This is probably roughly what you’re looking for:
if (screen.width >= 800) {
   var redirect = confirm("You are on the mobile site. Would you like to visit our full site instead?");
   if (redirect) {
       window.location.href="http://TEXTHIDDEN.com/"; 
   }
}
You should invest some time in learning some javascript basics, and use tools like JSLint to check your syntax in the future.
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solved Javascript Grammar Checking