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Error-First Callback in Node.js

Node Callback Concept

Last Updated : 08 Jan, 2025

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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!!!
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JavaScript

// 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!!!
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JavaScript

// 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:

Error-First Callback in Node.js

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Article Tags :

Node.js
Web Technologies
Node.js-Basics

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