Cara menggunakan async promise javascript

Question

The

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 operator is used to wait for a

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 and get its fulfillment value. It can only be used inside an async function or at the top level of a module.

Nội dung chính Show

Apa itu async Await Promise?
Apa Itu Promise di JavaScript?
Apa itu async Await JavaScript?
Kenapa menggunakan async Await?

await expression

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

9

A

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8, a , or any value to wait for.

The fulfillment value of the promise or thenable object, or, if the expression is not thenable, the expression's own value.

Throws the rejection reason if the promise or thenable object is rejected.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 is usually used to unwrap promises by passing a

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 as the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

9. Using

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 pauses the execution of its surrounding

async function f() {
  const thenable = {
    then(resolve, _reject) {
      resolve("resolved!");
    },
  };
  console.log(await thenable); // "resolved!"
}

f();

5 function until the promise is settled (that is, fulfilled or rejected). When execution resumes, the value of the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 expression becomes that of the fulfilled promise.

If the promise is rejected, the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 expression throws the rejected value. The function containing the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 expression will of the error. Otherwise, if the rejected promise is not awaited or is immediately returned, the caller function will not appear in the stack trace.

The

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

9 is resolved in the same way as

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

0: it's always converted to a native

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 and then awaited. If the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

9 is a:

Native

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 (which means

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

9 belongs to

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 or a subclass, and

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

6): The promise is directly used and awaited natively, without calling

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

7.

(including non-native promises, polyfill, proxy, child class, etc.): A new promise is constructed with the native

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

8 constructor by calling the object's

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

7 method and passing in a handler that calls the

async function f3() {
  const y = await 20;
  console.log(y); // 20

  const obj = {};
  console.log((await obj) === obj); // true
}

f3();

0 callback.

Non-thenable value: An already-fulfilled

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 is constructed and used.

Even when the used promise is already fulfilled, the async function's execution still pauses until the next tick. In the meantime, the caller of the async function resumes execution.

Because

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 is only valid inside async functions and modules, which themselves are asynchronous and return promises, the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 expression never blocks the main thread and only defers execution of code that actually depends on the result, i.e. anything after the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 expression.

If a

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 is passed to an

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 expression, it waits for the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 to be fulfilled and returns the fulfilled value.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

are resolved just the same as actual

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 objects.

async function f() {
  const thenable = {
    then(resolve, _reject) {
      resolve("resolved!");
    },
  };
  console.log(await thenable); // "resolved!"
}

f();

They can also be rejected:

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

If the value is not a

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8,

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 converts the value to a resolved

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8, and waits for it. The awaited value's identity doesn't change as long as it doesn't have a

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

2 property that's callable.

async function f3() {
  const y = await 20;
  console.log(y); // 20

  const obj = {};
  console.log((await obj) === obj); // true
}

f3();

If the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

8 is rejected, the rejected value is thrown.

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

You can handle rejected promises without a

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

4 block by chaining a

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

5 handler before awaiting the promise.

const response = await promisedFunction().catch((err) => {
  console.error(err);
  return "default response";
});
// response will be "default response" if the promise is rejected

This is built on the assumption that

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

6 never synchronously throws an error, but always returns a rejected promise. This is the case for most properly-designed promise-based functions, which usually look like:

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

However, if

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

6 does throw an error synchronously, the error won't be caught by the

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

5 handler. In this case, the

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

9 statement is necessary.

You can use the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 keyword on its own (outside of an async function) at the top level of a module. This means that modules with child modules that use

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 will wait for the child modules to execute before they themselves run, all while not blocking other child modules from loading.

Here is an example of a simple module using the Fetch API and specifying await within the

const response = await promisedFunction().catch((err) => {
  console.error(err);
  return "default response";
});
// response will be "default response" if the promise is rejected

2 statement. Any modules that include this will wait for the fetch to resolve before running any code.

// fetch request
const colors = fetch("../data/colors.json").then((response) => response.json());

export default await colors;

When an

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 is encountered in code (either in an async function or in a module), the awaited expression is executed, while all code that depends on the expression's value is paused and pushed into the microtask queue. The main thread is then freed for the next task in the event loop. This happens even if the awaited value is an already-resolved promise or not a promise. For example, consider the following code:

async function foo(name) {
  console.log(name, "start");
  console.log(name, "middle");
  console.log(name, "end");
}

foo("First");
foo("Second");

// First start
// First middle
// First end
// Second start
// Second middle
// Second end

In this case, the two async functions are synchronous in effect, because they don't contain any

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 expression. The three statements happen in the same tick. In promise terms, the function corresponds to:

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

0

However, as soon as there's one

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7, the function becomes asynchronous, and execution of following statements is deferred to the next tick.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

1

This corresponds to:

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

2

While the extra

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

7 handler is not necessary, and the handler can be merged with the executor passed to the constructor, the

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

7 handler's existence means the code will take one extra tick to complete. The same happens for

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7. Therefore, make sure to use

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 only when necessary (to unwrap promises into their values).

Other microtasks can execute before the async function resumes. This example uses

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

0 to demonstrate how the microtask queue is processed when each

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 expression is encountered.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

3

In this example, the

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

2 function is always called before the async function resumes, so the microtasks they each schedule are always executed in an intertwined fashion. On the other hand, because both

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 and

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

0 schedule microtasks, the order of execution is always based on the order of scheduling. This is why the "queueMicrotask() after calling async function" log happens after the async function resumes for the first time.

Sometimes, the

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 is omitted when a promise is directly returned from an async function.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

4

However, consider the case where

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

6 asynchronously throws an error.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

5

Only

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

6 appears in the stack trace, because the promise is rejected after it has already been returned from

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

8 — in some sense, the promise is unrelated to

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

8. To improve the stack trace, you can use

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

7 to unwrap the promise, so that the exception gets thrown into the current function. The exception will then be immediately wrapped into a new rejected promise, but during error creation, the caller will appear in the stack trace.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

6

However, there's a little performance penalty coming with

// fetch request
const colors = fetch("../data/colors.json").then((response) => response.json());

export default await colors;

1 because the promise has to be unwrapped and wrapped again.

Apa itu async Await Promise?

Apa Itu Async/Await? Async/Await merupakan sebuah syntax khusus yang digunakan untuk bekerja dengan Promise agar lebih nyaman dan mudah untuk digunakan.

Apa Itu Promise di JavaScript?

Promise merupakan salah satu fitur yang penting dari ES6. Objek promise mewakili penyelesaian atau sebuah kesalahan pada operasi Asynchronous.

Apa itu async Await JavaScript?

Async/await adalah fitur yang hadir sejak ES2017. Fitur ini mempermudah kita dalam menangani proses asynchronous.Async/Await merupakan sebuah syntax khusus yang digunakan untuk menangani Promise agar penulisan code lebih efisien dan rapih.

Kenapa menggunakan async Await?

Async-await bisa dikatakan sebagai cara mudah menggunakan JavaScript Promise yang agak sulit dipahami. JavaScript bersifat non blocking - tidak menunggu suatu fungsi selesai untuk melanjutkan ke fungsi yang lain. Kelebihannya - efisien dan cepat, karena tidak menunggu.

kode javascript

Cara menggunakan async promise javascript

Apa itu async Await Promise?

Apa Itu Promise di JavaScript?

Apa itu async Await JavaScript?

Kenapa menggunakan async Await?

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