Why new Array(4).join("ha") yields "hahahaha" and not "undefinedhaundefinedha .."

Question

Why new Array(4).join("ha") yields "hahaha" and not "undefinedhaundefinedha .." ?

var arr = new Array(4);
alert( arr[0] ); //  produces `undefined`

Answer 1

Elements of the array that are undefined or null are converted to the empty string. It's right there in the documentation.

If an element is undefined or null, it is converted to the empty string.