How to find arithmetic operator and assign it and its index to globally accessible variable?

I'm a web developer newbie and am trying to build a calculator in React as a practice project.

In order to do this, I need the user-selected arithmetic operations ("+", "/", "-" or "X") to be carried out when the user clicks "=".

I push all characters of the buttons that the user clicks to an array, including arithmetic operators.

So for example if the user clicks "3", then "+", then "6", the state array (called "numberList") is [3, "+", 6]. The problem I'm having is with identifying the index of the arithmetic operator. I have tried various combinations of loops, if statements, and array methods. This is my latest attempt:

      for (let i = 0; i < numberList.length; i++) {
        switch (numberList[i]) {
          case "+":
            operator = "+";
            operatorIndex = i;
            break;
          case "/":
            operator = "/";
            operatorIndex = i;
            break;
          case "-":
            operator = "-";
            operatorIndex = i;
            break;
          case "X":
            operator = "X";
            operatorIndex = i;
            break;
          default:
            return "ERR";
        }
      }
      //Find operators and their index in numberList
      console.log(numberList, operator, operatorIndex);

But when I run this the "ERR" default is returned even when there is a matching arithmetic operator in the numberList array.

Why doesn't the code continue after "operator" and "operatorIndex" have been assigned the values of the operator and its index?

Thanks, David