What is promise? What are the benefits of promise

1、 What is? Promise

promise Is the solution to asynchrony , It's essentially a constructor , You can instantiate an object with it . The object has resolve、reject、all, Prototype has then、catch Method .promise Objects have three states ：pending（ Have in hand ）、fulfilled（ success ）、rejected（ Failure ）

Promise The object has two characteristics ：

（1） The state of the object is not affected by the outside world .Promise The object represents an asynchronous operation , Only the result of asynchronous operation can determine Promise Object state , No other operation will change this state . This can be compared with Promise Its name is understood together ,Promise yes “ promise ” It means , Says nothing else can be changed .

（2） Once the status changes , Will not change again . And its state changes only by （pending->fulfilled、

pending->rejected） And as long as one of these two situations occurs , The state is fixed .

2、Promise The advantages of ：

（1） Support chain calls （ Asynchronous operations can be displayed synchronously ） Avoid going back to hell

What is callback hell ？

Multiple callback functions are nested layer by layer , The result returned asynchronously by the outer callback function is the execution condition of the inner callback function

The drawback of callback hell ？

1、 Readability 、 Poor maintainability

2、 Not convenient for exception handling

What is a callback function ？

The callback function is abbreviated as “ Callback ”, It can be understood as “ Go back and call the function ”. It is taken as a parameter , Pass it to another function , After that function is executed , Then execute the parameter passed in .

Why should there be a callback function ？

Because you don't know when the sub thread will finish executing , At this time, you need a callback function , Notify the main thread after it finishes executing .

（2） Specifying callback functions is more flexible ： Previously, we had to specify a callback before defining an asynchronous task , Now you can specify the callback after the asynchronous task is executed .

Promise The shortcomings of ？

1、 Cannot cancel it , Once established, it will be executed immediately , There is no way to cancel halfway

2、 If you don't set a callback function for it , Internal thrown errors cannot be displayed

3、 When in pending In state , It's impossible to know where the current progress is （ Is it just beginning or just ending ）

（ Because I am right Promise I haven't really understood , So record ）