例1
let asyncFunc = function(callback){let num = 100;setTimeout(function(){num += 100;callback(num);},2000);
};function foo(value){console.log(value);  //value => 200
}asyncFunc (foo);

例2
ajax(url1,function(value1){foo(value1);bar();
});
function foo(value){ajax(url2,function(value2){do something..ajax(url3,function(value3){...})});
}
function bar(){ do something.. };

例3
let p = new Promise(function(resolve,reject){let value = 100;setTimeout(function(){value += 100;resolve(value);},2000);
});
p.then(function(value){console.log(value);      //value => 200
},function(err){do something...
});

例4
let p1 = new Promise(function(resolve,reject){ajax(url1,function(value1){resolve(value1);});
});p1.then(function(value1){return new Promise(function(resolve,reject){ajax(url2,function(value2){do something..resolve(value2);});})
}).then(function(value2){return new Promise(function(resolve,reject){ajax(url3,function(value3){...});})
});
p1.then(bar);
function bar(){do something...};

new Promsie.then(null,function(){do something...
})

例5
let promiseArr = [new Promise(function(resolve,reject){setTimeout(function(){resolve(100)},1000)}),new Promise(function(resolve,reject){setTimeout(function(){resolve(200)},500)})
]
Promise.all(promiseArr).then(function(valArr){console.log(valArr)     // valArr  => [100,200]
},function(err){do something...
})

new Promise(function(resolve,reject){reject(reason);
})

 npm i -g promises-aplus-testspromises-aplus-tests Promise.js

function MyPromise(task) {const _this = this;_this.status = 'pending';  //设定初始状态_this.value = undefined;_this.onFulfilledsList = [];  //onFulfilled函数序列_this.onRejectedsList = [];  //onRejected函数序列function resolve(value) {if (value instanceof MyPromise) {return value.then(resolve, reject);}//异步执行resolve或reject方法，保证代码的统一性和注册的回调函数按照正确的顺序执行if (_this.status === 'pending') {_this.status = 'fulfilled';_this.value = value;_this.onFulfilledsList.forEach(cb => cb(value))}}function reject(reason) {if (_this.status === 'pending') {_this.status = 'rejected';_this.reason = reason;_this.onRejectedsList.forEach(cb => cb(reason))}}try {task(resolve, reject);} catch (err) {throw new Error(err);}
}function resolvePromise(promise2, x, resolve, reject) {if (x === promise2) {return reject(new TypeError('循环引用'));}//如果返回的是一个thenable对象，即一个拥有then方法的对象，那么使用它的then方法去获得它的最终返回值。目的是为了兼容其他Promise库if (x !== null && (typeof x === 'object' || typeof x === 'function')) {let then, called;try {then = x.then;if (typeof then === 'function') {then.call(x, function (newx) {if (called) return;   //防止重复调用called = true;resolvePromise(promise2, newx, resolve, reject);}, function (err) {if (called) return;called = true;return reject(err);});} else {resolve(x);}} catch (err) {if (called) return;called = true;reject(err);}} else {resolve(x);}
}MyPromise.prototype.then = function (onFulfilled, onRejected) {const _this = this;let promise2;onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : function (data) {return data;};onRejected = typeof onRejected === 'function' ? onRejected : function (data) {throw data;};//为了支持同步代码，当then方法注册的时候如果Promise的状态已经改变，那么立即执行对应的函数if (_this.status === 'fulfilled') {promise2 = new MyPromise(function (resolve, reject) {setTimeout(function () {let x;try {x = onFulfilled(_this.value);resolvePromise(promise2, x, resolve, reject);} catch (err) {reject(err);}})})}if (_this.status === 'rejected') {promise2 = new MyPromise(function (resolve, reject) {setTimeout(function () {let x;try {x = onRejected(_this.reason);resolvePromise(promise2, x, resolve, reject);} catch (err) {reject(err);}})})}if (_this.status === 'pending') {promise2 = new MyPromise(function (resolve, reject) {_this.onFulfilledsList.push(function (value) {setTimeout(function () {let x;try {x = onFulfilled(value);resolvePromise(promise2, x, resolve, reject);} catch (err) {reject(err);}});});_this.onRejectedsList.push(function (reason) {setTimeout(function () {try {let x = onRejected(reason);resolvePromise(promise2, x, resolve, reject);} catch (err) {reject(err);}});});})}return promise2;  //返回一个新的Promise实例，以便支持链式调用
};MyPromise.prototype.catch = function (onRejected) {this.then(null, onRejected);
};MyPromise.all = function (someValue) {let resolveValArr = [];let count = promiseLen = 0;let promise2;promise2 = new MyPromise(function (resolve, reject) {let iNow = 0;try {for (let item of someValue) {if (item !== null && typeof item === "object") {try {let then = item.then;let index = iNow;if (typeof then === 'function') {promiseLen++;then.call(item, function (value) {resolveValArr[index] = value;if (++count === promiseLen) {resolve(resolveValArr)}}, function (err) {reject(err);});}} catch (err) {resolveValArr[iNow] = item;}} else {resolveValArr[iNow] = item;}iNow++;}if (iNow === 0) {return resolve(someValue);}if (promiseLen === 0) {return resolve(resolveValArr);}} catch (err) {reject(new TypeError('无法遍历的类型!'));}});return promise2;
};MyPromise.race = function (someValue) {let promise2;promise2 = new MyPromise(function (resolve, reject) {let iNow = 0;try {for (let item of someValue) {if (item !== null && typeof item === "object") {try {let then = item.then;then.call(item, function (value) {resolve(value);}, function (err) {reject(err);});} catch (err) {resolve(item);break;}} else {resolve(item);break;}iNow++;}if (iNow === 0) {return resolve(someValue);}} catch (err) {reject(new TypeError('无法遍历的类型!'));}});return promise2;
};
MyPromise.resolve = function (value) {let promise2;if (value !== null && (typeof value === 'object' || typeof value === 'function')) {promise2 = new MyPromise(function (resolve, reject) {try {let then = value.then;if (typeof value.then === 'function') {then.call(value, function (data) {resolve(data);}, reject);} else {resolve(value);}} catch (err) {reject(err);}})} else {promise2 = new MyPromise(function (resolve) {resolve(value);})}return promise2;
};
MyPromise.reject = function (reason) {return new MyPromise(function (resolve, reject) {reject(reason);})
};
module.exports = MyPromise;//这是为了让代码能够测试而开放的接口，详见promises-aplus-tests中的相关描述
MyPromise.deferred = MyPromise.defer = function () {let deferred = {};deferred.promise = new MyPromise(function (resolve, reject) {deferred.resolve = resolve;deferred.reject = reject;});return deferred
};