手写promise
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const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";
 
function MyPromise(fn) {
    const self = this;
    self.value = null;
    self.error = null;
    self.status = PENDING;
    self.onFulfilledCallbacks = [];
    self.onRejectedCallbacks = [];
 
    function resolve(value) {
        if (value instanceof MyPromise) {
            return value.then(resolve, reject);
        }
        if (self.status === PENDING) {
            setTimeout(() => {
                self.status = FULFILLED;
                self.value = value;
                self.onFulfilledCallbacks.forEach((callback) => callback(self.value));
            }, 0)
        }
    }
 
    function reject(error) {
        if (self.status === PENDING) {
            setTimeout(function() {
                self.status = REJECTED;
                self.error = error;
                self.onRejectedCallbacks.forEach((callback) => callback(self.error));
            }, 0)
        }
    }
    try {
        fn(resolve, reject);
    } catch (e) {
        reject(e);
    }
}
 
function resolvePromise(bridgepromise, x, resolve, reject) {
    //2.3.1规范，避免循环引用
    if (bridgepromise === x) {
        return reject(new TypeError('Circular reference'));
    }
    let called = false;
    //这个判断分支其实已经可以删除，用下面那个分支代替，因为promise也是一个thenable对象
    if (x instanceof MyPromise) {
        if (x.status === PENDING) {
            x.then(y => {
                resolvePromise(bridgepromise, y, resolve, reject);
            }, error => {
                reject(error);
            });
        } else {
            x.then(resolve, reject);
        }
        // 2.3.3规范，如果 x 为对象或者函数
    } else if (x != null && ((typeof x === 'object') || (typeof x === 'function'))) {
        try {
            // 是否是thenable对象（具有then方法的对象/函数）
            //2.3.3.1 将 then 赋为 x.then
            let then = x.then;
            if (typeof then === 'function') {
            //2.3.3.3 如果 then 是一个函数，以x为this调用then函数，且第一个参数是resolvePromise，第二个参数是rejectPromise
                then.call(x, y => {
                    if (called) return;
                    called = true;
                    resolvePromise(bridgepromise, y, resolve, reject);
                }, error => {
                    if (called) return;
                    called = true;
                    reject(error);
                })
            } else {
            //2.3.3.4 如果 then不是一个函数，则 以x为值fulfill promise。
                resolve(x);
            }
        } catch (e) {
        //2.3.3.2 如果在取x.then值时抛出了异常，则以这个异常做为原因将promise拒绝。
            if (called) return;
            called = true;
            reject(e);
        }
    } else {
        resolve(x);
    }
}
 
MyPromise.prototype.then = function(onFulfilled, onRejected) {
    const self = this;
    let bridgePromise;
    onFulfilled = typeof onFulfilled === "function" ? onFulfilled : value => value;
    onRejected = typeof onRejected === "function" ? onRejected : error => { throw error };
    if (self.status === FULFILLED) {
        return bridgePromise = new MyPromise((resolve, reject) => {
            setTimeout(() => {
                try {
                    let x = onFulfilled(self.value);
                    resolvePromise(bridgePromise, x, resolve, reject);
                } catch (e) {
                    reject(e);
                }
            }, 0);
        })
    }
    if (self.status === REJECTED) {
        return bridgePromise = new MyPromise((resolve, reject) => {
            setTimeout(() => {
                try {
                    let x = onRejected(self.error);
                    resolvePromise(bridgePromise, x, resolve, reject);
                } catch (e) {
                    reject(e);
                }
            }, 0);
        });
    }
    if (self.status === PENDING) {
        return bridgePromise = new MyPromise((resolve, reject) => {
            self.onFulfilledCallbacks.push((value) => {
                try {
                    let x = onFulfilled(value);
                    resolvePromise(bridgePromise, x, resolve, reject);
                } catch (e) {
                    reject(e);
                }
            });
            self.onRejectedCallbacks.push((error) => {
                try {
                    let x = onRejected(error);
                    resolvePromise(bridgePromise, x, resolve, reject);
                } catch (e) {
                    reject(e);
                }
            });
        });
    }
}
MyPromise.prototype.catch = function(onRejected) {
    return this.then(null, onRejected);
}
// 执行测试用例需要用到的代码
MyPromise.deferred = function() {
    let defer = {};
    defer.promise = new MyPromise((resolve, reject) => {
        defer.resolve = resolve;
        defer.reject = reject;
    });
    return defer;
}
try {
    module.exports = MyPromise
} catch (e) {}
MyPromise.all = function(promises) {
    return new MyPromise(function(resolve, reject) {
        let result = [];
        let count = 0;
        for (let i = 0; i < promises.length; i++) {
            promises[i].then(function(data) {
                result[i] = data;
                if (++count == promises.length) {
                    resolve(result);
                }
            }, function(error) {
                reject(error);
            });
        }
    });
}
 
