asynchronous programming

异步（Asynchronous, async）是与同步（Synchronous, sync）相对的概念。

💥 异步编程 (opens new window)

异步编程

• callback
• 事件发布/订阅
• Promises/A+ (opens new window)
• async/await
• async和await:让异步编程更简单 (opens new window)
• generator

Promises/A+

Promises/A+ (opens new window)，An open standard for sound.

1. 概念

一个promise必须是三种状态之一：pending、fulfilled 或 rejected

1.1 Promise状态

• When pending, a promise:

  may transition to either the fulfilled or rejected state.

• When fulfilled, a promise:

  • must not transition to any other state.

  • must have a value, which must not change.

• When rejected, a promise:

  • must not transition to any other state.
  • must have a reason, which must not change.

1.2 then方法

一个Promise必须提供一个 then 方法去访问当前的最终值或reason，Promise的then方法接收两个参数：

promise.then(onFulfilled, onRejected)

async/await

结论

• 执行 async 函数，返回的都是 Promise 对象
• Promise.then 成功的情况，对应 await
• Promise.catch 异常的情况，对应 try...catch

https://www.bilibili.com/video/BV1mb4y1D7hH