一、概述
ES2015 Promise函数是对PromiseA+规范的完成，而且严格遵守该规范。
二、APIs
2.1 建立Promise对象
Promise组织函数的参数是个包括两个参数的函数，而且该函数的参数离别对应resolve, reject操纵（resolve，reject也是函数）。而且只能经由历程这两个参数转变Promise的状况。
function asynOperation(){
var promise = new Promise(function(resolve, reject){
setTimeout(function(){
reject(1,2,3); // 挪用reject操纵，并通报reason参数。虽然通报多个参数，但只把第一个作为reson值，相符Promise A+规范
}, 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);
})上面的代码用zpotojs完成：
/*
ZeptoJS Deferred
*/
function AsynOperation1(){
var deferred = $.Deferred();
setTimeout(function(){
deferred.reject(1, 2, 3);
}, 3000);
return deferred.promise();
}
AsynOperation1().then(function(val){
console.log('Fullfilled: ' + val);
}, function(reason){
console.log('Rejected: ' + reason);
})2.2 Promise.prototype.catch
catch要领用来增加Rejected状况回调，是then要领的一种子集。
asynOperation().catch(function(reason){
})
// 等价于
asynOperation().then(undefined, function(reason){
})2.3 Promise.reject(reason)
Promise组织函数的静态要领，建立个Promise对象并以reason值挪用reject句柄函数。换句话说该静态函数返回个处于“Rejected”状况的Promise对象。
var p = Promise.reject(1);
// 等价于
var p = new Promise(function(resolve, reject){
reject(1);
})2.4 Promise.resolve(value)
Promise组织函数的静态函数，建立一个Promise对象并以value值作为参数挪用resolve句柄函数。换句话说该静态函数返回个处于“fullfilled”状况的Promise对象。
var p = Promise.resolve(1);
// 等价于
var = new Promise(function(resolve){
resolve(1);
})2.5 Promise.all(iterable)
2.5.1 引见
Promise组织函数的静态函数，参数是Promise对象组成的可迭代数据鸠合。建立个Promise对象，而且参数指定的所以Promise都处理后，该Promise对象该被处理，反之假如个中存在rejected的Promise，则该Promise对象被rejected。
定义个异步操纵：
function asynOperation(value, reason){
var promise = new Promise(function(resolve, reject){
setTimeout(function(){
value === undefined ? reject(reason) : resolve(value);
}, 3000)
});
return promise;
}
var p1 = asynOperation(1),
p2 = asynOperation(2),
p3 = asynOperation(3);
Promise.all([p1, p2, p3]).then(function(value){
console.log('all resolved: value is ' + value); // value是[1, 2, 3]
})假如参数元素中发作rejected操纵，则立马reject返回的Promise：
var p1 = asynOperation(1),
p2 = asynOperation(undefined, 2), // reject操纵
p3 = asynOperation(undefined, 3); // reject操纵
p2.catch(function(reson){
console.log('p2')
})
p3.catch(function(reson){
console.log('p3')
})
Promise.all([p1, p2, p3]).then(function(value){
console.log('all resolved: value is ' + value)
}, function(reason){
console.log('one rejected: reason is ' + reason); // reson值即是发作reject操纵的promise对象的reason，即p2
})显现效果：
2.5.2 对照$.when
Promise.all和$.when很相似。主题功用差不多，参数通报体式格局不一样：ES2015中把一切的Promise对象的value构建个数组，而$.when是离别通报的。
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); // 注意Value的值
}, function(reason){
console.log('rejected ' + reason);
})2.6 Promise.race(iterable)
Promise组织函数的静态函数，参数是Promise对象组成的可迭代对象，建立个Promise对象。当参数恣意Promise对象fullfilled(rejected)时，则立马fullfill(reject)该Promise对象。
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)
})三、micro-task
Promise的回调经由历程micor-task完成的。
console.log(1)
setTimeout(function(){ console.log(2)}, 0)
Promise.resolve().then(function(){console.log(3)})
console.log(4)
// 输出应该是：1 4 3 2四、对照Promise A+ 规范
大部分Promise A+规范的完成对”Promise处理历程”规范的完成略有不同，比方zeptoJs Deferred就没斟酌thenable状况。然则ES2015 Promise函数完整严格遵守Promise A+规范，包括对“Promise 处理历程的完成。下面举例说明完成Promise处理历程（The Promise Resolution Procedure）：[[Resolve]](promise, x)
3.3.1 假如promise对象和x相称，则用TypeError对象作为reason 谢绝promise对象
var p1 = asynOperation(1);
var p2 = p1.then(function(value){
console.log('resolved: ' + value);
return p2; // 返回p2对象，
},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); // 返回个Promise对象，以该Prmoise对象状况决议p2的状况
},function(reason){
console.log('rejected: ' + reason);
});
p2.then(function(value){
console.log('resolved: ' + value);
},function(reason){
console.log('rejected: ' + reason);
})
3.3.3 假如x是thenable，promise状况的转变取决于x.then要领的实行中resolve,reject句柄的挪用状况。并不受x.then返回值的影响。
- A：then要领没有挪用resolve或许reject,而且没有返回值，则不会转变promose的状况
var p1 = asynOperation(1);
var p2 = p1.then(function(value){
console.log('resolved: ' + value);
return {name: 'john', then: function(resolve, reject){ // 返回值是个thenable对象，然则then要领没有挪用resolve或许reject,而且没有返回值
}};
},function(reason){
console.log('rejected: ' + reason);
});
p2.then(function(value){
console.log('resolved: ' + value);
},function(reason){
console.log('rejected: ' + reason);
})
- B:then要领中挪用resolve句柄，用新value y递归挪用Promise处理历程：[[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要领中挪用resolve句柄，用新value递归挪用Promise处理历程
}};
},function(reason){
console.log('rejected: ' + reason);
});
p2.then(function(value){
console.log('resolved: ' + value);
},function(reason){
console.log('rejected: ' + reason);
})- C：then要领中挪用reject句柄，则用雷同的reason谢绝promise.
var p1 = asynOperation(1);
var p2 = p1.then(function(value){
console.log('resolved: ' + value);
return {name: 'john', then: function(resolve, reject){
reject(2); // 挪用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要领中屡次挪用resolve, reject句柄，只要第一个挪用有用，其他的疏忽
- E：then要领中抛非常，假如抛非常之前已挪用resolve或许reject句柄，则疏忽该非常，反之则以该非常为reason谢绝promise
2.3.4 其他状况，则用x作为value完成promise。
var p1 = asynOperation(undefined, 1); // 谢绝的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);
})