Sum of sub-matrices (Brute force method)

sumSubBruteForce.js

function sumSubmatrices(mat){
    // n and m represent dimensions of the matrix
    let n = mat.length;
    let m = mat[0].length;
  
    // declaring a (n*m) matrix
    let sumMat = [...new Array(n)].map(row => [...new Array(m)]);
  
    for(let i = 0; i < n; i++){
        for(let j = 0; j < m; j++){
            sumMat[i][j] = 0; // initializing the cell to 0
            for(let ii = 0; ii <= i; ii++){
                for(let jj = 0; jj <= j; jj++){
                    sumMat[i][j] += mat[ii][jj];
                }
            }
        }
    }
  
    return sumMat;
}