From 218934fa2ed1c702a6d3923d2aa2cc6b43c48684 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:43:23 -0500 Subject: initial commit --- .../global_objects/promise/catch/index.html | 189 +++++++++++++++++++++ 1 file changed, 189 insertions(+) create mode 100644 files/zh-tw/web/javascript/reference/global_objects/promise/catch/index.html (limited to 'files/zh-tw/web/javascript/reference/global_objects/promise/catch') diff --git a/files/zh-tw/web/javascript/reference/global_objects/promise/catch/index.html b/files/zh-tw/web/javascript/reference/global_objects/promise/catch/index.html new file mode 100644 index 0000000000..d06036c11e --- /dev/null +++ b/files/zh-tw/web/javascript/reference/global_objects/promise/catch/index.html @@ -0,0 +1,189 @@ +--- +title: Promise.prototype.catch() +slug: Web/JavaScript/Reference/Global_Objects/Promise/catch +tags: + - EMCAScript 2015 + - JavaScript + - Method + - Promise + - Prototype +translation_of: Web/JavaScript/Reference/Global_Objects/Promise/catch +--- +
{{JSRef}}
+ +

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 {{jsxref("Promise.then", "Promise.prototype.then(undefined, onRejected)")}}。

+ +

語法

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

參數

+ +
+
onRejected
+
一個 {{jsxref("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 {{jsxref("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

+ + + + + + + + + + + + + + + + + + + +
SpecificationStatusComment
{{SpecName('ES2015', '#sec-promise.prototype.catch', 'Promise.prototype.catch')}}{{Spec2('ES2015')}}Initial definition in an ECMA standard.
{{SpecName('ESDraft', '#sec-promise.prototype.catch', 'Promise.prototype.catch')}}{{Spec2('ESDraft')}} 
+ +

瀏覽器相容性

+ + + +

{{Compat("javascript.builtins.Promise.catch")}}

+ +

參見

+ + -- cgit v1.2.3-54-g00ecf