Problems generating a variable in google TAG manager

I’m trying to generate a custom JavasCript varialbe in Google TAG Manager but I don’t know why its not working.

If I INSPECT the web and look for the variable it works fine. .

dataLayer[index]["gtm.element"][0].value

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But when I write the code in Custon Javascript variable in google tag managet it don’t return anything

function (){
   for (var i = 0; i < window.dataLayer.length; i++) {
      if (window.dataLayer[i].event === "gtm.formSubmit") {
        index = i;
      }
   }
     //return dataLayer[index];
     return dataLayer[index]["gtm.element"][0].value;
}

If I run the first option of return (return dataLayer[index];) it returns the index correctly

I need to know what I am doing wrong