Read lines from filename passed as argument or from stdin in Node.js

line-stream.mjs

import fs from "node:fs/promises";
import readline from "node:readline/promises";
const { error: debug } = console;

/**
 * Use as async iterator: `for await (const line of lineStream())`
 * Works for calling with filename argument or piping stdin
 *   $ node script.mjs filename.txt
 *   $ node script.mjs < cat filename.txt
 *   $ cat filename.txt | node script.mjs
 */
export async function* lineStream(
    filepath = process.argv.slice(2).shift(),
    stream = process.stdin,
) {
    if (filepath) {
        debug("reading:", filepath);
        try {
            let data = await fs.readFile(filepath, "utf8");
            for (let line of data.split(/\r\n|\n/)) {
                if (ignore(line)) continue;
                yield line;
            }
        } catch {
            debug("failed to read file", filepath);
            return;
        }
    } else {
        debug("reading: stdin");
        const lines = readline.createInterface({ input: stream });
        for await (let line of lines) {
            if (ignore(line)) continue;
            yield line;
        }
    }
}

function ignore(line) {
    // Supports single-line comments
    if (line.startsWith("#")) return true;
    // Ignores white-space
    if (line.trim() === "") return true;
    return false;
}