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 thisPromise. 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.

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 thethen 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});

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

Browser compatibility

See also

• Promise