javascript - What is .map() doing in this situation?

Question

Using the Chrome Console, this is my input and output:

[0].map(Array);

[[0, 0, [0]]]; // output

What is happening here?

EDIT

The reason this makes me curious is because something like

[0].map(String);

Will return

["0"];

And not

["0", "0", "String"]

Answer 1

The .map() function is calling the Array() function with three arguments, the value of the array element which is 0, the index of that element, also 0, and a reference to the whole array.

So it's like doing this:

var a = [0];
var index = 0
Array(a[index], index, a);   // create array with three elements

The array returned by Array() then becomes the first element of the array that .map() creates, hence the extra level of nesting in your [[0, 0, [0]]] result.

EDIT regarding your edit: when you say [0].map(String); that results in String() being called with the same three arguments like String(a[index], index, a), but the String() function ignores all but the first argument, whereas Array() uses all supplied arguments.