Promise.prototype.catch()
The catch() method returns a Promise
and deals with rejected cases only. It behaves the same as calling Promise.prototype.then(undefined, onRejected)
(en-US) (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(() => {})
.
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
The source for this interactive demo is stored in a GitHub repository. If you'd like to contribute to the interactive demo project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
Sözdizimi
p.catch(onRejected); p.catch(function(reason) { // rejection });
Parametreler
- onRejected
- A
Function
called when thePromise
is rejected. This function has one argument:reason
- The rejection reason.
catch()
is rejected ifonRejected
throws an error or returns a Promise which is itself rejected; otherwise, it is resolved.
Dönen değer
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
).
Note the examples below are throwing instances of Error. This is considered good practice in contrast to throwing Strings: Otherwise the part doing the catching would have to make checks to see if the argument was a string or an error, and you might lose valuable information like stack traces.
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()]
Açıklama
The catch
method can be useful for error handling in your promise composition.
Örnekler
Using and chaining the catch
method
var p1 = new Promise(function(resolve, reject) {
resolve('Success');
});
p1.then(function(value) {
console.log(value); // "Success!"
throw new Error('oh, no!');
}).catch(function(e) {
console.log(e.message); // "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 new Error('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 new Error('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 new Error('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
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Promise.prototype.catch' in that specification. |
Standard | Initial definition in an ECMA standard. |
ECMAScript (ECMA-262) The definition of 'Promise.prototype.catch' in that specification. |
Living Standard |
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