lowerTriangular_2.js

function isLowerTriangular(mat){
  let matSize = mat.length;
  for(let i = 0; i < matSize; i++){
    for(let j = i+1; j < matSize; j++){
      if(mat[i][j] != 0)
        return false;
    }
  }
  return true;
}

let mat1 = [[2, 0, 0, 0],
            [9, 5, 0, 0],
            [8, 2, 4, 0],
            [9, 4, 8, 3]];

let mat2 = [[1, 8, 3, 2],
            [0, 5, 7, 6],
            [7, 8, 4, 9],
            [8, 6, 3, 3]];

console.log(isLowerTriangular(mat1));
console.log(isLowerTriangular(mat2));

/*Output:
true
false
*/