Promise.prototype.catch()
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
js
p.catch(onRejected);
p.catch(function (reason) {
// rejection
});
參數
- onRejected
-
一個
Function
(en-US) ,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:
js
// 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
js
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
js
// 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
js
//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 |
---|
ECMAScript Language Specification # sec-promise.prototype.catch |
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