Promise.prototype.catch()

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句法
描述
范例
规范
浏览器兼容性
另请参阅
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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 an onRejected function even if you want to fall back to an 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.

句法

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

参数

onRejected


									Function

called when the


								Promise

is rejected. This function has one argument:

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.

返回值

Internally calls Promise.prototype.then on the object upon which it was called, passing the parameters undefined and the received onRejected handler. 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 perform 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.error('> > > > > > 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()]

描述

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() .

范例

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.error(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.error(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.error(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.error(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.error(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.error("catch p1!");
    console.error(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);
});

规范

规范
ECMAScript (ECMA-262) The definition of 'Promise.prototype.catch' in that specification.

浏览器兼容性

To contribute to this compatibility data, please write a pull request against this repository: https://github.com/mdn/browser-compat-data . 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`catch()`	Chrome 32	Edge 12	Firefox 29	IE No	Opera 19	Safari 8	WebView Android 4.4.3	Chrome Android 32	Firefox Android 29	Opera Android 19	Safari iOS 8	Samsung Internet Android 2.0	nodejs 0.12

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句法

参数

返回值

描述

范例

Using and chaining the catch method

Gotchas when throwing errors

If it is resolved

规范

浏览器兼容性

图例

另请参阅

元数据

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