How To Create A Pop Up using JavaScript and a SharePoint Content Editor Web Part

We once had a situation where users would forget to check in files after checking them out from a document library. So someone came with the idea of creating an "annoying" pop-up to remind the users to check the files back in once they were done using them. This is the result of that idea.

1. Edit the page

2. Add a Content Editor Web Part

3. Edit the Contend Editor web part properties, remove the chrome, and make any other changes as desired, rename the web part to something more meaningful than 'Content Editor,' (for example, 'Pop Up'), and then [Apply] and [OK]

4. Select Click here to add new comment at the Content Editor Web Part

5. Locate the HTML icon within the Markup section of the ribbon, select it, and then select Edit HTML Source

6. Type in or copy and paste the code below in the HTML Source web page dialog

    <script type='"text/javascript"'>
        <alert("Alert Message Here")
    </script>

7. Select [OK] and save the page (and publish if publishing is turned on for your site)

Note: Replace the Alert Message Here text with the appropriate message that you wish to  convey.