（六十）ES6新特征 - Promise
1：基本概念
表示一个异步操作的最终状态(完成或失败), 以及该异步操作的结果值
new Promise(
function(resolve, reject) {
/* executor */
}
);
2：基本用法
Promise接受一个函数作为参数, 该函数的参数分别是resolve和reject
let promise = new Promise(function(resolve, reject) {
console.log('new promise');
setTimeout(function() {
console.log('promise resolved');
return resolve("success");
}, 1000);
});
Promise实例生成后在then方法中指定resolved状态和rejected(可选)状态的回调函数
promise.then(function(value) {
console.log('resolved callback', value);
});
const promise1 = new Promise(function (resolve, reject) {
/* ... */
});
const promise2 = new Promise(function (resolve, reject) {
/* ... */
resolve(promise1);
})
