test: statically test types

[angeloashmore]

Types of changes

• [x] Chore (a non-breaking change which is related to package maintenance)
• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR adds static tests for all public APIs. It uses ts-expect in combination with the TypeScript compiler (tsc) to determine type validity.

Tests are structured using the following order:

• Exhaustive typeof check to ensure only permitted primitive types are allowed. For example, we can ensure a type is string | null by checking for typeof value against string and object. This verifies that a number cannot be passed without needing to manually check all possible types.
• Filled state (for types that support filled/empty states)
• Empty state (for types that support filled/empty states)
• Custom behavior implemented via generics

ts-expect was selected over other options because:

1. It provides a simple method for testing types.
2. It does not require separate compilers/analyzers to test types.
3. It does not introduce a large API.

These alternatives were considered:

• tsd is a CLI that runs the TypeScript type checker over assertions
• expect-type does type checks using a Jest-like API

Checklist:

• [ ] My change requires an update to the official documentation.
• [x] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [x] All new and existing tests are passing.

🎎

[codecov-commenter]

Codecov Report

Merging #15 (3025f5e) into master (05c7ea8) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           59        59           
=========================================
  Hits            59        59           

Impacted Files	Coverage Δ
src/customType.ts	100.00% <ø> (ø)
src/fields.ts	100.00% <ø> (ø)
src/index.ts	100.00% <ø> (ø)

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[angeloashmore]

+4,462 −55

For once, it's not package-lock.json causing a huge diff. 😅

You're right, it is verbose. Most of the code here is copy/pasted in each file to ensure the same test structure is used. This is something we won't need to touch often since the API is stable.

This is ready to merge if you think the general approach is good. I wanted to check with you @lihbr before merging.

[lihbr]

Yup, works like a charm, updated the CI workflow to run those.

Do you think we should get this setup available on the starter? Might encourage us to write a few of those elsewhere?