摘要：換句話說(shuō)該靜態(tài)函數(shù)返回個(gè)處于狀態(tài)的對(duì)象。等價(jià)于構(gòu)造函數(shù)的靜態(tài)函數(shù)，創(chuàng)建一個(gè)對(duì)象并以值作為參數(shù)調(diào)用句柄函數(shù)。等價(jià)于介紹構(gòu)造函數(shù)的靜態(tài)函數(shù)，參數(shù)是對(duì)象組成的可迭代數(shù)據(jù)集合。

ES2015 Promise函數(shù)是對(duì)PromiseA+標(biāo)準(zhǔn)的實(shí)現(xiàn)，并且嚴(yán)格遵守該標(biāo)準(zhǔn)。

Promise構(gòu)造函數(shù)的參數(shù)是個(gè)包含兩個(gè)參數(shù)的函數(shù)，并且該函數(shù)的參數(shù)分別對(duì)應(yīng)resolve, reject操作（resolve，reject也是函數(shù)）。并且只能通過(guò)這兩個(gè)參數(shù)改變Promise的狀態(tài)。

function asynOperation(){
    var promise = new Promise(function(resolve, reject){
        setTimeout(function(){
            reject(1,2,3); // 調(diào)用reject操作，并傳遞reason參數(shù)。雖然傳遞多個(gè)參數(shù)，但只把第一個(gè)作為reson值，符合Promise A+標(biāo)準(zhǔn)
        }, 3000)
    });
    return promise;
    
}
asynOperation().then(function(value){
    console.log("ES2015 Promise: resolved and value is " + value);
}, function(reason){
    console.log("ES2015 Promise: rejected and reason is " + reason);
})

上面的代碼用zpotojs實(shí)現(xiàn)：

/*
    ZeptoJS Deferred
*/
function AsynOperation1(){
    var deferred = $.Deferred();
    setTimeout(function(){
        deferred.reject(1, 2, 3);
    }, 3000);
    return deferred.promise();
}
AsynOperation1().then(function(val){
    console.log("Fullfilled: " + val);
}, function(reason){
    console.log("Rejected: " + reason);
})

catch方法用來(lái)添加Rejected狀態(tài)回調(diào)，是then方法的一種子集。

asynOperation().catch(function(reason){
})
// 等價(jià)于
asynOperation().then(undefined, function(reason){
})

Promise構(gòu)造函數(shù)的靜態(tài)方法，創(chuàng)建個(gè)Promise對(duì)象并以reason值調(diào)用reject句柄函數(shù)。換句話說(shuō)該靜態(tài)函數(shù)返回個(gè)處于“Rejected”狀態(tài)的Promise對(duì)象。

var p = Promise.reject(1);
// 等價(jià)于
var p = new Promise(function(resolve, reject){
  reject(1);
})

Promise構(gòu)造函數(shù)的靜態(tài)函數(shù)，創(chuàng)建一個(gè)Promise對(duì)象并以value值作為參數(shù)調(diào)用resolve句柄函數(shù)。換句話說(shuō)該靜態(tài)函數(shù)返回個(gè)處于“fullfilled"狀態(tài)的Promise對(duì)象。

var p = Promise.resolve(1);
// 等價(jià)于
var = new Promise(function(resolve){
  resolve(1);
})

Promise構(gòu)造函數(shù)的靜態(tài)函數(shù)，參數(shù)是Promise對(duì)象組成的可迭代數(shù)據(jù)集合。創(chuàng)建個(gè)Promise對(duì)象，并且參數(shù)指定的所以Promise都解決后，該P(yáng)romise對(duì)象該被解決，反之如果其中存在rejected的Promise，則該P(yáng)romise對(duì)象被rejected。
定義個(gè)異步操作：

function asynOperation(value, reason){
    var promise = new Promise(function(resolve, reject){
        setTimeout(function(){
            value === undefined ? reject(reason) : resolve(value);
        }, 3000)
    });
    return promise;
    
}

var p1 = asynOperation(1),
    p2 = asynOperation(2),
    p3 = asynOperation(3);
Promise.all([p1, p2, p3]).then(function(value){
    console.log("all resolved: value is " + value); // value是[1, 2, 3] 
})

如果參數(shù)元素中發(fā)生rejected操作，則立馬reject返回的Promise：

var p1 = asynOperation(1),
    p2 = asynOperation(undefined, 2), // reject操作
    p3 = asynOperation(undefined, 3); // reject操作
p2.catch(function(reson){
    console.log("p2")
})
p3.catch(function(reson){
    console.log("p3")
})
Promise.all([p1, p2, p3]).then(function(value){
    console.log("all resolved: value is " + value)
}, function(reason){
    console.log("one rejected: reason is " + reason); // reson值等于發(fā)生reject操作的promise對(duì)象的reason，即p2
})

顯示結(jié)果：

Promise.all和$.when很類似。主題功能差不多，參數(shù)傳遞方式不一樣：ES2015中把所有的Promise對(duì)象的value構(gòu)建個(gè)數(shù)組，而$.when是分別傳遞的。

function AsynOperation1(value, reason){
    var deferred = $.Deferred();
    setTimeout(function(){
        value === undefined ? deferred.reject(reason) : deferred.resolve(value);
    }, 3000);
    return deferred.promise();
}
var p1 = AsynOperation1(1),
    p2 = AsynOperation1(2),
    p3 = AsynOperation1(3);
$.when(p1, p2, p3).then(function(value){
    console.log("resolved " + value); // 留意Value的值
}, function(reason){
    console.log("rejected " + reason);
})

