Tests as a Programming Language Learning Tool

Most "hello world" programs for getting started with a language will have you log something to the console/terminal window. Though this can be useful to see what your program is doing as you dip your toes in, asserting program correctness is going to get clunky and messy if you rely on this as the program becomes more complex and you continue to add new features. As a former QA engineer my bias is showing, but I believe learning to automate tests to verify correctness of code you write is one of the most valuable skills you can sharpen as a software engineer. It proves things are working as intended while documenting how they are expected to work, which will allow you or other engineers to more easily refactor things later, should the need arise.

Instead of relying on logging when getting started with a new language, I'm going to propose a more pragmatic approach to getting up to speed: learn to write tests with the language's provided testing utilities. Learning this will benefit you in a few ways:

• You'll be thinking in terms of how to write code that is testable. Regardless of language, this tends to help you author more modular, easy-to-change code.
• You'll be able to quickly iterate on your code and rerun tests to verify it does what you think it does, allowing you to experiment with different approaches without getting bogged down with what you should or should not be logging, how to format what you're logging, etc.
• You'll know how to structure your project in a way that facilitates being able to write and run tests, instead of going through a lot of pain later trying to figure out how to tack them on. Bolting on quality assurance at the tail end of the SDLC is a big no-no 👎
• You'll be writing more code in the language you're learning to implement the tests themselves, which will help you build up some muscle memory and build familiarity with standard libraries and such. (I know, AI is gonna take over the world and you can just tell it what you want written... go try and run some Rust written by ChatGPT and get back to me on that. Furthermore, be aware that code from developers with access to AI assistance may be significantly less secure than code from those without it, so you still need to know your shit.)

Test Harness Demos

With all of the above in mind, I'd like to share some tips on how to scaffold out a simple project that can be used to learn a language this way (or just run experiments quickly in a language you already know). For now I'll focus on TypeScript, Go and Rust, but may add other language examples to this page later. The same simple functionality will be implemented for each language: a function that accepts an array of integers and detects if there is a duplicate. If you follow along with these examples, I encourage you to break things on purpose (comment out lines, return incorrect values, etc.) and rerun the tests to see how the output changes.

I'll go on a short tangent here to describe what prompted me to learn new languages this way, so if you're not a fan of story time you can skip ahead to the language-specific examples. OK, story time:

I was recently affected by some restructuring at my previous company, and found myself in the unenviable position of a software engineer competing for remote work at the national level. Though I'm confident in my abilities, potential employers have no reason to be until I've proven it somehow. Thus, I am beginning the "LeetCode grind" to prepare for whiteboard interviews and am also learning a couple of new languages to provide more value as a backend engineer. As part of these studies, I wanted to be able to write and run code locally on my machine to work on practice problems and verify my code's correctness with unit tests. That way when I go to work on actual projects, I'm not just familiar with the language but also up to speed on how to read/write/run tests and equipped to write reliable software that can be shipped frequently without regressions.

Language Example: TypeScript

For TypeScript, I opted to use the Bun runtime instead of Node, given Bun's out-of-the-box support for TypeScript. I didn't want to spend any time fighting to get TS configs working to be able to run scripts in Node, but do prefer strongly typed code to raw JS. That said, one caveat here is that if you're relying on Node standard libs that are in some way different from Bun's, you may need to fight that battle to get a project going.

It's pretty easy to scaffold a project once you've got the runtime installed:

mkdir <PROJECT_NAME>
cd <PROJECT_NAME>
bun init

From there, you can add .test.ts files where tests will be written. In this example, I've added a file at src/arrays/containsDuplicate.test.ts

// Import testing tools provided by the Bun runtime
import { describe, expect, test } from 'bun:test';

/**
 * Write a function we expect will do something. In this case, we will
 *   provide an array of numbers, and expect to receive a boolean that
 *   evaluates to `true` if any duplicates are found.
 */
function containsDuplicate(nums: number[]): boolean {
  const seen: Record<number, true> = {};

  for (const n of nums) {
    if (seen[n]) {
      return true;
    }

    seen[n] = true;
  }

  return false;
}

// Write some tests!
describe('arrays: containsDuplicate', () => {
  test('has duplicate', () => {
    expect(containsDuplicate([1, 2, 3, 1])).toBe(true);
  });

  test('has duplicates', () => {
    expect(containsDuplicate([1, 1, 1, 3, 3, 4, 3, 2, 4, 2])).toBe(true);
  });

  test('does not have duplicates', () => {
    expect(containsDuplicate([1, 2, 3, 4])).toBe(false);
  });
});

Normally, your functions under test would not live in the same file as the tests themselves, as is the case with containsDuplicate above. You'd write the function in another module and import it instead. In this demo, the function is declared in the same file to keep things simple.

