The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.
Promise 对象用于一个异步操作的最终完成（或失败）及其结果值的表示。简单点说，它就是用于处理异步操作的，异步处理成功了就执行成功的操作，异步处理失败了就捕获错误或者停止后续操作。
在promise之前处理异步回调的方式
function asyncFun(a,b,callback) { setTimeout(function () { callback(a+b); },200); } asyncFun(1,2, function (res) { if(res > 2) { asyncFun(res, 2, function (res) { if(res > 4) { asyncFun(res, 2, function (res) { console.log('ok'); console.log(res); }) } }) } });
从上面可以看出所谓的”回调地狱”的可怕
使用promise来优雅的处理异步
function asyncFun(a,b) { return new Promise(function (resolve, reject) { setTimeout(function() { resolve(a + b); },200); }) } asyncFun(1,2) .then(function (res) { if(res > 2) { return asyncFun(res, 2); } }) .then(function (res) { if(res > 4) { return asyncFun(res, 2); } }) .then(function (res) { console.log('ok'); console.log(res); }) .catch(function (error) { console.log(error); });
使用promise处理内部异常的举例
function asyncFun(a,b) { return new Promise(function (resolve, reject) { // 模拟异常判断 if(typeof a !== 'number' || typeof b !== 'number') { reject(new Error('no number')); } setTimeout(function() { resolve(a + b); },200); }) } asyncFun(1,2) .then(function (res) { if(res > 2) { return asyncFun(res, 2); } },function (err) { console.log('first err: ', err); }) .then(function (res) { if(res > 4) { return asyncFun(res, 'a'); } },function (err) { console.log('second err: ', err); }) .then(function (res) { console.log('ok'); console.log(res); },function (err) { console.log('third err: ', err); });
从上面可以看出通过then的第二个回调函数处理promise对象中的异常，通过reject返回异常的promise对象
通过catch统一处理错误，通过finally执行最终必须执行的逻辑
function asyncFun(a,b) { return new Promise(function (resolve, reject) { // 模拟异常判断 if(typeof a !== 'number' || typeof b !== 'number') { reject(new Error('no number')); } setTimeout(function() { resolve(a + b); },200); }) } asyncFun(1,2) .then(function (res) { if(res > 2) { return asyncFun(res, 2); } }) .then(function (res) { if(res > 4) { return asyncFun(res, 'a'); } }) .then(function (res) { console.log('ok'); console.log(res); }) .catch(function (error) { console.log('catch: ', error); }) .finally(function () { console.log('finally: ', 1+2); });
通过Promise.all()静态方法来处理多个异步
function asyncFun(a,b) { return new Promise(function (resolve, reject) { setTimeout(function() { resolve(a + b); },200); }) } var promise = Promise.all([asyncFun(1,2), asyncFun(2,3), asyncFun(3,4)]) promise.then(function (res) { console.log(res); // [3, 5, 7] });
通过Promise.race()静态方法来获取多个异步中最快的一个
function asyncFun(a,b,time) { return new Promise(function (resolve, reject) { setTimeout(function() { resolve(a + b); },time); }) } var promise = Promise.race([asyncFun(1,2,10), asyncFun(2,3,6), asyncFun(3,4,200)]) promise.then(function (res) { console.log(res); // 5 });
通过Promise.resolve() 静态方法来直接返回成功的异步对象
var p = Promise.resolve('hello'); p.then(function (res) { console.log(res); // hello });
等同于，如下：
var p = new Promise(function (resolve, reject) { resolve('hello2'); }) p.then(function (res) { console.log(res); // hello2 });
通过Promise.reject() 静态方法来直接返回失败的异步对象
var p = Promise.reject('err') p.then(null, function (res) { console.log(res); // err });
等同于，如下：
var p = new Promise(function (resolve, reject) { reject('err2'); }) p.then(null, function (res) { console.log(res); // err });
通过一个小例子来测试Promise在面向对象中应用
'use strict'; class User{ constructor(name, password) { this.name = name; this.password = password; } send() { let name = this.name; return new Promise(function (resolve, reject) { setTimeout(function () { if(name === 'leo') { resolve('send success'); }else{ reject('send error'); } }); }); } validatePwd() { let pwd = this.password; return new Promise(function (resolve, reject) { setTimeout(function () { if(pwd === '123') { resolve('validatePwd success'); }else{ reject('validatePwd error'); } }); }) } } let user1 = new User('Joh'); user1.send() .then(function (res) { console.log(res); }) .catch(function (err) { console.log(err); }); let user2 = new User('leo'); user2.send() .then(function (res) { console.log(res); }) .catch(function (err) { console.log(err); }); let user3 = new User('leo', '123'); user3.validatePwd() .then(function (res) { return user3.validatePwd(); }) .then(function (res) { console.log(res); }) .catch(function(error) { console.log(error); }); let user4 = new User('leo', '1234'); user4.validatePwd() .then(function (res) { return user4.validatePwd(); }) .then(function (res) { console.log(res); }) .catch(function(error) { console.log(error); });