Sevenify Problem

sevenify.js

/* This is my implementation of the problem regarding replacing the element in the row and column of each instance of a 7 with 7s. I did this without allocating extra memory by just keeping track of the rows and columns that needed to be 'sevenified' and I did it in the end in one pass, therefore not having to rewrite 7s over and over again. My efficiency for sevenify's element reassignment, at the worst case, is O(m+n) or O((m+n) + (m*n)) if you include the cycles to create the locations hash. 

I would test this by checking that: 
  - clone works properly (I tested to check for deep copying)

  - test fillRow (and fillColumn) work on a single row (and column) with no 7s present
  - test fillRow (and fillColumn) work on a single row (and column) with one 7 present
  - test fillRow (and fillColumn) work on a single row (and column) with all elements set to 7

  zahid mahir
*/

var cloneMatrix = function (matrixx) {
  var clonedMatrix = [];
  for (var i = 0; i < matrixx.length; i++) {
    clonedMatrix[i] = matrixx[i].slice(0);
 }
  return clonedMatrix;
};

var fillRow = function (arr, row) {
  for (var i = 0; i < arr[0].length; i++) {
    arr[row][i] = 7;
  }
};

var fillColumn = function (arr, column) {
  for (var i = 0; i < arr.length; i++) {
    arr[i][column] = 7;
  }
};

var matrixToString = function (matrixx) {
  var dump = '';
  matrixx.forEach(function (col) {
    col.forEach(function (element) {
      dump += element + ' ';
    });
    dump += '\n';
  });
  return dump;
};

var sevenify = function (matrixx) {
  var clonedMatrix = cloneMatrix(matrixx);
  var locations = {
    rows : {},
    columns : {}
  };
  for (var i = 0; i < matrixx.length; i++) {
    for (var j = 0; j < matrixx[0].length; j++) {
      if(matrixx[i][j] == 7) {
        locations.rows[i] = true;
        locations.columns[j] = true;
      }
    }
  }
  for (var row in locations.rows) {
    fillRow(clonedMatrix, row);
  }
  for (var column in locations.columns) {
    fillColumn(clonedMatrix, column);
  }
  return clonedMatrix;
};

// assume standard n x m matrix
var testMatrix = [
  [1, 8, 1, 4, -1],
  [2, 3, 8, 2, 1],
  [8, 3, 1, 7, 7]
];

console.log('original');
console.log(matrixToString(testMatrix));

var sevenifiedMatrix = sevenify(testMatrix);

console.log('sevenified');
console.log(matrixToString(sevenifiedMatrix));