title: Uncaught TypeError: Cannot set property 'value' of undefined at HTMLButtonElement.buttonClick
body: <p>I have a problem with the calculator I am trying to make working.
  After I click a button, the console shows such a message:</p>
  <p>&quot;Uncaught TypeError: Cannot set property 'value' of undefined at HTMLButtonElement.buttonClick&quot;.</p>
  <p>Here is a link to repository:</p>
  <p><a href="https://github.com/PRMK01/Calculator" rel="nofollow noreferrer">https://github.com/PRMK01/Calculator</a></p>
  <p>In this calculator, the bottom input will eventually be hidden and is used to write down and evaluate equations. I wanted the upper input to work similarly to Windows Calculator input.</p>
  <p>P.S.: I'm very green, thank you for your patience in advance!</p>