Promise.resolve()
The Promise.resolve()
method returns a
Promise
object that is resolved with a given value. If the value is a
promise, that promise is returned; if the value is a thenable (i.e. has a
"then" method
), the returned promise will "follow" that
thenable, adopting its eventual state; otherwise the returned promise will be fulfilled
with the value. This function flattens nested layers of promise-like objects (e.g. a
promise that resolves to a promise that resolves to something) into a single layer.
Syntax
Promise.resolve(value);
Parameters
value
-
Argument to be resolved by this
Promise
. Can also be aPromise
or a thenable to resolve.
Return value
A Promise
that is resolved with the given value, or the promise passed as
value, if the value was a promise object.
Description
The static Promise.resolve
function returns a Promise
that is
resolved.
Examples
Using the static Promise.resolve method
Promise.resolve('Success').then(function(value) {
console.log(value); // "Success"
}, function(value) {
// not called
});
Resolving an array
var p = Promise.resolve([1,2,3]);
p.then(function(v) {
console.log(v[0]); // 1
});
Resolving another Promise
var original = Promise.resolve(33);
var cast = Promise.resolve(original);
cast.then(function(value) {
console.log('value: ' + value);
});
console.log('original === cast ? ' + (original === cast));
// logs, in order:
// original === cast ? true
// value: 33
The inverted order of the logs is due to the fact that the then
handlers
are called asynchronously. See how then
works here.
Resolving thenables and throwing Errors
// Resolving a thenable object
var p1 = Promise.resolve({
then: function(onFulfill, onReject) { onFulfill('fulfilled!'); }
});
console.log(p1 instanceof Promise) // true, object casted to a Promise
p1.then(function(v) {
console.log(v); // "fulfilled!"
}, function(e) {
// not called
});
// Thenable throws before callback
// Promise rejects
var thenable = { then: function(resolve) {
throw new TypeError('Throwing');
resolve('Resolving');
}};
var p2 = Promise.resolve(thenable);
p2.then(function(v) {
// not called
}, function(e) {
console.error(e); // TypeError: Throwing
});
// Thenable throws after callback
// Promise resolves
var thenable = { then: function(resolve) {
resolve('Resolving');
throw new TypeError('Throwing');
}};
var p3 = Promise.resolve(thenable);
p3.then(function(v) {
console.log(v); // "Resolving"
}, function(e) {
// not called
});
Warning: Do not call Promise.resolve()
on a thenable that resolves to itself. That leads to infinite recursion, because it attempts to flatten an infinitely-nested promise.
let thenable = {
then: (resolve, reject) => {
resolve(thenable)
}
}
Promise.resolve(thenable) // Will lead to infinite recursion.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-promise.resolve |
Browser compatibility
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