promise理解

 1 API实现： 
 2 //立即执行 简单实例
 3 let promise = new Promise(function(resolve, reject) {
 4   console.log(‘Promise‘);
 5   resolve();
 6 });
 7 promise.then(function() { //成功时候的调用
 8   console.log(‘resolved.‘);
 9 }).catch(function (reason) { //异常时候的调用
10     console.log(‘失败：‘ + reason);
11 });

//串行调用：
p.then(multiply)
.then(add)
.then(multiply)
.then(add)
.then(function (result) {
    log(‘Got value: ‘ + result);
});

//并行调用： Promise.all
var p1 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 500, ‘P1‘);
});
var p2 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 600, ‘P2‘);
});
// 同时执行p1和p2，并在它们都完成后执行then:
Promise.all([p1, p2]).then(function (results) {
    console.log(results); // 获得一个Array: [‘P1‘, ‘P2‘]
});
 
//但是为了容错 就是用Promise.race 谁执行快就先用谁的
var p1 = new Promise(function (resolve, reject) {
setTimeout(resolve, 500, ‘P1‘);
});
var p2 = new Promise(function (resolve, reject) {
setTimeout(resolve, 600, ‘P2‘);
});
Promise.race([p1, p2]).then(function (result) {
console.log(result); // ‘P1‘
});