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This article shows you why js is called back. The content is concise and easy to understand, which will definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

In Javascript, a function is a first-class object, which means that a function can be used according to the first-class management like an object. Since functions are actually objects: they can be "stored" in variables, passed as function arguments, created in functions, and returned from functions. Because functions are first-class objects, we can use callback functions in Javascript. Let's take a look at the callback.

To put it simply: a callback is a function that is called after the execution of another function is completed.

To put it a little more complicated: in JavaScript, functions are also objects. Therefore, a function can be passed in as an argument, or it can be returned by other functions. Such functions are called higher-order functions. The function passed in as an argument is called a callback function.

Let's talk about why we need a callback.

There is a very important reason-JavaScript is an event-driven language. This means that JavaScript does not stop running because it is waiting for a response, but continues to execute while listening for other events.

Let's look at a basic example:

Function first () {console.log (1);} function second () {console.log (2);} first (); second ()

As you might expect, the first function is executed first, and then the second is executed-- the console outputs the following:

/ / 1// 2

But what if the function first contains some code that cannot be executed immediately?

For example, an API request where we have to send a request and wait for a response? To simulate this, we will use setTimeout, which is a JavaScript function that calls a function after a period of time. We delay the function by 500ms to simulate an API request, and the new code looks like this:

Function first () {/ / Simulation Code delay setTimeout (function () {console.log (1);}, 500);} function second () {console.log (2);} first (); second ()

It's not important to understand how setTimeout () works now, but it's important that you see that we've moved console.log (1); to the inside of the 500-second delay function. So what happens when you call a function now?

First (); second (); / / 2 Universe / 1

Even if we call the first () function first, the output we record comes after the second () function.

This is not a problem that JavaScript does not execute functions in the order we want, but that JavaScript does not wait for a response from first () before continuing with second ().

So why show you this?

Because you can't call functions one by one and want them to be executed in the correct order.

A callback is a way to ensure that one piece of code is executed before another.

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