javascript - Why are this two empty arrays of arrays being filled differently?

Question

I have this code. I even put console.logs to debug it. But I really don't understand why are they behaving in a different way. Why do the result1 array fill all the internal arrays with the last array values?

var myMatrix = [
  [0, 1, 2, 3],
  [4, 5, 6, 7],
  [8, 9, 10, 11],
  [12, 13, 14, 15]
];

function rotateMatrixClockwise(matrix) {
  var long = matrix.length;
  var result1 = new Array(long).fill([]);
  var result2 = [ [], [], [], [] ];
  console.log("Empty result1:");
  console.log(result1);
  console.log("Empty result2:");
  console.log(result2);
  for (var row = 0; row < long; row++) {
    for (var col = 0; col < long; col++) {
      result1[row][col] = matrix[long - col - 1][row];
      result2[row][col] = matrix[long - col - 1][row];
    }
  }
  console.log("Result1:");
  console.log(result1);
  console.log("Result2:");
  console.log(result2);
}

rotateMatrixClockwise(myMatrix);

Answer 1

You are filling the array with the same object reference of the array in

var result1 = new Array(long).fill([]);
//                                 ^^ same array for all elements

Any assignment later takes place in the same array.

In the working version,

var result2 = [[], [], [], []];
//             ^^  ^^  ^^  ^^  four different arrays

you takes four different arrays, which have no reference to the same array.

The reference for Array.fill states the static behavior, and while Javascript, objects are handed over by reference.

The fill() method fills all the elements of an array from a start index to an end index with a static value.

var myMatrix = [
  [0, 1, 2, 3],
  [4, 5, 6, 7],
  [8, 9, 10, 11],
  [12, 13, 14, 15]
];

function rotateMatrixClockwise(matrix) {
  var array = [];
  var long = matrix.length;
  var result1 = new Array(long).fill(array);
  var result2 = [[], [], [], []];
  console.log("Empty result1:");
  console.log(result1);
  console.log("Empty result2:");
  console.log(result2);
  for (var row = 0; row < long; row++) {
    for (var col = 0; col < long; col++) {
      result1[row][col] = matrix[long - col - 1][row];
      result2[row][col] = matrix[long - col - 1][row];
    }
  }
  console.log("Result1:");
  console.log(result1);
  console.log("Result2:");
  console.log(result2);
  console.log('array', array);
}

rotateMatrixClockwise(myMatrix)