自己实现promise功能
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
function Promise(executor) {
var _this = this
this.state = PENDING; //状态
this.value = undefined; //成功结果
this.reason = undefined; //失败原因
this.callback ={};
function resolve(value) {
if(_this.state === PENDING){
_this.state = FULFILLED
_this.value = value
if(_this.callback.onFulfilled){
_this.callback.onFulfilled(value)
}
}
}
function reject(reason) {
if(_this.state === PENDING){
_this.state = REJECTED
_this.reason = reason
if(_this.callback.onRejected){
_this.callback.onRejected(value)
}
}
}
try {
executor(resolve, reject);
} catch (e) {
reject(e);
}
}
Promise.prototype.then = function (onFulfilled, onRejected) {
if(this.state === FULFILLED){
onFulfilled(this.value)
}
if(this.state === REJECTED){
onRejected(this.reason)
}
if(this.state === PENDING){
this.callback ={onFulfilled,onRejected}
}
};<script src="promise.js"></script>
<script>
let p=new Promise((resolve,reject)=>{
setTimeout(() => {
resolve('ok');
}, 100);
});
p.then(value=>{
console.log(value);
},reason=>{
console.log(reason);
});
</script>
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