SOURCE

function red(){
    console.log('red')
}
function green(){
    console.log('green')
}
function blue()
{
    console.log('blue')
}//1.回调函数方法
function showTask(light,delay,callback){
    setTimeout(()=>{
        if(light==='red'){
            red()
        }else if(light==='green'){
            green()
        }else{
            blue()
        }
        callback()
    },delay)
}
function step(){
    showTask('red',3000,()=>{
        showTask('green',2000,()=>{
            showTask('blue',1000,step)//又调用step函数实现循环调用(打印)
        })
    })
}
//step()

//2.使用promise实现
function showTask2(light,delay){
   return new Promise((resolve,reject)=>{
       setTimeout(()=>{
           if(light==='red'){
               red()
           }else if(light==='green'){
               green()
           }else if(light==='blue'){
               blue()
           }
           resolve(light)
       },delay)
    })
}
function step2(){
    showTask2('red',3000)
    .then((value)=>showTask2('green',2000))
    .then((value)=>showTask2('blue',1000))
    .then(step2)
}
//step2()

//3.async/await
async function step3(){
    await showTask2('red',3000)
    await showTask2('green',2000)
    await showTask2('blue',1000)
    step3()
}
//step3()