https://leetcode.com/problems/valid-sudoku/

valid_sudoku.js

var isValidSudoku = function(board) {
    const rows = new Array(9).fill(0).map(() => new Set());
    const cols = new Array(9).fill(0).map(() => new Set());
    const boxes = new Array(9).fill(0).map(() => new Set());

    for (let i = 0; i < 9; i++) {
        for (let j = 0; j < 9; j++) {
            const num = board[i][j];
            if (num === '.') continue;

            const boxIndex = Math.floor(i / 3) * 3 + Math.floor(j / 3);

            if (rows[i].has(num) || cols[j].has(num) || boxes[boxIndex].has(num)) return false;

            rows[i].add(num);
            cols[j].add(num);
            boxes[boxIndex].add(num);
        }
    }

    return true;
};

the execution time of this solution dropped to a third just by doing loop unrolling and inlining variables.

(from ~13ms to ~4ms average)

https://medium.com/@kallel.front/desafios-do-leetcode-e-a-busca-pela-performance-19c7c3fa1d6a

edit: new version using a code similar to the original one but using sets to hold the state, ended up using a couple more MBs of memory than the unrolled one but keeps the legibility of the original with a ~4ms average execution time