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.