for await...of

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.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

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.

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 async generators implement the Async Iterator 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 iterator for a stream of data, then uses it to find the size of the response from the API.

async function* streamAsyncIterator(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 streamAsyncIterator(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');

Specifications

Browser compatibility

See also

• Symbol.asyncIterator
• for...of