asynchronously US: [e'sɪŋkrənəslɪ] 异步方式

use it to make a promise to do something, usually asynchronously.

任务完成后，是履行承诺还是不做

Promise is a constructor function, so you need to use the new keyword to create one

两个参数

resolve 方法，履行承诺

reject 方法，拒绝

const myPromise = new Promise(function(resolve, reject){

});

const myPromise = new Promise((resolve, reject) => {//省略了function 添加了=>

});

Promise 有三个状态 states

pending 等待

fulfilled 完成

rejected 拒绝

const myPromise = new Promise((resolve, reject) => {
  if(condition here) {
    resolve("Promise was fulfilled");
  } else {
    reject("Promise was rejected");
  }
}

//Promise 在任务时间未知的情况下特别有效
//promise is fulfilled with resolve，当promise 成功完成会立即调用then 方法
myPromise.then(function(result){
  console.log(result);
})
myPromise.then(result => {
  console.log(result);
})

//promise reject 的时候会调用catch 方法
myPromise.catch(error => {
  // do something with the error.
});

//chained
myPromise
	.then(result => {
	  console.log(result);
	})
	.catch(error => {
	  console.log(error);
	});