main.js

fs = require("fs");

const N = 2020;
const FILENAME = "input.txt";
const ENCODING = "utf8";

fs.readFile(FILENAME, ENCODING, (_, data) =>
  main(data.split("\n").map((line) => parseInt(line, 10)))
);

const main = (numbers) =>
  console.log(
    ((numbers, b) => (numbers.length ? numbers[0] : b))(
      numbers.flatMap((x) =>
        numbers.flatMap((y) => (x + y === N ? [x * y] : []))
      )
    )
  );