做完两道题，理解Promise

解题之前建议大家先看看promise的文档[https://segmentfault.com/a/1190000002452115]

第一题：promise应用的面试题

红灯三秒亮一次，绿灯一秒亮一次，黄灯2秒亮一次；如何让三个灯按照这个规律不断交替重复亮灯？（用Promse实现）三个亮灯函数已经存在：
function red(){
console.log('red');
}
function green(){
console.log('green');
}
function yellow(){
console.log('yellow');
}*

//已知函数
function red(){
    console.log('red');
}
function green(){
    console.log('green');
}
function yellow(){
    console.log('yellow');
}*
//亮灯一次 函数
var lightUp = function(timmer, cl){
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            cl();
            resolve();
        }, timmer);
    });
};

//promise 
var myPromise = new Promise(function(resolve, reject){resolve();});
var step = function(pro) {
    pro.then(function(){
        return lightUp(3000, red);
    }).then(function(){
        return lightUp(1000, green);
    }).then(function(){
        return lightUp(2000, yellow);
    }).then(function(){
        //递归
        step(pro);
    });
}
 
step(myPromise);

第二题：

实现一个LazyMan，可以按照以下方式调用:
LazyMan(“Hank”)输出:
Hi! This is Hank!
LazyMan(“Hank”).sleep(10).eat(“dinner”)输出
Hi! This is Hank!
//等待10秒..
Wake up after 10
Eat dinner~
LazyMan(“Hank”).eat(“dinner”).eat(“supper”)输出
Hi This is Hank!
Eat dinner~
Eat supper~
LazyMan(“Hank”).sleepFirst(5).eat(“supper”)输出
//等待5秒
Wake up after 5
Hi This is Hank!
Eat supper
以此类推。

//等待函数
function wait(second) {
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            resolve('Wake up after ' + second);
        }, second * 1000);
    });
}

//链式+promise
var  LazyMan = function(name) {
    var p = new Promise(function (resolve, reject) {
        resolve('Hi! This is ' + name + '!');
    });

    return {
        sleep: function (second) {
            p = p.then(function (msg) {
                console.log(msg);
                return wait(second);
            });
            return this;
        },
        sleepFirst: function (second) {
            var op = p;
            p = wait(second).then(function (msg) {
                console.log(msg);
                return op;
            });
            return this;
        },
        eat: function (part) {
            var pn = new Promise(function (resolve) {
                resolve('Eat ' + part + '~');
            });
            p = p.then(function (msg) {
                console.log(msg);
                return pn;
            });
            return this;
        },
        print: function () {
            return p.then(function (msg) {
                console.log(msg);
            });
        }
    };
}
exports.LazyMan = LazyMan;

本题有多种方案，感兴趣的可以尝试面向对象等其他方式来实现。