重写promise.1.html

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    <h2>重写promise</h2>


    <script>
        const PENDING = 'pending';
        const RESOLVED = 'resolved';
        const REJECTED = 'rejected';

        function MyPromise(fn) {
            console.log('enter..')
            var $this = this;
            this.state = PENDING;
            this.value = null;
            this.resolvedCallbacks = [];
            this.rejectedCallbacks = [];

            fn(resolve)

            function resolve(value) {  //resolve函数执行后更改state为RESOLVED
                if ($this.state == PENDING) {
                    $this.state = RESOLVED;
                    $this.value = value;
                    $this.resolvedCallbacks.forEach(cb => cb(value));
                }
            }
        }
        //resolve函数执行后更改state为RESOLVED
        MyPromise.prototype = {
            then: function (onFulfilled, onRejected) {
                
                onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;
                

                if (this.state === PENDING) {
                    this.resolvedCallbacks.push(onFulfilled);
                    this.rejectedCallbacks.push(onRejected);
                }
                if (this.state === RESOLVED) {
                    onFulfilled(this.value);
                }
                if (this.state === REJECTED) {
                    onRejected(this.value);
                }

            }
        }
        function getList() {
            return new MyPromise(resolve => {
                setTimeout(() => {
                    resolve([1, 2, 3, 4]);
                }, 10000)

            })
        }



        getList().then(res => console.log(res, 'result'));
    </script>
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