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promise2.html
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promise2.html
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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<script>
/* function Promise(fn) {
var value = null,
callbacks = []; //callbacks为数组，因为可能同时有很多个回调
this.then = function (onFulfilled) {
callbacks.push(onFulfilled);
};
function resolve(value) {
callbacks.forEach(function (callback) {
callback(value);
});
}
fn(resolve);
}
*/
//极简的实现
class Promise {
callbacks = []; // 存储回调函数
constructor(fn) {
fn(this._resolve.bind(this));
}
then(onFulfilled) {
this.callbacks.push(onFulfilled);
}
_resolve(value) {
this.callbacks.forEach(fn => fn(value));
}
}
//then 的说明
let p = new Promise(resolve => {
setTimeout(() => {
console.log('done');
resolve('5秒');
}, 5000);
});
p.then(tip => {
console.log('then1', tip);
});
p.then(tip => {
console.log('then2', tip);
});
</script>
</body>
</html>