When you're struggling with a programming problem you can:

- Look at other solutions to the problem to see how they did it

- Change the problem. Start with something even simpler than a calculator, then come back to it.

- Simplify the problem. Do you need a UI right away? Can you write a program that takes three hardcoded inputs (operator, operand1, operand2) and uses branching (if, switch) to match the operator, perform the operation on the operands and print the result? If you do that, you can then try to add a UI using a dropdown to select the operation and two number inputs. Then you could move to a keypad (I'll leave you to figure out how adding a keypad would change things).

E.g. simple calculator I cooked up just now:

// Hardcode the commented values first. 
// Later, wire up the DOM lookups with getElementById.
// I've shown both, using made up element IDs.

function doOp(op, operand1, operand2) {
  switch (op) {
    case 'add': return operand1 + operand2;
    case 'mul': return operand1 * operand2;
    case 'sub': return operand1 - operand2;
    case 'div': return operand1 / operand2;
  }
}

document.getElementById('btn-calc').onclick = function () {
  const op = document.getElementById('input-op').value; // 'add', or 'mul', 'sub', 'div'.
  const operand1 = document.getElementById('input-operand1').value; // 1, 2, 3 etc.
  const operand2 = document.getElementById('input-operand2').value; // 1, 2, 3 etc.

  // Calculate
  const result = doOp(op, operand1, operand2);

  // Show result
  document.getElementById('input-result').value = result; // alert(result);
};

Note: Code illustrative only.