The nethod -indexOf in js

hi everyone
I don't understand the method- IndexOf .
I know that is is concern with a str var but i do not understand what does it do.

this is my code:

<script type="text/javascript">var str="Hello world!";
document.write(str.indexOf("Hello") + "<br />");
document.write(str.indexOf("World") + "<br />");
document.write(str.indexOf("world"));
</script>

and this is the ouput

0
-1
6

why is it like that? i need a good explanation


thanx
pf

 

The indexOf() method returns the position of the first occurrence of a specified string value in a string.

http://www.w3schools.com/jsref/jsref_indexOf.asp

What do you not get?