You might have heard of TDD. But you haven't tried. Then you should better do it. 
TDD lets you write better code. This is one of the methods every developer must acquire. 
We all need habits to do things better. Like running, waking up early, eating...

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The 4 steps are what I think are what are needed in learning TDD. In the previous test, I actually omitted some important setups. This example app uses "pseudo" API to call data. So it runs perfectly on testing environment.

TDD (Test Driven Development) Basics Tutorial with TypeScript/React