exercises

Exercise:

Execute the function named myFunction.

function myFunction() {
  alert("Hello World!");
}
;

Start the Exercise

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