Manual:$wgAjaxSearch

Details
When true, this enables auto-suggestion for the search bar, using asynchronous javascript XML requests.

This also requires   to be true.

Setting   causes wfSajaxSearch to be added to  

  is used to deliver the content. The script defines where on the page, and when the ajax search results will be delivered.

This setting has been removed in favor of  , introduced in 1.13.0.