Node Callback Concept

A callback in Node is a non-blocking function that executes upon task completion, enabling asynchronous processing. It facilitates scalability by allowing Nodejs to handle multiple requests without waiting for operations to conclude, as exemplified in file I/O scenarios.

Explanation: The fs library is used for file-system operations. The readFileSync() function is synchronous, halting program execution until completion. This blocking behavior ensures that the program reads the file before progressing further.

Example 1: Code for reading a file synchronously (blocking code) in Nodejs. Create a text file inputfile1.txt with the following content:

Hello Programmer!!!
Learn NodeJS with GeeksforGeeks

// Write JavaScript code const fs = require("fs"); const filedata = fs.readFileSync('inputfile1.txt'); console.log(filedata.toString()); console.log("End of Program execution"); 

Output:

Explanation: The fs library is utilized for file-system operations. The asynchronous readFile() function allows the program to proceed immediately to the next instruction while the task runs in the background. A callback function is employed to execute upon the completion of the background task.

Example 2: Code for reading a file asynchronously (non-blocking code) in Nodejs. Create a text file inputfile1.txt with the following content.

Hello Programmer!!!
Learn NodeJS with GeeksforGeeks

// Write a JavaScript code const fs = require("fs"); fs.readFile('inputfile1.txt',  function (ferr, filedata) {  if (ferr) return console.error(ferr);  console.log(filedata.toString());  } ); console.log("End of Program execution"); 

Output: