Promise.prototype.catch()

The catch() method of Promise instances schedules a function to be called when the promise is rejected. It immediately returns an equivalent Promise object, allowing you to chain calls to other promise methods. It is a shortcut for Promise.prototype.then(undefined, onRejected).

Syntax

promiseInstance.catch(onRejected)

Parameters

onRejected
- : A function to asynchronously execute when this promise becomes rejected. Its return value becomes the fulfillment value of the promise returned by catch(). The function is called with the following arguments:
  - reason
    - : The value that the promise was rejected with.

Return value

Returns a new Promise. This new promise is always pending when returned, regardless of the current promise's status. It's eventually rejected if onRejected throws an error or returns a Promise which is itself rejected; otherwise, it's eventually fulfilled.

Description

The catch method is used for error handling in promise composition. Since it returns a Promise, it can be chained in the same way as its sister method, then().

If a promise becomes rejected, and there are no rejection handlers to call (a handler can be attached through any of then(), catch(), or finally()), then the rejection event is surfaced by the host. In the browser, this results in an unhandledrejection event. If a handler is attached to a rejected promise whose rejection has already caused an unhandled rejection event, then another rejectionhandled event is fired.

catch() internally calls then() on the object upon which it was called, passing undefined and onRejected as arguments. The value of that call is directly returned. This is observable if you wrap the methods.

// overriding original Promise.prototype.then/catch just to add some logs
((Promise) => {
  const originalThen = Promise.prototype.then;
  const originalCatch = Promise.prototype.catch;

  Promise.prototype.then = function (...args) {
    console.log("Called .then on %o with arguments: %o", this, args);
    return originalThen.apply(this, args);
  };
  Promise.prototype.catch = function (...args) {
    console.error("Called .catch on %o with arguments: %o", this, args);
    return originalCatch.apply(this, args);
  };
})(Promise);

// calling catch on an already resolved promise
Promise.resolve().catch(function XXX() {});

// Logs:
// Called .catch on Promise{} with arguments: Arguments{1} [0: function XXX()]
// Called .then on Promise{} with arguments: Arguments{2} [0: undefined, 1: function XXX()]

This means that passing undefined still causes the returned promise to be rejected, and you have to pass a function to prevent the final promise from being rejected.

Because catch() just calls then(), it supports subclassing.

Note: The examples below are throwing instances of Error. As with synchronous throw statements, this is considered a good practice; otherwise, the part doing the catching would have to perform checks to see if the argument was a string or an error, and you might lose valuable information such as stack traces.

Examples

Using and chaining the catch() method

const p1 = new Promise((resolve, reject) => {
  resolve("Success");
});

p1.then((value) => {
  console.log(value); // "Success!"
  throw new Error("oh, no!");
})
  .catch((e) => {
    console.error(e.message); // "oh, no!"
  })
  .then(
    () => console.log("after a catch the chain is restored"), // "after a catch the chain is restored"
    () => console.log("Not fired due to the catch"),
  );

// The following behaves the same as above
p1.then((value) => {
  console.log(value); // "Success!"
  return Promise.reject("oh, no!");
})
  .catch((e) => {
    console.error(e); // "oh, no!"
  })
  .then(
    () => console.log("after a catch the chain is restored"), // "after a catch the chain is restored"
    () => console.log("Not fired due to the catch"),
  );

Gotchas when throwing errors

Throwing an error will call the catch() method most of the time:

const p1 = new Promise((resolve, reject) => {
  throw new Error("Uh-oh!");
});

p1.catch((e) => {
  console.error(e); // "Uh-oh!"
});

Errors thrown inside asynchronous functions will act like uncaught errors:

const p2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    throw new Error("Uncaught Exception!");
  }, 1000);
});

p2.catch((e) => {
  console.error(e); // This is never called
});

Errors thrown after resolve is called will be silenced:

const p3 = new Promise((resolve, reject) => {
  resolve();
  throw new Error("Silenced Exception!");
});

p3.catch((e) => {
  console.error(e); // This is never called
});

catch() is not called if the promise is fulfilled

// Create a promise which would not call onReject
const p1 = Promise.resolve("calling next");

const p2 = p1.catch((reason) => {
  // This is never called
  console.error("catch p1!");
  console.error(reason);
});

p2.then(
  (value) => {
    console.log("next promise's onFulfilled");
    console.log(value); // calling next
  },
  (reason) => {
    console.log("next promise's onRejected");
    console.log(reason);
  },
);