<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>promise-polyfill</title>
</head>
<body>
<script type="text/javascript">
//Promise-polyfill
function Promise(fn) {
var state = 'pending';
var doneList = [];
this.then = function(done, fail) {
switch(state) {
case 'pending':
doneList.push(done);
return this;
break;
case 'fulfilled':
done();
return this;
break;
case 'rejected':
fail();
return this;
break;
}
//保存成功回調，異步
doneList.push(done);
//鏈式調用
return this;
}
function resolve(newValue) {
state = 'fulfilled';
//異步結果傳遞
var value = newValue;
//放到最後執行，確定donelist裏不爲空
setTimeout(function(){
var value = newValue;
for (var i = 0; i < doneList.length; i++){
var temp = doneList[i](value);
if(temp instanceof Promise){
var newP = temp;
for(i++; i < doneList.length; i++){
newP.then(doneList[i],failList[i]);
}
}else{
value = temp;
}
}
}, 0);
// doneList.forEach(function(fulfill) {
// value = fulfill(value);
// });
}
function reject(newValue) {
state = "rejected";
setTimeout(function() {
var value = newValue;
var tempRe = failList[0](value);
//如果reject裏面傳入了一個promise，那麼執行完此次的fail之後，將剩餘的done和fail傳入新的promise中
if(tempRe instanceof Promise) {
var newP = tempRe;
for(i = 1;i < doneList.length; i++) {
newP.then(doneList[i],failList[i]);
}
}else {
//如果不是promise，執行完當前的fail之後，繼續執行doneList
value = tempRe;
doneList.shift();
failList.shift();
resolve(value);
}
}, 0);
}
fn(resolve, reject);
}
var promise = new Promise(function(resolve) {
// console.log('hehehe')
resolve();
});
promise.then(function() {
console.log('resolve')
});
</script>
</body>
</html>Promise簡單實現
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