promise.catch()

promise.catch()是promise.then(undefined,onreject)的语法糖

使用链式语句的catch方法

question：如果通过Promise.reject()方法调用的catch，那么后面的.then方法是否还会发生调用 answer：是会调用的，promise.catch()是promise.then(undefined,onreject)的语法糖，与普通的链式调用没有区别

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');
});

// 以下行为与上述相同
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');
});

捕获抛出的异常

抛出一个错误，大多数时候将调用catch方法

var p1 = new Promise(function(resolve, reject) {
  throw 'Uh-oh!';
});

p1.catch(function(e) {
  console.log(e); // "Uh-oh!"
});

在异步函数中抛出的错误不会被catch捕获到

var p2 = new Promise(function(resolve, reject) {
  setTimeout(function() {
    throw 'Uncaught Exception!';
  }, 1000);
});

p2.catch(function(e) {
  console.log(e); // 不会执行
});

在resolve()后面抛出的错误会被忽略

var p3 = new Promise(function(resolve, reject) {
  resolve();
  throw 'Silenced Exception!';
});

p3.catch(function(e) {
   console.log(e); // 不会执行
});

如果已经决策

//创建一个新的 Promise ，且已决议
var p1 = Promise.resolve("calling next");

var p2 = p1.catch(function (reason) {
    //这个方法永远不会调用
    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);
});