Promise.resolve()

The Promise.resolve() method "resolves" a given value to a Promise. If the value is a promise, that promise is returned; if the value is a thenable, Promise.resolve() will call the then() 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

Syntax

Promise.resolve(value)

Parameters

value

Argument to be resolved by this Promise. Can also be a Promise 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. It may be either fulfilled or rejected — for example, resolving a rejected promise will still result in a rejected promise.

Description

The static Promise.resolve function returns a Promise that is resolved.

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

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 the then handlers are called asynchronously. See how then works here.

Resolving thenables and throwing Errors

// Resolving a thenable object
const p1 = Promise.resolve({
  then(onFulfill, onReject) {
    onFulfill('fulfilled!');
  },
});
console.log(p1 instanceof Promise) // true, object casted to a Promise

p1.then(
  (v) => {
    console.log(v); // "fulfilled!"
  },
  (e) => {
    // not called
  },
);

// Thenable throws before callback
// Promise rejects
const thenable = {
  then(onFulfilled) {
    throw new TypeError('Throwing');
    onFulfilled('Resolving');
  },
};

const p2 = Promise.resolve(thenable);
p2.then(
  (v) => {
    // not called
  },
  (e) => {
    console.error(e); // TypeError: Throwing
  },
);

// Thenable throws after callback
// Promise resolves
const thenable = {
  then(onFulfilled) {
    onFulfilled('Resolving');
    throw new TypeError('Throwing');
  },
};

const p3 = Promise.resolve(thenable);
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 call Promise.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.

Specifications

Specification
ECMAScript Language Specification
# sec-promise.resolve

Browser compatibility

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See also