js中宏任务，微任务，异步，同步，执行的顺序

setTimeout(function(){
    console.log(‘set1‘)
    new Promise(function(resolve){
        resolve()
    }).then(function(){
        new Promise(function(resolve){
            resolve()
        }).then(function(){
            console.log(‘then4‘)
        })
        console.log(‘then2‘)
    })
})

new Promise(function(resolve){
    console.log(‘pr1‘);
    resolve()
}).then(function(){
    console.log(‘then1‘)
})

setTimeout(function(){
    console.log(‘set2‘)
})

console.log(2)