then 2个参数 : 1.异步 2.同步
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>my</title>
</head>
<body>
<p>test my</p>
</body>
<script type="module">
// type="module" 严格模式
import MyPromise from './promise.js';
// 三种状态 1.pending 2.fulfilled 3.rejected
let myP = new MyPromise((resolve,reject)=>{
setTimeout(()=>{
console.log('====')
resolve("suc...");
});
// reject("err...");
});
// console.log(myP);
//then 2个参数
myP.then(res=>{
console.log(res);
},err=>{
console.log(err);
})
</script>
</html>
promise.js:
export default class MyPromise{
constructor(handle){
this.state = "pending";
this.result = undefined;
this.resolveFn = undefined;
this.rejectFn = undefined;
handle(this._resolve.bind(this),this._reject.bind(this));
};
_resolve(val){
this.state = "fulfilled";
this.result = val;
setTimeout(()=>{
this.resolveFn(val);
},0);
// 执行then的回调
// this.resolveFn(val);
};
_reject(val){
this.state = "rejected";
this.result = val;
setTimeout(()=>{
this.rejectFn(val);
},0);
// 执行then的回调
// this.rejectFn(val);
};
then(onResolved,onRejected){
// console.log(onResolved,onRejected,'-----');
//判断里面执行then里的参数，只能处理同步问题
// if(this.state === "fulfilled"){
// onResolved && onResolved(this.result)
// }else if(this.state === "rejected"){
// onRejected && onRejected(this.result)
// };
// 异步onResolved,onRejected不会立即执行；是在调取_resolve、_reject再执行
// 保存onResolved、onRejected
this.resolveFn = onResolved;
this.rejectFn = onRejected;
};
}
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