Can we build GWT app that does not have Captcha function but is still able to prevent spam?

I am building Gwt site for mobile devices. The screen size of the mobile devices is very small, so I think if i require users to enter Captcha when they register then it could be quite cumbersome.

I heard that we can use some Javascript technique to prevent the Spam without needing to use captcha.

GWT is all about Javascript, so I am not sure if GWT has any technology to prevent spam without using Captcha?

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