These answers point to dead links and reeks of Agile. They mention frameworks and MVC variations; from the start they've already bloated whatever they're build

These answers point to dead links and reeks of Agile. They mention frameworks and MVC variations; from the start they've already bloated whatever they're building and none of them address the real problem: complex sequences of state mutations. So not only are they having software that's orders of magnitude more complex than it should be, they're also completely avoiding the hard problems. You won't see a test like "enter a username, dont wait for server validation, enter an invalid email, optimistic updates, fix email, more optimistic updates, server response & state merges, advance to step 2, 3, come back to step 1, change email, advance to step 4, crash.. wat?!" That's where the bugs are. Not "click the button and the popup opens" because that's trivial. You're also probably not testing that button while a menu is open, or a dialog already opened, or between optimistic updates and server responses, or the combinations of these, or ALL the cases you're going to forget. You're not going back to test each new feature against every single possible state the app can be in. That's why I say TDD is overrated because the problems it fixes are trivial and it ignores the important ones.