Ask HN: What are some ways to test an UI focused web application

[mastershifu]

I am a fresh graduate and I have joined a startup where we are developing Ruby on Rails based web app with ReactJS. The project is fairly large in size, yet there is no testing framework setup. Everytime we push some code usually small bugs like null checks in javascript and backend occur but mostly at the UI side. We want to avoid this by using some better practises and would love to hear what readers have to say.

[IndianAstronaut]

I have seen a few "happy path" scripts that are written in Python using Selenium which goes in and does default checks of functionality to make sure they are working properly.