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Node Callback Concept
Last Updated :
12 Jul, 2025
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
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!!!
Learn NodeJS with GeeksforGeeks
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:
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