Why does new String * 1 return 0 in Javascript?

Question

In JavaScript, why is:

new String * 1
<- 0

What exactly makes it return 0, and why is the following equal to NaN?

function Foo() { this.bar = 0; }
new Foo * 5;
<- NaN

Answer 1

If you use arithmetic operators like *, JavaScript will try to convert the type to a number. An empty string becomes 0.

If you have, for example:

new String("foo") * 1

You will notice it returns NaN because the conversion to a number could not be completed. That's what happens in your second situation.