AsyncFunction
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 can be
obtained with the following code:
Object.getPrototypeOf(async function(){}).constructor
Syntax
new AsyncFunction(arg0, functionBody)
new AsyncFunction(arg0, arg1, functionBody)
new AsyncFunction(arg0, arg1, ...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 parameter (any of plain identifier, rest parameter, or destructured parameter, optionally with a default), or a list of such strings separated with commas.
As the parameters are parsed in the same way as function declarations, whitespace and comments are accepted. For example:
"x", "theValue = 42", "[a, b] /* numbers */"
— or"x, theValue = 42, [a, b] /* numbers */"
. ("x, theValue = 42", "[a, b]"
is also correct, though very confusing to read.) 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.
Examples
Creating an async function from an AsyncFunction() constructor
function resolveAfter2Seconds(x) {
return new Promise(resolve => {
setTimeout(() => {
resolve(x);
}, 2000);
});
}
let AsyncFunction = Object.getPrototypeOf(async function(){}).constructor
let 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 |
---|
ECMAScript Language Specification # sec-async-function-objects |
Browser compatibility
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