streams a non-binary file avoiding loading it all to memory

streamLines.mjs

import { createReadStream } from 'fs';

export function streamSplitBy(file, onMatch, splitBy='\n') {
    return new Promise((resolve) => {
        const stream = createReadStream(file);

        let ongoing = '';
        
        stream.on('data', buff => {
            const chunk = buff.toString();

            let chunkStartIndex = 0;
            let chunkEndIndex = 0;

            while (true) {
                chunkEndIndex = chunk.indexOf(splitBy, chunkStartIndex);
                if (chunkEndIndex !== -1) {
                    ongoing += chunk.substring(chunkStartIndex, chunkEndIndex);
                    onMatch(ongoing);
                    ongoing = '';
                    chunkStartIndex = chunkEndIndex + splitBy.length;
                } else {
                    ongoing += chunk.substring(chunkStartIndex);
                    break;
                }
            }
        });

        stream.on('end', () => {
            if (ongoing !== '') onMatch(ongoing);
            resolve();
        });
    });
}

export function streamLines(file, onMatch) {
    return streamSplitBy(file, onMatch, '\n');
}