Promise構(gòu)造函數(shù)的靜態(tài)函數(shù)，參數(shù)是Promise對(duì)象構(gòu)成的可迭代對(duì)象，創(chuàng)建個(gè)Promise對(duì)象。當(dāng)參數(shù)任意Promise對(duì)象fullfilled(rejected)時(shí)，則立馬fullfill(reject)該P(yáng)romise對(duì)象。

var p1 = asynOperation(1),
    p2 = asynOperation(undefined, 2),
    p3 = asynOperation(undefined, 3);
Promise.race([p1, p2, p3]).then(function(value){
    console.log("one resolved: value is " + value); // Value=1
}, function(reason){
    console.log("one rejected: reason is " + reason)
})

Promise的回調(diào)通過(guò)micor-task實(shí)現(xiàn)的。

console.log(1)
setTimeout(function(){ console.log(2)}, 0)
Promise.resolve().then(function(){console.log(3)})
console.log(4)
// 輸出應(yīng)該是：1 4 3 2

大部分Promise A+標(biāo)準(zhǔn)的實(shí)現(xiàn)對(duì)"Promise解決過(guò)程"標(biāo)準(zhǔn)的實(shí)現(xiàn)略有不同，比如zeptoJs Deferred就沒(méi)考慮thenable情況。但是ES2015 Promise函數(shù)完全嚴(yán)格遵守Promise A+標(biāo)準(zhǔn)，包含對(duì)“Promise 解決過(guò)程的實(shí)現(xiàn)。下面舉例說(shuō)明實(shí)現(xiàn)Promise解決過(guò)程（The Promise Resolution Procedure）：[[Resolve]](promise, x)

var p1 = asynOperation(1);
var p2 = p1.then(function(value){
        console.log("resolved: " + value);
        return p2; // 返回p2對(duì)象，
    },function(reason){
        console.log("rejected: " + reason);
        }};
    });
p2.then(function(value){
    console.log("resolved: " + value);
},function(reason){
    console.log("rejected: " + reason);

var p1 = asynOperation(1);
var p2 = p1.then(function(value){
        console.log("resolved: " + value);
         return asynOperation(2); // 返回個(gè)Promise對(duì)象，以該P(yáng)rmoise對(duì)象狀態(tài)決定p2的狀態(tài)
    },function(reason){
        console.log("rejected: " + reason);
    });
p2.then(function(value){
    console.log("resolved: " + value);
},function(reason){
    console.log("rejected: " + reason);
})

A：then方法沒(méi)有調(diào)用resolve或者reject,并且沒(méi)有返回值，則不會(huì)改變promose的狀態(tài)

var p1 = asynOperation(1);
var p2 = p1.then(function(value){
        console.log("resolved: " + value);
        return {name: "john", then: function(resolve, reject){ // 返回值是個(gè)thenable對(duì)象，但是then方法沒(méi)有調(diào)用resolve或者reject,并且沒(méi)有返回值
            
        }};
    },function(reason){
        console.log("rejected: " + reason);
        
    });
p2.then(function(value){
    console.log("resolved: " + value);
},function(reason){
    console.log("rejected: " + reason);
})

B:then方法中調(diào)用resolve句柄，用新value y遞歸調(diào)用Promise解決過(guò)程：[[Resolve]](promise, y)

var p1 = asynOperation(1);
var p2 = p1.then(function(value){
        console.log("resolved: " + value);
        return {name: "john", then: function(resolve, reject){
            resolve(2); // then方法中調(diào)用resolve句柄，用新value遞歸調(diào)用Promise解決過(guò)程
        }};
    },function(reason){
        console.log("rejected: " + reason);
    });
p2.then(function(value){
    console.log("resolved: " + value);
},function(reason){
    console.log("rejected: " + reason);
})

C：then方法中調(diào)用reject句柄，則用相同的reason拒絕promise.

var p1 = asynOperation(1);
var p2 = p1.then(function(value){
        console.log("resolved: " + value);
        return {name: "john", then: function(resolve, reject){
            reject(2); // 調(diào)用reject句柄
        }};
    },function(reason){
        console.log("rejected: " + reason);
    });
p2.then(function(value){
    console.log("resolved: " + value);
},function(reason){
    console.log("rejected: " + reason);
})

D：then方法中多次調(diào)用resolve, reject句柄，只有第一個(gè)調(diào)用有效，其他的忽略

E：then方法中拋異常，如果拋異常之前已經(jīng)調(diào)用resolve或者reject句柄，則忽略該異常，反之則以該異常為reason拒絕promise

var p1 = asynOperation(undefined, 1); // 拒絕的promise
var p2 = p1.then(function(value){
        console.log("resolved: " + value);
        return {name: "john"};
    },function(reason){
        console.log("rejected: " + reason);
        return 2;
    });
p2.then(function(value){
    console.log("resolved: " + value);
},function(reason){
    console.log("rejected: " + reason);
})

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