一道面试题去理解：promise、async和await 三者

async function async1(){
    console.log('async1 start')
    await async2()
    console.log('async1 end')
}
async function async2(){
    console.log('async2')
}
console.log('script start')
setTimeout(function(){
    console.log('setTimeout')
},0)  
async1();
new Promise(function(resolve){
    console.log('promise1')
    resolve();
}).then(function(){
    console.log('promise2')
})
console.log('script end')

// ES6.html: 61 script start
// ES6.html: 61 async1 start
// ES6.html: 67 async2
// ES6.html: 77 promise1
// ES6.html: 83 script end
// ES6.html: 63 async1 end
// ES6.html: 80 promise2
// ES6.html: 71 setTimeout