jsonl-to-csv.js

import { readFile } from "node:fs/promises";

const jsonl = await readFile("/dev/stdin", "utf-8");

const lines = jsonl.split("\n").filter((s) => !!s.trim());
const objects = lines.map((l) => JSON.parse(l));

let keys = [];
for (const object of objects) {
  for (const key of Object.keys(object)) {
    if (!keys.includes(key)) keys.push(key);
  }
}

console.log(keys.join(","));

for (const object of objects) {
  console.log(
    keys
      .map(
        (key) =>
          `"${("" + object[key]).replaceAll('"', '""').replaceAll("\n", "\\n")}"`
      )
      .join(",")
  );
}