Testing Type Definitions

Type-level tests check what TypeScript accepts or rejects, not what JavaScript prints. A well-typed library should compile valid usage and reject invalid usage. Dedicated tooling or compiler fixtures can verify inferred types, exported declarations, and expected error diagnostics.

TestString resolves to true, satisfying the Assert constraint. Changing the tested input to number causes a compile-time type error before JavaScript executes.

A passing runtime unit test cannot prove that the compiler inferred the intended generic signature for users of a library.

Save as testing-type-definitions.ts. The following program passed strict TypeScript compilation, and the displayed output was captured by executing its emitted JavaScript with Node.js 22.16.0.

TypeScript source

type IsString<T> = T extends string ? true : false;
type Assert<T extends true> = T;
type TestString = Assert<IsString<string>>;
const checked: TestString = true;
console.log(checked);

Verified output

true

Compile: npx tsc --strict --target es2022 --module commonjs testing-type-definitions.ts; run: node testing-type-definitions.js. For Node.js API examples, install @types/node first. For browser projects, use the browser-specific instructions below.

The example demonstrates the code-level behavior; the related configuration or test commands are shown separately when needed.

TestString resolves to true, satisfying the Assert constraint. Changing the tested input to number causes a compile-time type error before JavaScript executes.

Predict what changes if you alter one input. Then run the code to compare your reasoning with the real output.

A passing runtime unit test cannot prove that the compiler inferred the intended generic signature for users of a library.

Identify whether the failure is a type-checking issue, a configuration issue, or a runtime behavior issue before choosing a fix.

Write an Assert test that verifies ReturnType of a function is number; change it to string and observe the error.

Write down your expected result first. For configuration examples, compare compiler output and diagnostics; for test examples, make the assertion fail deliberately to verify that the tool reports it.

Type-level tests check what TypeScript accepts or rejects, not what JavaScript prints.

Run the primary example, then complete the practice task or environment-specific lab.