核心思想即把then后的函數提取出來等待resolve后執行callbacks

function _Promise(fn, callbacks) {var value = null;var callbacks = callbacks || [];var state = 'pending';var _this = this;//注冊所有的回調函數this.then = function (fulfilled, rejected) {//鏈式promisereturn new _Promise(function (resolv, rejec) {try {if (state == 'pending') {callbacks.push(fulfilled);return;}if (state == 'fulfilled') {var data = fulfilled(value) || value;resolv(data);return;}if (state == 'rejected') {var data = rejected(value);resolv(data);return;}} catch (e) {_this.catch(e);}}, callbacks);}//執行所有的回調函數function resolve(valueNew) {value = valueNew;state = 'fulfilled';execute();}//執行所有的回調函數function reject(valueNew) {value = valueNew;state = 'rejected';execute();}function execute() {//加入延時機制 防止promise里面有同步函數 導致resolve先執行 then還沒注冊上函數setTimeout(function () {callbacks.forEach(function (cb) {value = cb(value);});}, 0);}this.catch = function (e) {console.error(JSON.stringify(e));}fn(resolve, reject); } 復制代碼

轉載于:https://juejin.im/post/5d47b6ee518825052a032173

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