Promise.prototype.catch()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.

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 ，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® 2027 Language Specification
# sec-promise.prototype.catch

瀏覽器相容性

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參見

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