You're adding an empty string between the words, if the answer is "camel Casing", you are returning "camelCasing". But more importantly, this only solves the case of single word inputs or the input "camelCasing". You need to solve the general case so that it also handles "fooBar", "thisWordHere", and whatever else the user enters.

I timed out as well with this same solution. How did this one pass I wonder?

Perfectly servicable and efficient. Wouldn't best practices encourage adding a return line? Obviously not necessary for such simple code.