line-by-line

Read or parse node.js streams line by line without loading the entire file to memory. Use async iterator in a for await loop, or object-mode stream transform.

Initially, line-by-line used readline package internally, but because of its shortcomings (inability to specify encoding), it now implements a custom line-reading algorithm.

Why not just use the built-in readline package?

Because it does not offer Transform stream, and because its async iterator does not propagate stream errors. So if you are parsing a file or network stream and an error happens, you better manually catch .on('error') and additionally use that to break from the for await iterator loop.

Additionally, readline is hard-coded to utf8 encoding, so you cannot use it with other encodings.

Installation

With npm do:

$ npm install @mangosteen/line-by-line

Usage (string array)

import fs from 'fs/promises';
import { splitStringLines } from '@mangosteen/line-by-line';

(async () => {
    const fileBuffer: Buffer = await fs.readFile('./shakespeare.txt');
    const text: string = fileBuffer.toString('utf8');
    const lines: string[] = splitStringLines(text);

    for (const line of lines) {
        console.log('Line:', line);
    }
})();

splitStringLines splits the text into lines array.

This may potentially consume a lot of memory, because at one point you need to hold both the entire input string and the entire output array of lines. Thus, we generally recommend using below functions instead.

Usage (string iterator)

import fs from 'fs/promises';
import { iterateStringLines } from '@mangosteen/line-by-line';

(async () => {
    const fileBuffer: Buffer = await fs.readFile('./shakespeare.txt');
    const text: string = fileBuffer.toString('utf8');
    const iterator: Iterable<string> = iterateStringLines(text);

    for (const line of iterator) {
        console.log('Line:', line);
    }
})();

iterateStringLines is a generator function that lazily yields lines one by one.

You still need to hold the entire input string in memory, but the output lines can be processed efficiently.

Usage (stream iterator)

import fs from 'fs';
import { iterateStreamLines } from '@mangosteen/line-by-line';

(async () => {
    const inputStream = fs.createReadStream('./shakespeare.txt');
    const iterator: AsyncIterable<string> = iterateStreamLines(inputStream, 'utf8');

    for await (const line of iterator) {
        console.log('Line:', line);
    }
})();

iterateStreamLines is async generator function that lazily yields lines one by one.

This is the most efficient method of reading lines. The input is a stream and can be processed on-demand. The output is generated on-demand as well, one line at a time.

When the stream iterator returned by iterateStreamLines is consumed (via for await), it will automatically close and destroy the input stream, and fully propagate input stream errors. You won't need to do anything more to clean up the input stream.

When you break, return or throw from within the for await loop, everything gets cleaned up automatically. Errors thrown by the stream work the same way.

You cannot reuse the same input stream for multiple for await loops or multiple lineByLine iterators, because everything gets cleaned up automatically.

Usage (transform stream)

import fs from 'fs';
import stream from 'stream';
import { promisify } from 'util';
import { createLineByLineStream } from '@mangosteen/line-by-line';

const pipeline = promisify(stream.pipeline);

(async () => {
    await pipeline(
        fs.createReadStream('./shakespeare.txt'),
        createLineByLineStream('utf8'),
        createSinkStream(),
    );
})();

function createSinkStream(): stream.Writable {
    return new stream.Writable({
        objectMode: true,
        highWaterMark: 0,
        write(line: string, _encoding, callback): void {
            console.log('Line:', line);
            callback();
        },
    });
}

The createLineByLineStream transform stream's Writable side expects a standard non-objectMode stream. The Readable side runs in an objectMode, where each object is a line string. You can specify an encoding to decode any Buffers the transform stream receives.