Promise.resolve()

value

Argument to be resolved by this Promise. Can also be a Promise or a thenable to resolve.

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.

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.

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 resolver 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 resolver function receives another thenable object, it will be resolved again, so that the eventual fulfillment value of the promise will never be thenable.

Promise.resolve() reuses existing Promise 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 the then handlers are called asynchronously. See the then() reference for more information.

// 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
});

const thenable = {
  then(onFulfilled, onRejected) {
    onFulfilled(thenable);
  },
};

Promise.resolve(thenable); // Will lead to infinite recursion.

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:

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 resolver 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 resolver.

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] }"

• Promise