Describe the Issue
Example JavaScript Calculator accepts invalid input.
Affected Page
https://javascript-calculator.freecodecamp.rocks/
Your code
Expected behavior
It should ignore invalid input.
Screenshots

System
Device: Laptop OS: Windows 18.04.5 Browser: Chrome
Additional context
https://www.freecodecamp.org/learn/front-end-development-libraries/front-end-development-libraries-projects/build-a-javascript-calculator