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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