Mobile first responsive vanilla js, css-flex

This is a project build with pure js and CSS. This was a fun challenge and at the same time challenging. I have tried to implement all the possible functionality. Any suggestion would be appreciated.

• No consecutive operators are allowed.
  eg: +/-*
• operator gets replaced with a later one.
  eg: 1+- -> 1+ or 2-* -> 2*
• A operand can only have a single decimal.
  eg: 0.1 or 1.0 but not 0..1 or 1..0
• Infinity if divisor equals zero.
• Throw an error if the initial operand is '/' or '*'