The Why

TDD just works, it's simple to implement. Gives you confidence on the code you are writing. It can be a great form of documentation. Moreover on complex problems it can help you get started by breaking problem down.

Why now?

It's never to late start including tests. Regardless of the fact that you are scaling your operations, encountering a lot of bugs, lacking documentation, or starting a tight deadline feature, etc. From my humble experience, any time you save by not writing tests is offset by fixing bugs later on, most times more time that it would take to write the test. Plus, ain't nothing more scarier than pushing a change and wondering if something broke while your fixing a bug.

What?

TDD, is just an iterative process of building software.

How?

This is just how I do it, I'm always open to make it better.

1. Start with the simplest iteration of the function. Ex:

// testing a function that checks for prime
test('should true for 2', () => {
  const got = isPrime(2)
  
  expect(got).to.be(true)
})

2. Make the test pass, keep it simple and focus on quick wins. Ex:

// function
function isPrime(num) {
  if (num === 2) return true
  return false
}
 
// this might seem trivial and unnecessary but will get you going

3. Refactor (if neccessary)

4. Go to step 1