🎯

type-checking-vs-testing

🎯Skill

from marius-townhouse/effective-typescript-skills

What it does

Guides developers in strategically using type checking and unit testing to catch different types of errors and validate code assumptions.

📦

Part of

marius-townhouse/effective-typescript-skills(83 items)

type-checking-vs-testing

Installation

Quick InstallInstall with npx

npx skills add marius-townhouse/effective-typescript-skills --all

Quick InstallInstall with npx

npx skills add marius-townhouse/effective-typescript-skills -s prefer-unknown-over-any exhaustiveness-checking

Quick InstallInstall with npx

npx skills add marius-townhouse/effective-typescript-skills -a opencode claude-code

Quick InstallInstall with npx

npx skills add marius-townhouse/effective-typescript-skills -l

git cloneClone repository

git clone https://github.com/marius-townhouse/effective-typescript-skills.git

📖 Extracted from docs: marius-townhouse/effective-typescript-skills

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AddedFeb 4, 2026

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Skill Details

SKILL.md

Use when deciding between types and tests. Use when catching logic errors. Use when testing edge cases. Use when building test suites. Use when validating assumptions.

Overview

# Understand the Relationship Between Type Checking and Unit Testing

Overview

Type checking and unit testing catch different kinds of errors. Type checking catches type mismatches at compile time. Unit testing catches logic errors and edge cases at runtime. They complement each other - types reduce the need for some tests, but can't replace tests for logic.

When to Use This Skill

Deciding between types and tests
Catching logic errors
Testing edge cases
Building test suites
Validating assumptions

The Iron Rule

Use types to catch type errors at compile time. Use tests to catch logic errors and edge cases at runtime. They complement each other.

Example

```typescript

// Type checking catches:

function add(a: number, b: number): number {

return a + b;

}

add(1, '2'); // Compile error - types don't match

// Tests catch:

function divide(a: number, b: number): number {

return a / b; // Logic error: no check for division by zero

}

// Type system can't catch this - need tests:

test('divide by zero', () => {

expect(() => divide(1, 0)).toThrow();

});

```

Reference

Effective TypeScript, 2nd Edition by Dan Vanderkam
Item 77: Understand the Relationship Between Type Checking and Unit Testing

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