Promise 简单实现

来源：互联网发布：软件测试团队管理编辑：程序博客网时间：2024/05/12 14:18
/** * Created by wikid on 3/6/17. */;(function (f) {    /**     * 模块 引入方式     */    if (typeof exports === 'object') module.exports = f();    else if (typeof define === 'function' && define.amd) define(f);    else if (typeof window !== undefined) window.Promise = f();})(function () {    function Promise(fn) {        /*            value: resolve 的结果值(主要要来传递)            state: promise 的状态                    1. pending  等待中                    2. fulfilled 已完成                    3. rejected 失败            deferred: resolve 的回调栈         */        var value = null,            state = 'pending',            deferreds = [];        /**         * 为了兼容 promise 的串行，所以同一 then 方法同一返回 Promise 类型的实例         * @param fulfilledCall         * @param rejectedCall         * @returns {Promise}         */        this.then = function (fulfilledCall, rejectedCall) {            /**             *  每一次调用 then 方法时，都会创建一个 bridge promise             */            return new Promise(function (resolve) {                handle({                    fulfilledCall: fulfilledCall || null,                    rejectedCall: rejectedCall || null,                    resolve: resolve,                    reject: reject                })            });        };        function handle(deferred) {            if (state === 'pending') {                deferreds.push(deferred);                return;            }            var cb = state === 'fulfilled' ? deferred.fulfilledCall : deferred.rejectedCall,                ret;            // 兼容异常处理            if (cb === null) {                cb = state === 'fulfilled' ? deferred.resolve : deferred.reject;                cb(value);                return            }            try {                ret = cb(value);                deferred.resolve(ret);            } catch (e) {                deferred.resolve(e);            }        }        function resolve(newValue) {            //            if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) {                var then = newValue.then;                if (typeof then === 'function') {                    then.call(newValue, resolve, reject);                    return;                }            }            state = 'fulfilled';            value = newValue;            finale();        }        function reject(reason) {            state = 'rejected';            value = reason;            finale();        }        function finale() {            /**             * 异步执行，主要是兼容，创建 promise 实例时，fn 内直接将 promise 的状态置为 fulfilled，             * 而 后面的 then 方法还未执行，从而导致 deferreds 为 空数组             */            setTimeout(function () {                deferreds.forEach(function (deferred, i) {                    deferred(value);                });            }, 0)        }        fn(resolve);    }    return Promise;});
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