async function

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Syntax

async function name([param[, param[, ... param]]]) {
   statements
}

Parameters

name

The function name.

param

The name of an argument to be passed to the function.

statements

The statements comprising the body of the function.

Return value

A Promise which will be resolved with the value returned by the async function, or rejected with an uncaught exception thrown from within the async function.

Description

An async function can contain an await expression that pauses the execution of the async function and waits for the passed Promise's resolution, and then resumes the async function's execution and returns the resolved value.

Remember, the await keyword is only valid inside async functions. If you use it outside of an async function's body, you will get a SyntaxError.

Examples

Simple example

var resolveAfter2Seconds = function() {
  console.log("starting slow promise");
  return new Promise(resolve => {
    setTimeout(function() {
      resolve(20);
      console.log("slow promise is done");
    }, 2000);
  });
};

var resolveAfter1Second = function() {
  console.log("starting fast promise");
  return new Promise(resolve => {
    setTimeout(function() {
      resolve(10);
      console.log("fast promise is done");
    }, 1000);
  });
};

var sequentialStart = async function() {
  console.log('==SEQUENTIAL START==');

  // If the value of the expression following the await operator is not a Promise, it's converted to a resolved Promise.
  const slow = await resolveAfter2Seconds();

  const fast = await resolveAfter1Second();
  console.log(slow);
  console.log(fast);
}

var concurrentStart = async function() {
  console.log('==CONCURRENT START with await==');
  const slow = resolveAfter2Seconds(); // starts timer immediately
  const fast = resolveAfter1Second();

  console.log(await slow);
  console.log(await fast); // waits for slow to finish, even though fast is already done!
}

var stillConcurrent = function() {
  console.log('==CONCURRENT START with Promise.all==');
  Promise.all([resolveAfter2Seconds(), resolveAfter1Second()]).then((messages) => {
    console.log(messages[0]); // slow
    console.log(messages[1]); // fast
  });
}

var parallel = function() {
  console.log('==PARALLEL with Promise.then==');
  resolveAfter2Seconds().then((message)=>console.log(message));
  resolveAfter1Second().then((message)=>console.log(message));
}

sequentialStart(); // after 2 seconds, logs "slow", then after 1 more second, "fast"
// wait above to finish
setTimeout(concurrentStart, 4000); // after 2 seconds, logs "slow" and then "fast"
// wait again
setTimeout(stillConcurrent, 7000); // same as concurrentStart
// wait again
setTimeout(parallel, 10000); // trully parallel: after 1 second, logs "fast", then after 1 more second, "slow"

Rewriting a promise chain with an async function

An API that returns a Promise will result in a promise chain, and it splits the function into many parts. Consider the following code:

function getProcessedData(url) {
  return downloadData(url) // returns a promise
    .catch(e => {
      return downloadFallbackData(url)  // returns a promise
    })
    .then(v => {
      return processDataInWorker(v); // returns a promise
    });
}

it can be rewritten with a single async function as follows:

async function getProcessedData(url) {
  let v;
  try {
    v = await downloadData(url); 
  } catch(e) {
    v = await downloadFallbackData(url);
  }
  return processDataInWorker(v);
}

Note that in the above example, there is no await statement on the return statement, because the return value of an async function is implicitly wrapped in Promise.resolve.

Specifications

Browser compatibility

See also

• async function expression
• AsyncFunction object
• await
• "Decorating Async Javascript Functions" on "innolitics.com"