Promise.resolve()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

The Promise.resolve() static 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

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 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. 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. See Promise 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 the value expression may synchronously throw an error, this error won't be caught and wrapped in a rejected promise by Promise.resolve(). Consider using Promise.try(() => value) in this case.

Promise.resolve() is generic and supports subclassing, which means it can be called on subclasses of Promise, and the result will be a promise of the subclass type. To do so, the subclass's constructor must implement the same signature as the Promise() constructor — accepting a single executor function that can be called with the resolve and reject callbacks as parameters.

Promise.resolve() special-cases native Promise instances. If value belongs to Promise or a subclass, and value.constructor === Promise, then value is directly returned by Promise.resolve(), without creating a new Promise instance. Otherwise, Promise.resolve() is essentially a shorthand for new Promise((resolve) => resolve(value)).

The bulk of the resolving logic is actually implemented by the resolve function passed by the Promise() 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 through Promise.resolve() without creating a wrapper, the then method is not called on native promises.) If the resolve 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

js
Promise.resolve("Success").then(
  (value) => {
    console.log(value); // "Success"
  },
  (reason) => {
    // not called
  },
);

Resolving an array

js
const p = Promise.resolve([1, 2, 3]);
p.then((v) => {
  console.log(v[0]); // 1
});

Resolving another Promise

Promise.resolve() reuses existing Promise instances. If it's resolving a native promise, it returns the same promise instance without creating a wrapper.

js
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 the then() reference for more information.

Resolving thenables and throwing Errors

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

// Thenable throws after callback
// Promise resolves
const 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.

js
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.

js
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 the Promise() constructor. For example, we can call it on a constructor that passes it console.log as resolve:

js
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 the resolve function of the Promise() constructor, so if you call it on another constructor, nested thenables may not be flattened, depending on how that constructor implements its resolve function.

js
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] }"