Why does this very simple calculator not work?

Question

https://code.sololearn.com/WNiInYvwF4wU/?ref=app

Answer 1

Corection:

<span class="equa" onclick="document.begin.screen.value = eval(document.begin.screen.value)">=</span>

Answer 2

Look at this example
https://code.sololearn.com/Wujjm0cdh4D1/?ref=app