Having a problem translating part of a script which uses C# delegates into JS

I'm following this tutorial series on procedural terrain generation. For the tutorial, it uses C#, but I'm translating it to JS since that's the coding language I am most fluent with.

I've gotten through a lot of the tutorial series, but now I'm having a problem translating this specific part of the script: (you can tell I already partially translated it)

 public function RequestMapData(callback:Action.<MapData>) {
     var threadStart:ThreadStart = delegate { 
         MapDataThread(callback); 
     };
 }

The part I can't get translated is "var threadStart:ThreadStart = delegate { ... };". I read up on this and apparently you can't use delegates in JS, so I have no idea what to do here. Is there some JS equivalent of delegates?

Any help translating this chunk of code would really be appreciated, thanks so much.