参考

var PENDING = undefined,    FULLFILLED = 1,    REJECTED = 2;var Promise = function(resolver) {    var promise = this;    this.value;    this.reason;    this.status = PENDING;    this.resolver;    this.rejecter;    var resolve = function(value) {        promise.value = value;        promise.status = FULLFILLED;        if (promise.resolver || promise.rejecter) {            var ret = promise.resolver.call(null, promise.value);            if(ret instanceof Promise) {                ret.then(promise.resolver_relsolve);            } else {                promise.resolver_relsolve(promise.value);            }        }    }    var reject = function(err) {        promise.value = err;        promise.status = REJECTED;        if (promise.resolver || promise.rejecter) {            var ret = promise.rejecter.call(null, promise.value);            if(ret instanceof Promise) {                ret.then(function(){}, promise.rejecter_reject);            } else {                promise.rejecter_reject(promise.value);            }        }    }        resolver(resolve, reject);}// Promise then 方法Promise.prototype.then = function(onFulfilled, onRejected) {    var promise = this;    if (promise.status == FULLFILLED) {        var ret = onFulfilled(promise.value);        if(ret instanceof Promise) {            return ret;        } else {            return new Promise(function(resolve) {                resolve(ret);            });        }    } else if (promise.status == REJECTED) {        var ret = onRejected(promise.value);        if(ret instanceof Promise) {            return ret;        } else {            return new Promise(function(resolve, reject) {                reject(ret);            });        }        }    return new Promise(function(resolve, reject) {        promise.resolver = onFulfilled;        promise.resolver_relsolve = resolve;        promise.rejecter = onRejected;        promise.rejecter_reject = reject;    });}

测试

var p = new Promise(function(resolve, reject) {    console.log('before');    setTimeout(function() {        resolve(10);    }, 2000);});p.then(function() {    console.log('after', arguments);}).then(function() {    return new Promise(function(resolve, reject) {        setTimeout(function() {            resolve(20)        }, 2000);    });}).then(function() {    console.log(arguments);});

输出

// 两秒beforeafter [10]// 两秒[20]