How Can I Read a File Line by Line in Node.js?

Read a File One Line at a Time in Node.js

Node.js provides efficient mechanisms for processing large files one line at a time. This capability is essential for memory-intensive operations or when dealing with files that exceed server memory.

To read a file line by line in Node.js, you can utilize the following approaches:

Using the readline Core Module (Node.js v0.12 )

Node.js offers a readline core module for seamless file line iteration. Consider the following code:

const fs = require('fs');
const readline = require('readline');

async function processLineByLine() {
  const fileStream = fs.createReadStream('input.txt');

  const rl = readline.createInterface({
    input: fileStream,
    crlfDelay: Infinity
  });
  // Note: we use the crlfDelay option to recognize all instances of CR LF
  // ('\r\n') in input.txt as a single line break.

  for await (const line of rl) {
    // Each line in input.txt will be successively available here as `line`.
    console.log(`Line from file: ${line}`);
  }
}

processLineByLine();

Using the 'readline' Package

For Node.js versions prior to v0.12, the 'readline' package provides an alternative solution:

var lineReader = require('readline').createInterface({
  input: require('fs').createReadStream('file.in')
});

lineReader.on('line', function (line) {
  console.log('Line from file:', line);
});

lineReader.on('close', function () {
    console.log('all done, son');
});

In both approaches, the last line is read correctly even without a trailing newline character.

Additional Considerations

• Ensure that the file path provided in 'createReadStream()' is accurate.
• Handle errors appropriately using try-catch blocks or event listeners.
• The readline module's 'close' event is emitted when the end of the file is reached or an error occurs.

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