1.promise.all
目的：此方法在集合多个 promise 的返回结果时很有用
普通用法：
var p1 = Promise.resolve(3);
var p2 = 1337;
var p3 = new Promise((resolve, reject) => {
// 时间一到，‘foo’作为参数传给resolve
setTimeout(resolve, 100, 'foo');
});
Promise.all([p1, p2, p3]).then(values => {
console.log(values); // [3, 1337, "foo"]
});
同步异步
Promise.all 当且仅当传入的可迭代对象为空时为同步：
var p = Promise.all([]); // will be immediately resolved
var p2 = Promise.all([1337, "hi"]); // non-promise values will be ignored, but the evaluation will be done asynchronously
console.log(p);
console.log(p2)
setTimeout(function(){
console.log('the stack is now empty');
console.log(p2);
});
// logs
// Promise { <state>: "fulfilled", <value>: Array[0] }
// Promise { <state>: "pending" }
// the stack is now empty
// Promise { <state>: "fulfilled", <value>: Array[2] }
遇到false
有一个立即调用失败函数，那么 Promise.all 将立即变为失败。
var p4 = new Promise((resolve, reject) => {
setTimeout(resolve, 4000, 'four');
});
var p5 = new Promise((resolve, reject) => {
reject('reject');
});
Promise.all([p1, p2, p3, p4, p5]).then(values => {
console.log(values);
}, reason => {
console.log(reason)
});
//From console:
//"reject"
