How to Read a File Line by Line using Node.js ?

The ability to Read a File Line by Line using Node allows us to read large files without entirely storing them in memory. It is useful in saving resources and improving the efficiency of the application. It allows us to look for the information that is required and once the relevant information is found, we can stop the search process and can prevent unwanted memory usage.

Reading a file line by line is a common task in Node.js applications, especially when dealing with large text files or log files. This article provides a comprehensive guide on how to read a file line by line using different methods and libraries available in Node.js.

Readline is a native module of Node.js, it was developed specifically for reading the content line by line from any readable stream. It can be used to read data from the command line. Since the module is the native module of Node.js, it doesn’t require any installation and can be imported as –

const readline = require('readline');

Since the readline module works only with Readable streams, we need to first create a readable stream using the fs module.

const file = readline.createInterface({
    input: fs.createReadStream('source_to_file'),
    output: process.stdout,
    terminal: false
});

Now, listen for the line event on the file object. The event will trigger whenever a new line is read from the stream:

file.on('line', (line) => {
    console.log(line);
});

Example: Implementation to read a file using readline module.

Output:

The line-reader module is an open-source module for reading file line by line in Node.js. It is not the native module, so you need to install it using npm(Node Package Manager) using the command:

npm install line-reader --save

The line-reader module provides eachLine() method which reads the file line by line. It had got a callback function which got two arguments: the line content and a boolean value that stores, whether the line read, was the last line of the file.

const lineReader = require('line-reader');

lineReader.eachLine('source-to-file', (line, last) => {
    console.log(line);
});

Example: Implementation to read a file using line reader module.

Output:

• Error Handling: Always handle errors appropriately, especially when dealing with file I/O operations.
• Memory Management: For large files, consider using streaming or buffering techniques to avoid loading the entire file into memory at once.
• Code Readability: Write clear and understandable code with appropriate comments to improve readability and maintainability.

Reading a file line by line in Node.js is a straightforward process, thanks to the built-in readline module and external libraries like readline-promise. By following the steps outlined in this guide, you can efficiently read and process large text files in your Node.js applications, ensuring optimal performance and scalability. Whether you prefer using core modules or external libraries, Node.js provides flexible options to meet your specific requirements for file handling tasks.