[SalesForce] VS Code and custom Labels

This is a great question, and Custom Labels and the $Label global are the right place to start! Instead of evaluating Visualforce inside your JavaScript, you can do a little bit of JavaScript inside your Visualforce.

How do I get evaluated Visualforce variables into dumb static JavaScript?

1. Load your Custom Labels into the platform using the normal interface,

2. Create what I like to call a "JavaScript bridging component" that loads before your script, like so:

   <script>
       window.$Label = window.$Label || {};
       $Label.MyError = '{!JSENCODE($Label.MyError)}';
       $Label.MyPrompt = '{!JSENCODE($Label.MyPrompt)}';
       $Label.MyMessage = '{!JSENCODE($Label.MyMessage)}';
   </script>
   

3. Now in your javascript (in a static resource) use the variables just as if they were VF without {!}

   <script src="{!URLFOR($Resource.ScriptZip, '/my.js')}">
       //NOTE: this example code is for clarity,
       //really he lives in the static resource
       function errorHandler() {
           console.log($Label.MyError);
           alert($Label.MyMessage);
       }
   </script>
   

This is in the spirit of the platform: there is no need to interpret any variation on the token names and you can pluck the values straight out in your JavaScript without inventing anything :-)

Just tested, Test class can access custom label. No need to enable seeAllDate=true.