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This article introduces the relevant knowledge of "how to apply the JavaScript callback function". Many people will encounter such a dilemma in the operation of actual cases, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Callback function

A callback function is a function that tells us what to do when we start an asynchronous task: what are you going to do when you finish the task? In this way, the main thread hardly cares about the state of the asynchronous task, and he will finish what he has started.

Case study

Function print () {

Document.getElementById ("demo") [xss_clean] = "RUNOOB!"

}

SetTimeout (print, 3000)

SetTimeout in this program is a process that takes a long time (3 seconds). Its first argument is a callback function, and the second argument is milliseconds. After this function is executed, a child thread is generated, which waits for 3 seconds, and then executes the callback function "print" to output "RUNOOB!" on the command line.

Of course, the JavaScript syntax is very friendly, we don't have to define a function print separately, we often write the above program as:

Case study

SetTimeout (function () {

Document.getElementById ("demo") [xss_clean] = "RUNOOB!"

}, 3000)

Note: since setTimeout waits in the child thread for 3 seconds, the main thread does not stop after the setTimeout function executes, so:

Case study

SetTimeout (function () {

Document.getElementById ("demo1") [xss_clean] = "RUNOOB-1!"

}, 3000)

Document.getElementById ("demo2") [xss_clean] = "RUNOOB-2!"

Console.log ("2")

The execution result of this program is:

RUNOOB-1!

RUNOOB-2!

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