Creating multi dimensional array with map method: javascript

Question

Here is how I tried creating a 2D array:

var data = new Array(10); // an array of length 10 all elements undefined

var twoDArray = data.map(function (d){
    return new Array(20);    // expected to be 2d array, but 10 elements all undefined
});

But now if I do the following I get results as expected:

var data = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
var twoDArray = data.map(function (d){
    return new Array(20);
});

Answer 1

That's because .map won't iterate over the values because they are all undefined.

You could use:

Array.apply(null, Array(10)).map(function() {
   return new Array(20);
});

Answer 2

From MDN:

map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes that are undefined, those which have been deleted or which have never been assigned values.

Thus you can't use map with an array full of undefined.