We do one thing - we break everything else?

Question

Software testing

Who faced such that your development department (php, js, css) makes a feature/page/method/functionality more or less successfully, but what was implemented earlier constantly breaks down?
Why is this happening? How to deal with it? Need to enter documentation? Testing? Do developers need to be strictly tied to specific guidelines and frameworks?
I would be grateful for your examples of solving similar situations and useful links.

Answer 1

We write our code according to TDD
After the feature is implemented, it runs all tests .
If the tests fail, we decide how to write our part, so that tests that do not relate to our part do not fall

Answer 2

Let's be honest, products with real food value have so many states that some of them will always be broken.
Regardless of frameworks, testing, etc. There will be bugs, and the more changes you make, the more likely you are to introduce bugs.
However, there are quite standard methods to minimize this pain.
These are testing practices (not only by testers) and increasing code culture, code reviews, product status monitoring, normal release practices, and so on and so forth.
There is no single way to "fix everything at once".

Answer 3

It happens, depending on the frequency, and it's normal and not.
Struggling on test side: Regression autotests
On developer side: Unit testing, run on package build or each developer runs locally before pushing changes