MyPromise.race = function(promises) {
    return new MyPromise(function(resolve, reject) {
        for (let i = 0; i < promises.length; i++) {
            promises[i].then(function(data) {
                resolve(data);
            }, function(error) {
                reject(error);
            });
        }
    });
}
 
MyPromise.resolve = function(value) {
    return new MyPromise(resolve => {
        resolve(value);
    });
}
 
MyPromise.reject = function(error) {
    return new MyPromise((resolve, reject) => {
        reject(error);
    });
}
MyPromise.promisify = function(fn) {
    return function() {
        var args = Array.from(arguments);
        return new MyPromise(function(resolve, reject) {
            fn.apply(null, args.concat(function(err) {
                err ? reject(err) : resolve(arguments[1])
            }));
        })
    }
}

　　

一、Promise是什么？

Promise是最早由社区提出和实现的一种解决异步编程的方案，比其他传统的解决方案（回调函数和事件）更合理和更强大。

ES6 将其写进了语言标准，统一了用法，原生提供了Promise对象。
ES6 规定，Promise对象是一个构造函数，用来生成Promise实例。

二、Promise是为解决什么问题而产生的？

promise是为解决异步处理回调金字塔问题而产生的

三、Promise的两个特点

1、Promise对象的状态不受外界影响

1）pending 初始状态

2）fulfilled 成功状态

3）rejected 失败状态

Promise 有以上三种状态，只有异步操作的结果可以决定当前是哪一种状态，其他任何操作都无法改变这个状态

2、Promise的状态一旦改变，就不会再变，任何时候都可以得到这个结果，状态不可以逆，只能由 pending变成fulfilled或者由pending变成rejected

四、Promise的三个缺点

1）无法取消Promise,一旦新建它就会立即执行，无法中途取消
2）如果不设置回调函数，Promise内部抛出的错误，不会反映到外部
3）当处于pending状态时，无法得知目前进展到哪一个阶段，是刚刚开始还是即将完成

五、Promise在哪存放成功回调序列和失败回调序列？

1）onResolvedCallbacks 成功后要执行的回调序列 是一个数组

2）onRejectedCallbacks 失败后要执行的回调序列 是一个数组

以上两个数组存放在Promise 创建实例时给Promise这个类传的函数中，默认都是空数组。
每次实例then的时候 传入 onFulfilled 成功回调 onRejected 失败回调，如果此时的状态是pending 则将onFulfilled和onRejected push到对应的成功回调序列数组和失败回调序列数组中，如果此时的状态是fulfilled 则onFulfilled立即执行，如果此时的状态是rejected则onRejected立即执行

上述序列中的回调函数执行的时候 是有顺序的，即按照顺序依次执行

六、Promise的用法

1、Promise构造函数接受一个函数作为参数，该函数的两个参数分别是resolve和reject。它们是两个函数，由 JavaScript 引擎提供，不用自己部署。

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const promise = new Promise(function(resolve, reject) {
   // ... some code
 
   if (/* 异步操作成功 */){
     resolve(value);
   } else {
     reject(error);
   }
 });

　　

2、resolve函数的作用是，将Promise对象的状态从“未完成”变为“成功”（即从 pending 变为 resolved），在异步操作成功时调用，并将异步操作的结果，作为参数传递出去；reject函数的作用是，将Promise对象的状态从“未完成”变为“失败”（即从 pending 变为 rejected），在异步操作失败时调用，并将异步操作报出的错误，作为参数传递出去。

3、Promise实例生成以后，可以用then方法分别指定resolved状态和rejected状态的回调函数。

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promise.then(function(value) {
    // success
  }, function(error) {
    // failure
  });

　　

then方法可以接受两个回调函数作为参数。第一个回调函数是Promise对象的状态变为resolved时调用，第二个回调函数是Promise对象的状态变为rejected时调用。其中，第二个函数是可选的，不一定要提供。这两个函数都接受Promise对象传出的值作为参数。

