JS callback function--easy to understand there are instances

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Beginner JS, the callback function is very dizzy, now back to summarize what is the callback function.

Let's take a look at the definition of the callback in English: A callback is a function, a passed as an argument to another function and is executed after its parent function has completed.

In a literal sense, a callback function is a parameter that passes the function as a parameter to another function, and then executes the function passed in when the function is executed. This process is called a callback.

In fact, it is very well understood, right, callback, callback, is called back to the meaning. The main function is done in advance, and then the function passed in is called back. But I've seen a lot of blogs before, and they always explain the callback function in a foggy, very advanced way.

Take an example: when you send your girlfriend home at the end of the date, when you leave, you'll say, "I'm worried about you when I get home and give me a message." "No, then your girlfriend really sent you a message when she came home. Boy, you have a chance. In fact, this is the process of a callback. You left a parametric function (asking your girlfriend to give you A clockwork message) to your girlfriend, and then your girlfriend goes home, and the action to go home is the main function. She had to get home first. After the main function is finished, the function passed in is executed, and then you receive a message.

Now basically understand the meaning of the callback function. It doesn't matter if you don't understand, we talk in code.

 //  Define main function, callback function as parameter  function        A (callback) {callback ();      Console.log ( ' I am the main function '  //  define callback function   function   B () {setTimeout ( "Console.log (' I am the callback function ')", //  mimic time-consuming operation  }  //  call the main function, pass function B in  a (B);  //  output result   I am the main function I am the callback function   

In the above code, we define the main function and the callback function, then call the main function, and pass the callback function in.

When defining the main function, we let the code execute the callback () callback function first, but the output is the content of the back output callback function. This means that the main function does not have to wait for the callback function to complete, and then executes its own code. So the general callback function is used on the time-consuming operation. such as Ajax requests, such as processing files.

JS callback function--easy to understand there are instances