Array.apply(null, new Array(3)).map(function() {return 0;});

Array.apply(null, [undefined, undefined, undefined]).map(function() {return 0;});

var sparseArr = [3,,,4,1,,],
    denseArr = Array.apply(null, sparseArr).map(function(e) {
      return e === undefined ? 0 : e;
    });

// denseArr = [3,0,0,4,1,0]

[] // empty array
[,] // empty array
[,,] // [undefined x 1]
[,,,] // [undefined x 2]

Array.apply(null, [3,,4,1]) ≡ Array(3, undefined, 4, 1)
Array.apply(null, [3,4]) ≡ Array(3, 4)
Array.apply(null, [1]) ≡ Array(1)

new Array(1, 2, 3)
=> [1, 2, 3]

new Array(2)
=> [undefined, undefined]