json2lines/ts/index.ts

Takes a JSON file input and a JSONStream filter and produces a JSONL line-seperated file.

#!/usr/bin/env node
import es from "event-stream";
import fs from "fs";
import { parse } from "JSONStream";
import { Readable, Writable } from "stream";
import { pipeline } from "stream/promises";
import options from "./options.js";

/**
 * Processes a single Object from the upstream JSONStream, stringifying it.
 * @param this
 * @param data A single JSON object
 * @param callback
 */
function processLine(
    this: any,
    data: any,
    callback: (err: null, json: any) => void
) {
    callback(null, JSON.stringify(data));
}

/**
 * Takes a Readable that contains a valid JSON file, and outputs JSON Lines to
 * the Writable.
 * @param input Readable stream with JSON contents
 * @param output Writable stream that will recieve JSON Lines
 * @param filter The JSONStream filter to use
 */
export default async function read(
    input: Readable,
    output: Writable,
    filter: string
) {
    await pipeline(input, parse(filter), es.map(processLine), output);
}

/**
 * Gets the input stream source
 * @returns Either a fail stream
 */
function getInputStream() {
    if (options.input === "-") {
        return process.stdin;
    } else {
        return fs.createReadStream(options.input);
    }
}

/**
 * Gets the output stream source
 * @returns
 */
function getOutputStream() {
    if (options.output === "-") {
        return process.stdout;
    } else {
        return fs.createWriteStream(options.output);
    }
}

// Bootstrap code for CLI usage
const { input, output, filter } = options;

if (input && output && filter) {
    read(getInputStream(), getOutputStream(), filter);
}