七、按照Promise A+规范写Promise的简单实现原理

// 第一步：Promise构造函数接受一个函数作为参数，该函数的两个参数分别是resolve和reject。它们是两个函数，由 JavaScript 引擎提供，不用自己部署。
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function Promise(task) {
 
        let that = this; // 缓存this
        that.status = 'pending'; // 进行中的状态
        that.value = undefined; //初始值
 
        that.onResolvedCallbacks = []; // 存放成功后要执行的回调函数的序列
        that.RejectedCallbacks = []; // 存放失败后要执行的回调函数的序列
        // 该方法是将Promise由pending变成fulfilled
        function resolve (value) {
            if (that.status == 'pending') {
                that.status = 'fulfilled';
                that.value = value;
                that.onResolvedCallbacks.forEach(cb => cd(that.value))
            }
 
        }
        // 该方法是将Promise由pending变成rejected
        function reject (reason) {
          if (that.status == 'pending') {
                that.status = 'rejected';
                that.value = reason;
                that.onRjectedCallbacks.forEach(cb => cd(that.value))
            }
        }
 
        try {
        // 每一个Promise在new一个实例的时候 接受的函数都是立即执行的
            task(resolve, reject)
        } catch (e) {
            reject(e)
        }
    }

　　

 // 第二部 写then方法，接收两个函数onFulfilled onRejected，状态是成功态的时候调用onFulfilled 传入成功后的值，失败态的时候执行onRejected，传入失败的原因，pending 状态时将成功和失败后的这两个方法缓存到对应的数组中，当成功或失败后 依次再执行调用 Promise.prototype.then = function(onFulfilled, onRejected) { let that = this; if (that.status == 'fulfilled') { onFulfilled(that.value); } if (that.status == 'rejected') { onRejected(that.value); } if (that.status == 'pending') { that.onResolvedCallbacks.push(onFulfilled); that.onRjectedCallbacks.push(onRejected); } }

八、Promise 链式写法

我们先来看一个例子，根据例子得出结论，然后再写源码的实现部分来验证结论

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let promise = new Promise(function (resolve, reject) {
       resolve(100);// reject(100)
   });
 
   promise.then(function (data) {
       return data+100;
 
   },function (err) {
      return 'ssss';
 
   }).then(function (data) {
       console.log(data);// 200  // undefined // sss
   })

　　

从上面的例子可以看出:

当第一个promise的成功的回调里返回 200时，第二个promise的成功回调的参数就是200
当将resolve(100)改成reject(100)的时候，因为失败回调中什么也没有返回所以第二个promise的成功回调中的参数是undefined
当失败的回调中返回sss时，第二个promise的成功回调中的参数是sss

由此我们可以看出，第一个promise不管成功回调还是失败回调，他的返回值作为第二个promise中的成功时回调函数的参数值

链式写法能一直then下去的原因：链式调用靠的是返回新的promise，来保证可以一直走成功或失败

九、 Promise.catch

Promise.prototype.catch方法是.then(null, rejection)的别名，用于指定发生错误时的回调函数。

    //catch原理就是只传失败的回调
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Promise.prototype.catch = function(onRejected){
       this.then(null,onRejected);
   }

　　

十、 Promise.all 方法

参数：接受一个数组，数组内都是Promise实例
返回值：返回一个Promise实例，这个Promise实例的状态转移取决于参数的Promise实例的状态变化。当参数中所有的实例都处于resolve状态时，返回的Promise实例会变为resolve状态。如果参数中任意一个实例处于reject状态，返回的Promise实例变为reject状态

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Promise.all = function(promises){
    return new Promise(function(resolve,reject){
      let done = gen(promises.length,resolve);
      for(let i=0;i<promises.length;i++){
        promises[i].then(function(data){
          done(i,data);
        },reject);
      }
    });
   }

　　

十一、Promise.resolve

返回一个Promise实例，这个实例处于resolve状态。
根据传入的参数不同有不同的功能：

值(对象、数组、字符串等)：作为resolve传递出去的值
Promise实例：原封不动返回

    //返回一个立刻成功的promise
    //别人提供 给你一个方法，需要你传入一个promise,但你只有一个普通的值，你就可以通过这个方法把这个普通的值(string number object)转成一个promise对象
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Promise.resolve = function(value){
    return new Promise(function(resolve){
      resolve(value);
    });
  }

　　

十二、 Promise.reject

返回一个Promise实例，这个实例处于reject状态。

参数一般就是抛出的错误信息。

    //返回一个立刻失败的promise
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Promise.reject = function(reason){
      return new Promise(function(resolve,reject){
        reject(reason);
      });
    }

　　

十三、 Promise.race

参数：接受一个数组，数组内都是Promise实例
返回值：返回一个Promise实例，这个Promise实例的状态转移取决于参数的Promise实例的状态变化。当参数中任何一个实例处于resolve状态时，返回的Promise实例会变为resolve状态。如果参数中任意一个实例处于reject状态，返回的Promise实例变为reject状态。

