Adding following lines at the start of the .ascx file in a visual webpart enables Intellisense in Visual Studio while writing ECMA code.
<script type="text/javascript" src="/_layouts/MicrosoftAjax.js"></script>
<script type="text/javascript" src="/_layouts/SP.Debug.js"></script>
<script type="text/javascript" src="/_layouts/SP.Core.debug.js">
If reference to SP.Core.debug.js is not included, then we wont get IntelliSense on list objects. Following is the list of debug .js files present in _layouts folder and depending upon the objects you are working with, you may need to add a reference to any of them to get IntelliSense.
· SP.debug.js
· SP.Core.debug.js
· SP.Ribbon.debug.js
· SP.Runtime.debug.js
· JsGrid.debug.js
· JsGrid.Gantt.debug.js
Note: Remember to exclude these references while deploying the code to production server.