variance-check

This repository contains the sample code used in the variance article.
You can verify TypeScript’s behavior under different variance-related compiler settings.

covariant.ts and invariant.ts intentionally contain type errors for variance testing.

Setup

pnpm install

How to run the checks

Run the following commands to compare strict (theoretical) variance checks with TypeScript’s more permissive behavior:

pnpm exec tsc -p tsconfig.json
pnpm exec tsc -p tsconfig.strict.json

• tsconfig.json: strictFunctionTypes: false — shows TypeScript’s bivariant function parameter behavior
• tsconfig.strict.json: strictFunctionTypes: true — closer to the theoretical, sound variance rules

Use these two modes to observe how arrays, function parameters, and generic types behave differently under each configuration.