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Promise.race = function(promises){
    return new Promise(function(resolve,reject){
      for(let i=0;i<promises.length;i++){
        promises[i].then(resolve,reject);
      }
    });
  }

　　

 

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# es6 class版
class Promise{
    constructor(executor) {
        this.state = "pending";
        this.value = undefined;
        this.reason = undefined;
        this.onResolvedCallbacks = [];
        this.onRejectedCallbacks = [];
        let resolve = value => {
            if(this.status === "pending") {
                this.state = "fulfilled";
                this.value = value;
                this.onResolvedCallbacks.forEach(fn => fn())
            }
        }
        ket reject = reason => {
            if(this.status === "pending") {
                this.state = "rejected";
                this.reason = reason;
                this.onRejectedCallbacks.forEach(fn => fn())
            }
        }
        try{
            executor(resolve, reject);
        } catch(err) {
            reject(err)
        }
    }
    then(onFulfilled, onRejected) {
        onFulfilled = typeof onFulfilled === "function" ? onFulfilled : value => value;
        onRejected = typeof onRejected === "function" ? onRejected: err => { throw err};
        let promise2 = new Promise((resolve, reject) => {
            if(this.status === "fulfilled") {
                setTimeout(() => {
                    try{
                        let x = onFulfilled(this.value);
                        resolvePromise(promise2, x, resolve, reject);
                    }catch(err) {
                        reject(err)
                    }
                }, 0)
            }
            if(this.status === "rejected") {
                setTimeout(() => {
                    try{
                        let x = onRejected(this.value);
                        resolvePromise(promise2, x, resolve, reject);
                    }catch(err) {
                        reject(err)
                    }
                }, 0)
            }
            if(this.status === "pending") {
                this.onResolvedCallbacks.push(() => {
                    setTimeout(() => {
                        try{
                            let x = onFulfilled(this.value);
                            resolvePromise(promise2, x, resolve, reject);
                        }catch(err) {
                            reject(err)
                        }
                    }, 0)
                })
                this.onRejectedCallbacks.push(() => {
                    setTimeout(() => {
                        try{
                            let x = onRejected(this.value);
                            resolvePromise(promise2, x, resolve, reject);
                        }catch(err) {
                            reject(err)
                        }
                    }, 0)
                })
            }
        })
        return Promise2;
    }
    catch(err) {
        return this.then(null, fn)
    }
}
 
function resolvePromise(promise2, x, resolve, reject) {
    if(x === promise2) {
        return reject(new TypeError("chanining cycle detected for promise"))
    }
    let called ;
    if(x != null && (typeof x === "object" || typeof x === "function")) {
        try{
            let then = x.then;
            if(typeof then === "function") {
                then.call(x, y=> {
                    if(called) return;
                    called = true;
                    resolvePromise(promise2, y, resolve, reject)
                }, err => {
                    if(called) return;
                    called = true;
                    reject(err)
                } )
            }
        } catch(e) {
            if(called) return;
            called = true;
            reject(e)
        }
         
    } else {
        resolve(x)
    }
}
 
Promise.resolve = function(val) {
    return new Promise((resolve, reject) => {
        resolve(val)
    })
}
 
Promise.reject = function(val) {
    return new Promise((resolve, reject) => {
        reject(val)
    })
}
 
Promise.all = function(promises) {
    let arr = [];
    let i = 0;
    function processData(index,data) {
        arr[index] = data;
        i++;
        if(i == promises.length) {
            resolve(arr)
        }
    }
    return new Promise((resolve, reject) => {
        for(let i=0;i<promises.length;i++) {
            promises[i].then(data => {
                processData(i, data)
            }, reject)
        }
    })
}
 
Promise.race = function(promises) {
    return new Promise((resolve, reject) => {
        for(let i=0;i<promises.length;i++) {
            promises[i].then(resolve, reject)
        }
    })
}
 
Promise.defer = Promise.defered = function() {
    let dfd = {};
    dfd.promise = new Promise((resolve, reject) => {
        dfd.resolve = resolve;
        dfd.reject = reject;
    })
    return dfd;
}
 
Promise.promisify = function (fn) {
    return function() {
        var args = Array.from(arguments);
        return new Promise((resolve, reject) => {
            fn.apply(null, args.concat((err)=>{
                err ? reject(err) : resolve(argumnets[1])
            }))
        })
    }
}
Promise.promisify = fn =>(...args)=>{
    return new Promise((resolve,reject)=>{
        fn(...args,function(err,data){
            if(err) reject(err);
            resolve(data);
        })
    })
}

　　




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