Read a file and return an array of lines

Parsing text files is one of the most common tasks in programming. Luckily, Node.js provides simple ways to read files synchronously or asynchronously. Connecting the dots to convert the result into an array of lines is straightforward.

Read a text file

Reading text files can be accomplished either using fs.readFileSync() or fs.readFile(). The former is synchronous and blocks the event loop, while the latter is asynchronous and non-blocking, returning a Promise.

Both methods return a Buffer object, which can be converted to a string using Buffer.prototype.toString(). Alternatively, you can specify the encoding as the second argument to return a string directly.

import { readFileSync } from 'fs';
import { readFile } from 'fs/promises';

readFileSync('test.txt').toString('UTF8');
// 'line1\nline2\nline3\n'
readFile('test.txt', 'UTF8');
// 'line1\nline2\nline3\n'

readFile('test.txt').then(data => data.toString());
// Promise {'line1\nline2\nline3\n'}
readFile('test.txt', 'UTF8');
// Promise {'line1\nline2\nline3\n'}

Read file and convert to an array of lines

To convert the contents of a file to an array of lines, you can use String.prototype.split() to split the string by the newline character (\n).

const fileContents = 'line1\nline2\nline3\n';

fileContents.split('\n');
// ['line1', 'line2', 'line3']

Read file lines asynchronously

Putting the pieces together, you can read a file asynchronously and return an array of lines. Make sure to handle the resulting Promise accordingly, using await or Promise.prototype.then().

import { readFile } from 'fs/promises';

const readFileLines = path =>
  readFile(path, 'UTF8').then(data => data.split('\n'));

readFileLines('test.txt').then(lines => console.log(lines));
// ['line1', 'line2', 'line3']

Read file lines synchronously

If you prefer synchronous code, you can read the file synchronously and convert it to an array of lines. The resulting array can be used directly.

import { readFileSync } from 'fs';

const readFileLinesSync = path =>
  readFileSync(path, 'UTF8').toString().split('\n');

const lines = readFileLinesSync('test.txt');
console.log(lines);
// ['line1', 'line2', 'line3']