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The AsyncFunction constructor creates a new async function object. In JavaScript every asynchronous function is actually an AsyncFunction object.

Note that AsyncFunction is not a global object. It could be obtained by evaluating the following code.

Object.getPrototypeOf(async function(){}).constructor

Syntax

new AsyncFunction([arg1[, arg2[, ...argN]],] functionBody)

Parameters

arg1, arg2, ... argN
Names to be used by the function as formal argument names. Each must be a string that corresponds to a valid JavaScript identifier or a list of such strings separated with a comma; for example "x", "theValue", or "a,b".
functionBody
A string containing the JavaScript statements comprising the function definition.

Description

async function objects created with the AsyncFunction constructor are parsed when the function is created. This is less efficient than declaring an async function with an async function expression and calling it within your code, because such functions are parsed with the rest of the code.

All arguments passed to the function are treated as the names of the identifiers of the parameters in the function to be created, in the order in which they are passed.

Note: async functions created with the AsyncFunction constructor do not create closures to their creation contexts; they are always created in the global scope. When running them, they will only be able to access their own local variables and global ones, not the ones from the scope in which the AsyncFunction constructor was called. This is different from using eval with code for an async function expression.

Invoking the AsyncFunction constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

Properties

AsyncFunction.length
The AsyncFunction constructor's length property whose value is 1.
AsyncFunction.prototype
Allows the addition of properties to all async function objects.

AsyncFunction prototype object

Properties

AsyncFunction.constructor
The initial value is AsyncFunction.
AsyncFunction.prototype[@@toStringTag]
Returns "AsyncFunction".

AsyncFunction instances

AsyncFunction instances inherit methods and properties from AsyncFunction.prototype. As with all constructors, you can change the constructor's prototype object to make changes to all AsyncFunction instances.

Examples

Creating an async function from an AsyncFunction constructor

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

var AsyncFunction = Object.getPrototypeOf(async function(){}).constructor

var a = new AsyncFunction('a', 
                          'b', 
                          'return await resolveAfter2Seconds(a) + await resolveAfter2Seconds(b);');

a(10, 20).then(v => {
  console.log(v); // prints 30 after 4 seconds
});

Specifications

Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'AsyncFunction object' in that specification.
Draft Initial definition in ES2017.

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
Basic supportChrome Full support 55Edge ? Firefox Full support 52IE No support NoOpera Full support 42Safari ? WebView Android Full support 55Chrome Android Full support 55Edge Mobile ? Firefox Android Full support 52Opera Android Full support 42Safari iOS ? Samsung Internet Android Full support 6.0nodejs Full support 7.6.0
Full support 7.6.0
Full support 7.0.0
Disabled
Disabled From version 7.0.0: this feature is behind the --harmony runtime flag.
prototypeChrome Full support 55Edge ? Firefox Full support 52IE No support NoOpera Full support 42Safari ? WebView Android ? Chrome Android Full support 55Edge Mobile ? Firefox Android Full support 52Opera Android Full support 42Safari iOS ? Samsung Internet Android Full support 6.0nodejs Full support 7.6.0
Full support 7.6.0
Full support 7.0.0
Disabled
Disabled From version 7.0.0: this feature is behind the --harmony runtime flag.

Legend

Full support  
Full support
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No support
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See also

Etiquetas do documento e contribuidores

Contribuidores para esta página: Jiang-Xuan, fscholz, nmve, DanielHerr, arai
Última atualização por: Jiang-Xuan,