Try str = parseInt( str, 10 )
(note: the second parameter signifies the radix for parsing; in this case, your number will be parsed as a decimal)
That being said, however, the term "type" is used quite loosely in JavaScript.
Could you explain why are you concerned with variable's type at all?
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…