Why doesn't an array variable have to be declared with `new Array`?

Question

While reading a book about JavaScript I stumbled across an example:

var names = new Array("Paul","Catherine","Steve");
var ages = new Array(31,29,34);
var concatArray;
concatArray = names.concat(ages);

My question is, why doesn't the variable concatArray need to be define as a new Array() in order to store the concatenated data for both arrays name and ages, but when I try to treat the concatArray as an array by adding another line of code "document.write(concatArray[0])", it works just like an array and shows me the data stored in the first element. I just wonder why I'm not declaring the concatArray as a new array, yet it still works as one.

Answer 1

You are declaring concatArray as a new array but the declaration is implicit. The concat function returns a new array which contains concatenated copies of the original two arrays. The type of concatArray is inferred from the return type of the concat function.