promise.catch

The catch() method returns a Promise and deals with rejected cases only. It behaves the same as calling Promise.prototype.then(undefined, onRejected) (in fact, calling obj.catch(onRejected) internally calls obj.then(undefined, onRejected)). This means, that you have to provide onRejected function even if you want to fallback to undefined result value - for example obj.catch(() => {}).

Syntax

p.catch(onRejected);

p.catch(function(reason) {
   // rejection
});

Parameters

onRejected

A Function called when the Promise is rejected. This function has one argument:

reason

The rejection reason.

The Promise returned by catch() is rejected if onRejected throws an error or returns a Promise which is itself rejected; otherwise, it is resolved.

Return value

Internally calls Promise.prototype.then on the object upon which is called, passing the parameters undefined and the onRejected handler received; then returns the value of that call (which is a Promise).

Demonstration of the internal call:

// overriding original Promise.prototype.then/catch just to add some logs
(function(Promise){
    var originalThen = Promise.prototype.then;
    var originalCatch = Promise.prototype.catch;
    
    Promise.prototype.then = function(){
        console.log('> > > > > > called .then on %o with arguments: %o', this, arguments);
        return originalThen.apply(this, arguments);
    };
    Promise.prototype.catch = function(){
        console.log('> > > > > > called .catch on %o with arguments: %o', this, arguments);
        return originalCatch.apply(this, arguments);
    };

})(this.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()]

Description

The catch method can be useful for error handling in your promise composition.

Examples

Using and chaining the catch method

var p1 = new Promise(function(resolve, reject) {
  resolve('Success');
});

p1.then(function(value) {
  console.log(value); // "Success!"
  throw 'oh, no!';
}).catch(function(e) {
  console.log(e); // "oh, no!"
}).then(function(){
  console.log('after a catch the chain is restored');
}, function () {
  console.log('Not fired due to the catch');
});

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

Gotchas when throwing errors

// Throwing an error will call the catch method most of the time
var p1 = new Promise(function(resolve, reject) {
  throw 'Uh-oh!';
});

p1.catch(function(e) {
  console.log(e); // "Uh-oh!"
});

// Errors thrown inside asynchronous functions will act like uncaught errors
var p2 = new Promise(function(resolve, reject) {
  setTimeout(function() {
    throw 'Uncaught Exception!';
  }, 1000);
});

p2.catch(function(e) {
  console.log(e); // This is never called
});

// Errors thrown after resolve is called will be silenced
var p3 = new Promise(function(resolve, reject) {
  resolve();
  throw 'Silenced Exception!';
});

p3.catch(function(e) {
   console.log(e); // This is never called
});

If it is resolved

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

var p2 = p1.catch(function (reason) {
    //This is never called
    console.log("catch p1!");
    console.log(reason);
});

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

Specifications

Browser compatibilityUpdate compatibility data on GitHub

See also

• Promise
• Promise.prototype.then()