参考：https://www.jianshu.com/p/473cd754311f
<!DOCTYPE html> <html> <head> <meta charset="utf-8" /> <meta http-equiv="X-UA-Compatible" content="IE=edge"> <title>Page Title</title> <meta name="viewport" content="width=device-width, initial-scale=1"> </head> <body> <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> </body> </html>