JavaScript是单线程执行代码的,但是浏览器可以多线程
异步编程:
https://www.ruanyifeng.com/blog/2012/12/asynchronous%EF%BC%BFjavascript.html
Promise:
https://es6.ruanyifeng.com/#docs/promise
https://www.runoob.com/w3cnote/javascript-promise-object.html
Ajax: