JavaScript_code_snippets/promise_implement.html at master · dendoink/JavaScript_code_snippets · GitHub

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<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<script>
    class Promise {
        constructor(excutor) {
            // 需要确保 excutor 为函数
            if (typeof excutor !== 'function') {
                throw new Error('excutor must be a function')
            }
            // 当前promise的状态
            this.state = 'pending' //fulfilled、rejected
            // 当状态为 fulfilled 时的 value
            this.value = undefined
            // 当状态为 rejected 时的 reason
            this.reason = undefined
            // 缓存 Resolved 状态下的回调函数
            this.onResolvedCallbacks = []
            // 缓存 Rejected 状态下的回调函数
            this.onRejectedCallbacks = []
            let resolve = (value) => {
                if (value instanceof Promise) {
                    return value.then(resolve, reject);
                }
                // 实践中要确保 onFulfilled 和 onRejected 方法异步执行，且应该在 then 方法被调用的那一轮事件循环之后的新执行栈中执行。
                setTimeout(() => {
                    // 调用resolve 回调对应onFulfilled函数
                    if (this.state === 'pending') {
                        this.state = 'fulfilled'
                        this.value = value
                        this.onResolvedCallbacks.forEach(fn => fn())
                    }
                });
            }
            let reject = (reason) => {
                setTimeout(() => {
                    if (this.state === 'pending') {
                        this.state = 'fulfilled、rejected'
                        this.reason = reason
                        this.onRejectedCallbacks.forEach(fn => fn())
                    }
                })
            }
            try {
                excutor(resolve, reject)
            } catch (error) {
                reject(error)
            }
        }
        then(onFulfilled, onRejected) {
            if (this.state === 'fulfilled') {
                onFulfilled(this.value)
            }
            if (this.state === 'rejected') {
                onRejected(this.reason)
            }
            if (this.state === 'pending') {
                this.onResolvedCallbacks.push(() => {
                    onFulfilled(this.value)
                })
                this.onRejectedCallbacks.push(() => {
                    onRejected(this.reason)
                })
            }
        }
    }
</script>

</html>