https://www.scalawilliam.com/unit-testing-is-simple/

Is unit testing difficult?

No, it is not. You don't even need a testing framework for it.
You don't even need to write the test in a separate file or class.

You just need to exit with code 1. This is enough to fail a Jenkins build and a Makefile!

Why is a unit test useful?

• Prevent regressions when you change the code.
• Document expected input and output as code.
• Replace error-prone println-driven development with test-driven development.

What do you test? when the logic is "complex enough" I know it could be broken 6 months down the line, due to "that one small change" - or that I'd spend time building it with print statements. Example from my own code.

Here, I'll show you the most basic way to unit testing with four languages: Bash, Python, Node.js, Scala, just to prove the point.

These examples are for a very basic piece of code and are intentionally minimalistic, to demonstrate that you needn't use a test framework to get started with - though one will be very necessary when you scale your application.

Bash

Here's a bash script inc.sh with a basic assertion and some main code:

#!/bin/bash

increment() {
    echo $(("$1"+1))
}

# If this assertion fails, the whole thing quits 
[ 3 -eq $(increment 2) ] || exit 1

# Increment a stream of numbers
while read n; do
    increment $n;
done

If we run:

$ seq 1 5 | bash inc.sh
2
3
4
5
6
$ echo $?
0

Exit code is 0. But if we broke the function, and put 2 instead of 1, we get:

$ seq 1 5 | bash inc.sh
$ echo $?
1

Python

This is easy to achieve in any language really. Python assertions:

Broken test in file inc.py:

def increment(n):
  return n + 2

assert increment(2) == 3

$ python inc.py
Traceback (most recent call last):
  File "inc.py", line 4, in <module>
    assert increment(2) == 3
AssertionError
$ echo $?
1

Node.js

Comes with assertions built in.

Broken test in file inc.js:

const assert = require('assert');

function increment(n) {
  return n + 2;
}

assert.equal(2, increment(1));

$ node inc.js

assert.js:81
  throw new assert.AssertionError({
  ^
AssertionError: 2 == 3

...
$ echo $?
1

Scala

Comes with assert.

def inc(n: Int) = n + 2
assert(inc(1) == 2)

$ scala inc.scala
java.lang.AssertionError: assertion failed
$ echo $?
1

Why is this important?

You can get started light with several assertions and work your way up to proper test suites.

So what are test frameworks for?

To make testing more organised. You get:

• Organisation
• Detailed error messages
• Detailed test reports
• Neat assertions and human readable language (see ScalaTest)
• Incremental/TDD type of development

Conclusion

Now you have no excuse for not writing at least a few tests in your code.