You can replace the require(‘fs’) on the first line with the input and run it in your browser console as well.

const input = require('fs').readFileSync('input-day2.txt', 'utf-8');

let idsPart1 = [];
let idsPart2 = [];

input.split(',').forEach(range => {
    const [start, end] = range.split('-').map(v => parseInt(v, 10));
    let cursor = start;

    while (cursor <= end) {
        const cursorString = cursor.toString(10);

        // part 1 check
        let halfLength = Math.floor(cursorString.length / 2);
        let left = cursorString.slice(0, halfLength);
        let right = cursorString.slice(halfLength, cursorString.length);
        if (left === right) {
            idsPart1.push(cursor);
        }

        // part 2 check
        let sequenceLength = 1;
        while (sequenceLength <= halfLength) {
            const sequence = cursorString.slice(0, sequenceLength);
            let builtString = sequence;
            while (builtString.length < cursorString.length) {
                builtString = `${builtString}${sequence}`;
            }
            if (builtString === cursorString) {
                idsPart2.push(cursor);
                break;
            }

            sequenceLength += 1;
        }

        cursor += 1;
    }
})

const answer1 = idsPart1.flat().reduce((acc, cur) => acc += cur, 0);
const answer2 = idsPart2.flat().reduce((acc, cur) => acc += cur, 0);

console.log(`Part 1 Answer: ${answer1}`);
console.log(`Part 2 Answer: ${answer2}`);