Now, with tests written, you can run them:

bun test

You can learn more about options such as filtering, watch mode, etc. for this command in Bun's documentation. Without changing anything from the above code, here's what you should see:

bun test v1.0.35 (940448d6)

src/arrays/containsDuplicate.test.ts:
✓ arrays: containsDuplicate > has duplicate [0.30ms]
✓ arrays: containsDuplicate > has duplicates [0.05ms]
✓ arrays: containsDuplicate > does not have duplicates [0.03ms]

 3 pass
 0 fail
 3 expect() calls
Ran 3 tests across 1 files. [59.00ms]

Nice! Notice how we don't have any console.log statements, but can easily verify which inputs our function correctly handles, and are in a good position to be able to refactor if desired.

Language Example: Go

I'm very new to Go, so if you spot anything wacky in my setup I am open to being rebuked! Here's how I got it up and running. Assuming you have Go installed on your machine, initialize a project:

mkdir <PROJECT_NAME>
cd <PROJECT_NAME>
go mod init github.com/<GITHUB_USERNAME>/<REPO_NAME>

In case it's not clear, if you're not using GitHub to host your repo, you'd use a different module path. If you're going to keep the code on your machine locally with no intention of pushing it anywhere, you can always just use a dummy path like example.com/my-module.

With the module initialized, you can now add a package. You can learn more about how to use the Go testing package in their docs. In this example, I've created a file at arrays/contains_duplicate_test.go, with the following contents:

// Name this file as a package
package contains_duplicate_test

// Import testing tools provided by the testing package
import (
 "testing"
)

/**
 * Write a function we expect will do something. In this case, we will
 *   provide an array of numbers, and expect to receive a boolean that
 *   evaluates to `true` if any duplicates are found.
 */
func ContainsDuplicate(nums []int) bool {
 mapped := make(map[int]bool)

 for _, n := range nums {
  if mapped[n] {
   return true
  }

  mapped[n] = true
 }

 return false
}

// Write some tests!
func TestHasDuplicate(t *testing.T) {
 got := ContainsDuplicate([]int{1, 2, 3, 1})
 if !got {
  t.Error("should be true:", got)
 }
}

func TestHasDuplicates(t *testing.T) {
 got := ContainsDuplicate([]int{1, 1, 1, 3, 3, 4, 3, 2, 4, 2})
 if !got {
  t.Error("should be true:", got)
 }
}

func TestDoesNotHaveDuplicates(t *testing.T) {
 got := ContainsDuplicate([]int{1, 2, 3, 4})
 if got {
  t.Error("should be false:", got)
 }
}

Normally, your functions under test would not live in the same file as the tests themselves, as is the case with ContainsDuplicate above. You'd write the function in another file and import it instead. In this demo, the function is declared in the same file to keep things simple.

Now, with tests written, you can run them:

go test ./arrays/contains_duplicate_test.go

You can learn more about options for this command in Go's documentation or by running go test help. Without changing anything from the above code, here's what you should see:

ok      command-line-arguments  0.767s

Right on! Notice how we don't have any fmt.Println statements but can still verify our code works, and are in a good position to be able to refactor if desired.

Language Example: Rust

I've been dabbling in Rust for a while now but am not proficient yet, so if you spot anything wacky in my setup I am open to being admonished!

As an aside: Rust is the language I have enjoyed learning most aside from TS. Part of what thrills me about it is that it has challenged me in ways that other languages I've used before (e.g. JS, TS, Ruby, Python, PHP) have not, because it forces one to think much more deeply about how data is being managed in memory and what the implications are for accessing variables and values at runtime. It's such a nifty language! I hope that someday in the near future I'll get to actually build some useful things with it.

Here's how I got it up and running. Assuming you have Rust/Cargo installed on your machine, initialize a lib crate:

mkdir <PROJECT_NAME>
cd <PROJECT_NAME>
cargo init --lib

In this example I wanted to be able to write code as if it were functions available from a library, not an actual program that runs, so I included the --lib argument. You may decide to structure your test harness project(s) differently.

At this point, Cargo has conveniently added an example of how a test case would be written in the src/lib.rs file. I went ahead and cleared out all the contents of lib.rs, since it's easier for the sake of this language learning exercise to write tests under the tests folder.

