js 求解释注意除法取整，这里直接使用字符串处理

/**
 * @param {number} threshold
 * @param {number} rows
 * @param {number} cols
 * @return {number}
 */
var movingCount = function(k, rows, cols) {
    const dx = [-1, 0, 1, 0], dy = [0, 1, 0, -1];
    const vt = new Array(rows).fill(0).map(() => new Array(cols).fill(false));

    const dfs = (r, c) => {
        const t = (''+r+c).split('').reduce((sum, cur) => sum += +cur, 0);
        if (r < 0 || r >= rows || c < 0 || c >= cols || vt[r][c] || t > k) {
            return 0;
        }
        vt[r][c] = true;
        let ans = 1;
        for (let d = 0; d < 4; ++d) {
            let x = dx[d] + r, y = dy[d] + c;
            ans += dfs(x, y);
        }
        return ans;
    }

    return dfs(0, 0);
};