app calculator with pure JavaScript

Hello everyone, My second project is coming!

I haven't finished the project yet, but I submitted it because I think I have a lot of doubts to ask you.

The functionality works fine, But i encounter some style features problems.

And I will continue to polish my existing code afterwards.

The following are the questions:

• How to make a plan to build a project like this and What should i think first ?

• What should I consider to make my project as perfect as possible？

BTW（I learned to use a markdown skill this time, but i don't know how to input a emoji, which i think could express our emotion much better.）