What parts of a web application need to be tested?

Question

JavaScript

Now I am writing a small application (first practice) and at this stage I decided to master
testing with Jest;
The stack is:
Express, Mongoose, React, Redux, Socket.io.
I actively use promises when building interaction Frontend => socket.io => mongoose;
And because I am new in this business. I would like to know which parts of the application need to be tested? Should we write tests for absolutely all functions/components, or are there some subjective opinions about what exactly needs to be tested?
I would be glad to have links to foreign articles (or Runet articles).
Thank you all in advance for your help.

Answer 1

According to TDD - first write the test, and then write the working code that will pass this test. Read books by Robert Martin, he writes about it in each of them. For example, "Ideal programmer" and "Clean code". Ideally, each of your components should be covered by tests. We write in Google "React testing" and get a sea of ​​articles in Russian, write "React testing" and get in English, and so on and so forth. If something can be tested, then it should be tested. You should not look for an excuse for your laziness in this matter, it is better to overdo it.