With the crate initialized, you can now add test modules. In this example, I've created a few files:

• tests/tests.rs which will import the arrays testing module
• tests/arrays/mod.rs which will import all test modules from the arrays folder
• tests/arrays/contains_duplicate.rs which is where our demo test will be written

Here's what goes into each file.

tests/tests.rs

mod arrays;

tests/arrays/mod.rs

mod contains_duplicate;

tests/arrays/contains_duplicate.rs

/**
 * Write a function we expect will do something. In this case, we will
 *   provide an array of numbers, and expect to receive a boolean that
 *   evaluates to `true` if any duplicates are found.
 */
fn contains_duplicate(arr: &[i32]) -> bool {
    let mut seen = std::collections::HashSet::new();

    for &num in arr {
        if !seen.insert(num) {
            return true;
        }
    }

    false
}

// Write some tests!
#[test]
fn contains_duplicate_when_duplicate_present() {
    assert!(contains_duplicate(&[1, 2, 3, 1]))
}

#[test]
fn contains_duplicate_when_duplicates_present() {
    assert!(contains_duplicate(&[1, 1, 1, 3, 3, 4, 3, 2, 4, 2]))
}

#[test]
fn contains_duplicate_when_no_duplicates_present() {
    assert!(!contains_duplicate(&[1, 2, 3, 4]))
}

Normally, your functions under test would not live in modules in the tests folder as is the case with contains_duplicate above. You'd write the functions in the lib and import it into these test files instead. In the demo, the function is declared in the same file to keep things simple.

The Rust example a little bit more involved than writing tests in other languages, so let's review some of what's going on here before running the tests.

In order to be able to run individual test files from the command line, we're authoring them as "integration tests" that are external to the library, which allows Cargo to compile each test file as an individual crate. For the purposes of this tutorial, we just have a monolithic "crate" specified by tests/tests.rs. That crate is importing the arrays module, and thus any tests that are written into the arrays module. Similarly, the arrays module is importing modules that are in the arrays directory.

For now we only have the arrays module and its child contains_duplicate module. If we added more .rs files under tests/arrays, we'd need to declare those modules as included in the tests/arrays/mod.rs if we want them to run in the test suite. Similarly if we added a new submodule folder such as tests/strings, we'd need a test/strings/mod.rs file and to declare that module as included in the tests/tests.rs file.

Perhaps as I begin moving along on my learning journey I'll find or write a nifty little script to handle the tedium of declaring each new file as included in its parent module, but hopefully all of this at least makes sense for now.

With all of that out of the way, we're ready to run tests. To run tests for the arrays::contains_duplicate module:

cargo test arrays::contains_duplicate

You can learn more about options for this command in Rust's documentation or by running cargo test --help. Without changing anything from the above code, here's what you should see:

   Compiling leetcode_rust v0.1.0 (/Users/jb/Repos/rust/leetcode_rust)
    Finished test [unoptimized + debuginfo] target(s) in 0.61s
     Running unittests src/lib.rs (target/debug/deps/leetcode_rust-b2231803eb8049e1)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/tests.rs (target/debug/deps/tests-4efc5d223fa6cb0c)

running 3 tests
test arrays::contains_duplicate::contains_duplicate_when_duplicates_present ... ok
test arrays::contains_duplicate::contains_duplicate_when_duplicate_present ... ok
test arrays::contains_duplicate::contains_duplicate_when_no_duplicates_present ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Wowza! 🤩 That may have been a little more effort than in the other languages covered so far, but how incredibly rewarding! Notice how we don't have any println statements but can still verify our code works, and are in a good position to be able to refactor if desired.

Language Example: Python

I have not used Python extensively for several years so my code is bound to be atrocious, rebuke away!

Setting up the Python project is dead simple, assuming you already have Python installed:

mkdir <PROJECT_NAME>
cd <PROJECT_NAME>
mkdir -p tests/arrays
touch tests/arrays/contains_duplicate.py

In tests/arrays/contains_duplicate.py, add the following content:

import unittest

def contains_duplicate(nums):
  seen = {}

  for n in nums:
    if seen.get(n):
      return True

    seen[n] = True

  return False

class TestContainsDuplicate(unittest.TestCase):
  def test_has_duplicate(self):
    self.assertTrue(contains_duplicate([1, 2, 3, 1]))

  def test_has_duplicates(self):
    self.assertTrue(contains_duplicate([1, 1, 1, 3, 3, 4, 3, 2, 4, 2]))

  def test_does_not_have_duplicates(self):
    self.assertFalse(contains_duplicate([1, 2, 3, 4]))

if __name__ == '__main__':
  unittest.main()

As mentioned in other language examples, your functions under test would not normally live in the same file as the tests themselves, as is the case with contains_duplicate above. You'd write the function in another file and import it instead. In this demo, the function is declared in the same file to keep things simple.

To run test files, just execute them with python, as so:

python tests/arrays/contains_duplicate.py

Easy peasy! Notice how we don't have any print statements but can still verify our code works, and are in a good position to be able to refactor if desired.

Wrapping Up

I have enjoyed dipping my toes into new languages while solving coding challenges, and am glad to know how to run tests in each new language I'm learning. I hope you have learned something new and are confident to begin writing tests as part of your adoption of any of the languages I've covered here. Thanks for reading!