面试----你可以手写一个promise吗

参考：https://www.jianshu.com/p/473cd754311f
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    <meta charset="utf-8" />
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    <title>Page Title</title>
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    <script>
        function Promise(fn) {
            var state = 'pending';
            var doneList = [];
            var failList = [];
            this.then = function (done, fail) {
                switch (state) {
                    case "pending":
                        doneList.push(done);
                        //每次如果没有推入fail方法，我也会推入一个null来占位
                        failList.push(fail || null);
                        return this;
                        break;
                    case 'fulfilled':
                        done();
                        return this;
                        break;
                    case 'rejected':
                        fail();
                        return this;
                        break;
                }
            }
            function resolve(newValue) {
                state = "fulfilled";
                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);
            }
            function reject(newValue) {
                state = "rejected";
                setTimeout(function () {
                    var value = newValue;
                    var tempRe = failList[0](value);
                    //如果reject里面传入了一个promise，那么执行完此次的fail之后，将剩余的done和fail传入新的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，执行完当前的fail之后，继续执行doneList
                        value = tempRe;
                        doneList.shift();
                        failList.shift();
                        resolve(value);
                    }
                }, 0);
            }
            fn(resolve, reject);
        }
        var p = function () {
            return new Promise(function (resolve, reject) {
                setTimeout(function () {
                    reject('p 的结果');
                }, 100);
            });
        }
        var p2 = function (input) {
            return new Promise(function (resolve) {
                setTimeout(function () {
                    console.log('p2拿到前面传入的值：' + input)
                    resolve('p2的结果');
                }, 100);
            });
        }
        p()
            .then(function (res) { console.log('p的结果:' + res); return 'p then方法第一次返回' }, function (value) { console.log(value); return 'p then方法第一次错误的返回' })
            .then(function (res) { console.log('p第一次then方法的返回：' + res); return 'p then方法第二次返回' })
            .then(p2)
            .then(function (res) { console.log('p2的结果：' + res) });


    </script>

    <script>
       // ES6中使用promise
        const promise = new Promise(function (resolve, reject) {
            // ... some code

            if (/* 异步操作成功 */) {
                resolve(value);
            } else {
                reject(error);
            }
        });

        promise.then(function (value) {
            // success
        }, function (error) {
            // failure
        });

    </script>


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