Javascript – Uncaught TypeError: X.setValue is not a function… but writing out the value (string) works fine

The line: sseTopicString.setValue(sseValueNumber); is throwing a Type error:
Uncaught TypeError: sseTopicString.setValue is not a function.

If I write out the value (string) of sseTopicString before the . everything works as expected.

What am I not seeing? I tried async / await as well, not expecting much of it.
I just don’t see why using the variable would make such a difference. Pls help.

All the conversions to strings and numbers are experimental – none of them made a difference.

  let sse = new EventSource("http://localhost:3000/sse-stream");
  // sse.onmessage = console.log


    sse.onmessage = function (event) {
    let jdata = JSON.parse(event.data);
    console.log(jdata);

    let rawSseTopic = jdata.topicname;
    let sseTopic = removeSpecialCharacters(rawSseTopic);
    let sseTopicString = sseTopic.toString()
    console.log(sseTopicString);
    
    let sseValue = jdata.message;
    let sseValueNumber = Number(sseValue);
    console.log(sseValueNumber);
  
    sseTopicString.setValue(sseValueNumber);

// sseTopicString has the assigned string value "Gauge12"
// changing the line to: Gauge12.setValue(sseValueNumber); works (Gauge changes to value)
// sseTopicString.setValue(sseValueNumber); throws the error
// how can I use te variable sseTopicString to achieve the desired effect?

  };

  function removeSpecialCharacters(string) {
    let cleanString = string.replace(/,|.|-|_||s|//g, "");
    return cleanString;
  }