Node.js Filesystem I/O Reading a file into a Buffer using streams


While reading content from a file is already asynchronous using the fs.readFile() method, sometimes we want to get the data in a Stream versus in a simple callback. This allows us to pipe this data to other locations or to process it as it comes in versus all at once at the end.

const fs = require('fs');

// Store file data chunks in this array
let chunks = [];
// We can use this variable to store the final data
let fileBuffer;

// Read file into stream.Readable
let fileStream = fs.createReadStream('text.txt');

// An error occurred with the stream
fileStream.once('error', (err) => {
    // Be sure to handle this properly!

// File is done being read
fileStream.once('end', () => {
    // create the final data Buffer from data chunks;
    fileBuffer = Buffer.concat(chunks);
    // Of course, you can do anything else you need to here, like emit an event!

// Data is flushed from fileStream in chunks,
// this callback will be executed for each chunk
fileStream.on('data', (chunk) => {
    chunks.push(chunk); // push data chunk to array

    // We can perform actions on the partial data we have so far!