catch() 方法只處理 Promise 的被拒絕狀態,並回傳一個新的 Promise 物件。此方法的行為等同於呼叫 Promise.prototype.then(undefined, onRejected)

語法

p.catch(onRejected);

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

參數

onRejected
一個 Function ,在 Promise 被拒絕時被呼叫。這個函式有一個引數:
reason
失敗訊息。
若 onRejected 拋出一個錯誤或回傳一個被拒絕的 Promise,則 catch() 回傳的 Promise 被拒絕;其他情形都是被實現。

回傳值

呼叫(catch 的 promise)物件,內部呼叫 Promise.prototype.then,傳入引數 undefined 及 onRejected;接著以之結果回傳(結果為 Promise)。

內部呼叫演示:

// 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()]

描述

catch 方法在處理 promise 組合的錯誤時很有幫助。

範例

使用及串接 catch 方法

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');
});

拋出例外時的陷阱

// 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
});

如果 Promise 被實現

//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);
});

規範

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 Latest Draft (ECMA-262)
The definition of 'Promise.prototype.catch' in that specification.
Living Standard  

瀏覽器相容性

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic Support32.0(Yes)29.0No197.1
FeatureAndroidChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic Support4.4.432.0(Yes)29No(Yes)8.0

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