for await...of - JavaScript 编辑

The for await...of statement creates a loop iterating over async iterable objects as well as on sync iterables, including: built-in String, Array, Array-like objects (e.g., arguments or NodeList), TypedArray, Map, Set, and user-defined async/sync iterables. It invokes a custom iteration hook with statements to be executed for the value of each distinct property of the object. Like the await operator, the statement can only be used inside an async function.

for await...of doesn't work with async iterators that are not async iterables.

Syntax

for await (variable of iterable) {
  statement
}

variable

On each iteration a value of a different property is assigned to variable. variable may be declared with const, let, or var.

iterable

Object whose iterable properties are to be iterated over.

Examples

Iterating over async iterables

You can also iterate over an object that explicitly implements async iterable protocol:

const asyncIterable = {
  [Symbol.asyncIterator]() {
    return {
      i: 0,
      next() {
        if (this.i < 3) {
          return Promise.resolve({ value: this.i++, done: false });
        }

        return Promise.resolve({ done: true });
      }
    };
  }
};

(async function() {
   for await (let num of asyncIterable) {
     console.log(num);
   }
})();

// 0
// 1
// 2

Iterating over async generators

Since the return values of async generators conform to the async iterable protocol, they can be looped using for await...of.

async function* asyncGenerator() {
  let i = 0;
  while (i < 3) {
    yield i++;
  }
}

(async function() {
  for await (let num of asyncGenerator()) {
    console.log(num);
  }
})();
// 0
// 1
// 2

For a more concrete example of iterating over an async generator using for await...of, consider iterating over data from an API.

This example first creates an async iterable for a stream of data, then uses it to find the size of the response from the API.

async function* streamAsyncIterable(stream) {
  const reader = stream.getReader();
  try {
    while (true) {
      const { done, value } = await reader.read();
      if (done) {
        return;
      }
      yield value;
    }
  } finally {
    reader.releaseLock();
  }
}
// Fetches data from url and calculates response size using the async generator.
async function getResponseSize(url) {
  const response = await fetch(url);
  // Will hold the size of the response, in bytes.
  let responseSize = 0;
  // The for-await-of loop. Async iterates over each portion of the response.
  for await (const chunk of streamAsyncIterable(response.body)) {
    // Incrementing the total response length.
    responseSize += chunk.length;
  }

  console.log(`Response Size: ${responseSize} bytes`);
  // expected output: "Response Size: 1071472"
  return responseSize;
}
getResponseSize('https://jsonplaceholder.typicode.com/photos');

Iterating over sync iterables and generators

for await...of loop also consumes sync iterables and generators. In that case it internally awaits emitted values before assign them to the loop control variable.

function* generator() {
  yield 0;
  yield 1;
  yield Promise.resolve(2);
  yield Promise.resolve(3);
  yield 4;
}

(async function() {
  for await (let num of generator()) {
    console.log(num);
  }
})();
// 0
// 1
// 2
// 3
// 4

// compare with for-of loop:

for (let numOrPromise of generator()) {
  console.log(numOrPromise);
}
// 0
// 1
// Promise { 2 }
// Promise { 3 }
// 4

Note: be aware of yielding rejected promises from sync generator. In such case for await...of throws when consuming rejected promise and DOESN'T CALL finally blocks within that generator. This can be undesireable if you need to free some allocated resources with try/finally.

function* generatorWithRejectedPromises() {
  try {
    yield 0;
    yield 1;
    yield Promise.resolve(2);
    yield Promise.reject(3);
    yield 4;
    throw 5;
  } finally {
    console.log('called finally')
  }
}

(async function() {
  try {
    for await (let num of generatorWithRejectedPromises()) {
      console.log(num);
    }
  } catch (e) {
    console.log('catched', e)
  }
})();
// 0
// 1
// 2
// catched 3

// compare with for-of loop:

try {
  for (let numOrPromise of generatorWithRejectedPromises()) {
    console.log(numOrPromise);
  }
} catch (e) {
  console.log('catched', e)
}
// 0
// 1
// Promise { 2 }
// Promise { <rejected> 3 }
// 4
// catched 5
// called finally

To make  finally blocks of a sync generator to be always called use appropriate form of the loop, for await...of for the async generator and for...of for the sync one and await yielded promises explicitly inside the loop.

(async function() {
  try {
    for (let numOrPromise of generatorWithRejectedPromises()) {
      console.log(await numOrPromise);
    }
  } catch (e) {
    console.log('catched', e)
  }
})()
// 0
// 1
// 2
// catched 3
// called finally

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Symbol.asyncIterator
• for...of