Async Function: basics

async defines an asynchronous function

can be created by putting async before

a function expression

const AsyncFunction = async function () {}.constructor; const f = function() {}; assert.equal(f instanceof AsyncFunction, true);

a function declaration

const AsyncFunction = async function () {}.constructor; function f() {} assert.equal(f instanceof AsyncFunction, true);

an arrow function

const AsyncFunction = async function () {}.constructor; const f = () => {}; assert.equal(f instanceof AsyncFunction, true);

an object method

const AsyncFunction = async function () {}.constructor; const obj = {f: () => void 0}; assert.equal(obj.f instanceof AsyncFunction, true);

the return value

is always a Promise

const f = 'nö'; assert.equal(f() instanceof Promise, true);

wraps the return value in a Promise

const f = () => 42; return f().then(v => assert.equal(v, 42));

is a rejected Promise when the async function throws

const f = async () => 0; return f() .then(() => assert(false, 'Promise must reject (not pass)!')) .catch(v => assert.equal(v, 23)) ;

Links

mdn docs A short description of async function.
spec The actual chapter about `async`, you need to dive in from here.

Related Katas

function API

• function.length (as introduced in ES1)
• function.bind()
• function.length (with ES5 features)
• function.length (with ES6 features)
• function.name

Arrow functions

• basics
• function binding

Async Function

• basics

Difficulty Level

INTERMEDIATE

First Published

9 June 2017

Stats

7 tests to solve