核心思想即把then后的函數(shù)提取出來(lái)等待resolve后執(zhí)行callbacks

function _Promise(fn, callbacks) {var value = null;var callbacks = callbacks || [];var state = 'pending';var _this = this;//注冊(cè)所有的回調(diào)函數(shù)this.then = function (fulfilled, rejected) {//鏈?zhǔn)絧romisereturn 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(zhí)行所有的回調(diào)函數(shù)function resolve(valueNew) {value = valueNew;state = 'fulfilled';execute();}//執(zhí)行所有的回調(diào)函數(shù)function reject(valueNew) {value = valueNew;state = 'rejected';execute();}function execute() {//加入延時(shí)機(jī)制 防止promise里面有同步函數(shù) 導(dǎo)致resolve先執(zhí)行 then還沒注冊(cè)上函數(shù)setTimeout(function () {callbacks.forEach(function (cb) {value = cb(value);});}, 0);}this.catch = function (e) {console.error(JSON.stringify(e));}fn(resolve, reject); } 復(fù)制代碼

轉(zhuǎn)載于:https://juejin.im/post/5d47b6ee518825052a032173

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