test-driven-development

Test driven development is a software engineering philosophy - is isn't the only one (see List of software development philosophies - Wikipedia).

Criticism of TDD

Why most unit testing is a waste

Why test

tests can tell you if you new code breaks anything
tests make you write better code
tests help others understand your code

1 - Verifying that you didn't break anything

A test suite gives you verification that functions are not broken

check for correctness by failing to show incorrectness

It doesn't prove that everything is perfect!

2 - Writing better code

Tests force you to decompose code into functions

code that is difficult to test may be too tightly coupled

We must design the function so it can be tested

think about what kinds of data structures being used - favour simple (i.e. JSON) so that a test stub can be used
if the code is hard to test because of too many use cases, this suggests the function is doing too much

Test also force you to think about dependencies (you shouldn't need to run the database to test the UI)

3 - Make code easier for others to understand

Tests also act as a form of documentation - executable documentation that a computer can run (and other developers can read)

Test code is as important as your source code

A good test suite is

Fast

Easy to run

Automatically run (pre commit git hook)

Deterministic

Types of tests

Unit test = tests a function/class in isolation

Integration test = several functions/classes

System test = entire system mimicking how a user would use the system