Perform refactoring of JavaScript code

You need a JavaScript specialist. You need to refactor JavaScript code. Our company is engaged in the manufacture of internal stairs for private houses for individual projects. We have our own computer-aided staircase design system (Sapr), made in the form of a web application on javascript based on the 3D graphics library three.js. Based on the input data entered by the user, the browser builds a 3D model of the ladder, creates working drawings and specifications for production. The system automates the work of the design and technological departments of our production.
We are now actively using it, but due to the initially misorganized development process, it turned out to be unsupported. That is, correcting errors and implementing new functionality is very difficult and correcting old errors creates new ones. Auto tests have not been implemented at the moment either.
It is necessary to cover the code with tests and refactor.

We need two more people on the team. The task requires deep immersion in the subject area (design of internal staircases for private houses). The amount of work is about six months for 4 people. I will provide more information upon request.