实现 Promise 的简化版，满足以下需求即可
function Promise(???){
???
return ???
}
var promise = new Promise(function(x,y){
setTimeout(()=>{
x(101)
}, 3000)
})
promise.then((z)=>{
console.log(z) // 101
})
Promise 简化版本
function Promise(fn){
var status = 'pending'
function successNotify(){
status = 'resolved'
toDoThen.apply(undefined, arguments)
}
function failNotify(){
status = 'rejected'
toDoThen.apply(undefined, arguments)
}
function toDoThen(){
setTimeout(()=>{ // 保证回调是异步执行的
if(status === 'resolved'){
for(let i =0; i< successArray.length;i ++) {
successArray[i].apply(undefined, arguments)
}
}else if(status === 'rejected'){
for(let i =0; i< failArray.length;i ++) {
failArray[i].apply(undefined, arguments)
}
}
})
}
var successArray = []
var failArray = []
fn.call(undefined, successNotify, failNotify)
return {
then: function(successFn, failFn){
successArray.push(successFn)
failArray.push(failFn)
return undefined // 简化
}
}
}
时序图
