JS array matrix implementation

array-matrix.js

// ES5
Array.matrix = function(numRows, numCols, initialValue) {
  var matrix = [];
  for (var i = 0; i < numRows; ++i) {
    var columns = [];
    for (var j = 0; j < numCols; ++j) {
      columns[j] = initialValue;
    }
    matrix[i] = columns;
  } 
  return matrix;
};

// ES6
Array.matrix = (numRows, numCols, initialValue) => {
  let matrix = [];
  for (let i = 0; i < numRows; ++i) {
    let columns = [];
    for (let j = 0; j < numCols; ++j) {
      columns[j] = initialValue;
    }
    matrix[i] = columns;
  }
  return matrix;
};