Promise.resolve()
BaselineWidely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
ThePromise.resolve()
static method "resolves" a given value to aPromise
. If the value is a promise, that promise is returned; if the value is athenable,Promise.resolve()
will call thethen()
method with two callbacks it prepared; otherwise the returned promise will be fulfilled with the value.
This function flattens nested layers of promise-like objects (e.g., a promise that fulfills to a promise that fulfills to something) into a single layer — a promise that fulfills to a non-thenable value.
Try it
const promise1 = Promise.resolve(123);promise1.then((value) => { console.log(value); // Expected output: 123});
Syntax
Promise.resolve(value)
Parameters
value
Argument to be resolved by this
Promise
. Can also be aPromise
or a thenable to resolve.
Return value
APromise
that is resolved with the given value, or the promise passed as value, if the value was a promise object. A resolved promise can be in any of the states — fulfilled, rejected, or pending. For example, resolving a rejected promise will still result in a rejected promise.
Description
Promise.resolve()
resolves a promise, which is not the same as fulfilling or rejecting the promise. SeePromise description for definitions of the terminology. In brief,Promise.resolve()
returns a promise whose eventual state depends on another promise, thenable object, or other value.
Note:If evaluating thevalue
expression may synchronously throw an error, this error won't be caught and wrapped in a rejected promise byPromise.resolve()
. Consider usingPromise.try(() => value)
in this case.
Promise.resolve()
is generic and supports subclassing, which means it can be called on subclasses ofPromise
, and the result will be a promise of the subclass type. To do so, the subclass's constructor must implement the same signature as thePromise()
constructor — accepting a singleexecutor
function that can be called with theresolve
andreject
callbacks as parameters.
Promise.resolve()
special-cases nativePromise
instances. Ifvalue
belongs toPromise
or a subclass, andvalue.constructor === Promise
, thenvalue
is directly returned byPromise.resolve()
, without creating a newPromise
instance. Otherwise,Promise.resolve()
is essentially a shorthand fornew Promise((resolve) => resolve(value))
.
The bulk of the resolving logic is actually implemented bytheresolve
function passed by thePromise()
constructor. In summary:
- If a non-thenable value is passed, the returned promise is already fulfilled with that value.
- If a thenable is passed, the returned promise will adopt the state of that thenable by calling the
then
method and passing a pair of resolving functions as arguments. (But because native promises directly pass throughPromise.resolve()
without creating a wrapper, thethen
method is not called on native promises.) If theresolve
function receives another thenable object, it will be resolved again, so that the eventual fulfillment value of the promise will never be thenable.
Examples
Using the static Promise.resolve method
Promise.resolve("Success").then( (value) => { console.log(value); // "Success" }, (reason) => { // not called },);
Resolving an array
const p = Promise.resolve([1, 2, 3]);p.then((v) => { console.log(v[0]); // 1});
Resolving another Promise
Promise.resolve()
reuses existingPromise
instances. If it's resolving a native promise, it returns the same promise instance without creating a wrapper.
const original = Promise.resolve(33);const cast = Promise.resolve(original);cast.then((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 thethen
handlers are called asynchronously. See thethen()
reference for more information.
Resolving thenables and throwing Errors
// Resolving a thenable objectconst p1 = Promise.resolve({ then(onFulfill, onReject) { onFulfill("fulfilled!"); },});console.log(p1 instanceof Promise); // true, object casted to a Promisep1.then( (v) => { console.log(v); // "fulfilled!" }, (e) => { // not called },);// Thenable throws// Promise rejectsconst p2 = Promise.resolve({ then() { throw new TypeError("Throwing"); },});p2.then( (v) => { // not called }, (e) => { console.error(e); // TypeError: Throwing },);// Thenable throws after callback// Promise resolvesconst p3 = Promise.resolve({ then(onFulfilled) { onFulfilled("Resolving"); throw new TypeError("Throwing"); },});p3.then( (v) => { console.log(v); // "Resolving" }, (e) => { // not called },);
Nested thenables will be "deeply flattened" to a single promise.
const thenable = { then(onFulfilled, onRejected) { onFulfilled({ // The thenable is fulfilled with another thenable then(onFulfilled, onRejected) { onFulfilled(42); }, }); },};Promise.resolve(thenable).then((v) => { console.log(v); // 42});
Warning:Do not callPromise.resolve()
on a thenable that resolves to itself. That leads to infinite recursion, because it attempts to flatten an infinitely-nested promise.
const thenable = { then(onFulfilled, onRejected) { onFulfilled(thenable); },};Promise.resolve(thenable); // Will lead to infinite recursion.
Calling resolve() on a non-Promise constructor
Promise.resolve()
is a generic method. It can be called on any constructor that implements the same signature as thePromise()
constructor. For example, we can call it on a constructor that passes itconsole.log
asresolve
:
class NotPromise { constructor(executor) { // The "resolve" and "reject" functions behave nothing like the // native promise's, but Promise.resolve() calls them in the same way. executor( (value) => console.log("Resolved", value), (reason) => console.log("Rejected", reason), ); }}Promise.resolve.call(NotPromise, "foo"); // Logs "Resolved foo"
The ability to flatten nested thenables is implemented by theresolve
function of thePromise()
constructor, so if you call it on another constructor, nested thenables may not be flattened, depending on how that constructor implements itsresolve
function.
const thenable = { then(onFulfilled, onRejected) { onFulfilled({ // The thenable is fulfilled with another thenable then(onFulfilled, onRejected) { onFulfilled(42); }, }); },};Promise.resolve.call(NotPromise, thenable); // Logs "Resolved { then: [Function: then] }"
Specifications
Specification |
---|
ECMAScript® 2026 Language Specification # sec-promise.resolve |