I want to know if i write a Google Chrome extension, will anyone be able to use my code?
It looks like people will be able to see your code. If they can see your code, they can potentially use it.
C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\Google\Chrome\User Data\Default\Extensions
~/Library/Application Support/Google/Chrome/Default/Extensions/
or ~/.config/google-chrome/Default/Extensions/
The source code is available for inspection/debugging/experimenting & whatever people might want to do. You may try to use a JavaScript obfuscator.