Run this in the Chrome console (Ctrl-Shift-J) on the input file.

aoc2020_day1_part1.js

const numbers    = document.body.textContent.trim().split('\n').map(Number);
const set        = new Set(numbers);
const goalSum    = 2020;
const difference = n => goalSum - n;
const subtrahend = numbers.find(n => set.has(difference(n)));
const minuend    = difference(subtrahend);
const product    = subtrahend * minuend;
console.log(product);