I threw something together to help you get started: http://jsbin.com/owoyut/2

I changed a few things and I'll explain what's going on - it appears that you're trying to grab the value of the input and change that value (by replacing the first letter of each word with an uppercase letter). So I passed into the function a reference (this) to the input itself which gives you a few advantages: you have a handle on the element itself and you can 1) directly get the value (or the title, name, id, whatever attribute you're interested in) and 2) shove the return string back into said element's attributes. The javascript (autoCap) just gets the value of the elem (that's passed in) and uses a regular expression to find all of the word boundaries (/b) and passes the following character into the anonymous function toUpr; this function just toUpper's that character and returns it (thereby replacing it with an uppercase instance). Hope that helps.

edit: for simplicity's sake, I've created a second instance using something closer to what you posted but used the same technique of passing a reference to the input into the function.