What does callback mean in nodejs

In nodejs, a callback is a function passed as a parameter to another function, and is usually called after the first function is completed; it is an asynchronous equivalent function, and all APIs of Node support it. Callback is such a way.

The operating environment of this article: Windows 10 system, nodejs version 12.19.0, Dell G3 computer.

What does callback mean in nodejs

Callback is an asynchronous equivalent function. The callback function is called when a specific task is completed. Node makes extensive use of callbacks. All Node APIs support callbacks.

For example, a function reading a file might start reading the file and allow the next instruction to be executed to immediately return control to the execution environment. Once the file I/O is complete, it calls the callback function, passing the callback function the contents of the file as a parameter. Therefore, there is no blocking or waiting for file I/O. This makes Node.js highly scalable as it can handle a large number of requests without waiting for any function to return a result.

Blocking code example

Create a file named input.txt with the following text

Yiibai Point is giving self learning content
to teach the world in simple and easy way!!!!!

Create a js file named main. js contains the following code:

var fs = require("fs");
var data = fs.readFileSync('input.txt');
console.log(data.toString());
console.log("Program Ended");

Now run main.js to see the result:

$ node main.js

Verification output

Yiibai Point is giving self learning content
to teach the world in simple and easy way!!!!!
Program Ended

Non-blocking code example

Create a file named input.txt with the following text

Yiibai Point is giving self learning content
to teach the world in simple and easy way!!!!!

Update the main.js file with the following code:

var fs = require("fs");
fs.readFile('input.txt', function (err, data) {
    if (err) return console.error(err);
    console.log(data.toString());
});
console.log("Program Ended");

Now run main.js to see what you see Result:

$ node test.js

Verification output

Program Ended
Yiibai Point is giving self learning content
to teach the world in simple and easy way!!!!!

These two examples illustrate the concepts of blocking and non-blocking calls. The first example illustrates that the program blocks until it reads the file and then only advances to where the program ends. In the second example, the program does not wait for the file to be read, but it just prints "Program Ended" and in the meantime the program continues reading the file without blocking.

Therefore, blocking programs execute in sequence, which makes it easier to implement logic from a programming point of view, but non-blocking programs do not execute in sequence. In this case, a program needs to use any data for processing, it You should keep using the same blocks so that they are executed sequentially.

Recommended learning: "nodejs video tutorial"

The above is the detailed content of What does callback mean in nodejs. For more information, please follow other related articles on the PHP Chinese website!