strictFunctionTypes

strictFunctionTypes checks function parameter compatibility more safely, especially when functions are stored in variables. A callback that only understands Dogs should not be treated as a callback that can receive any Animal. There are intentional compatibility exceptions for method syntax, so learn the rule in context.

describeAnimal accepts any Animal, so it can safely handle a Dog. The reverse assignment can be unsafe because a generic Animal may lack bark.

Parameter variance is not simply interchangeable: accepting a broader input is different from requiring a narrower one.

Save as strictfunctiontypes.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 Animal = { name: string };
type Dog = Animal & { bark: () => void };
const describeAnimal: (a: Animal) => string = a => a.name;
const describeDog: (d: Dog) => string = describeAnimal;
const dog: Dog = { name: "Rex", bark: () => {} };
console.log(describeDog(dog));

Verified output

Rex

Compile: npx tsc --strict --target es2022 --module commonjs strictfunctiontypes.ts; run: node strictfunctiontypes.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.

describeAnimal accepts any Animal, so it can safely handle a Dog. The reverse assignment can be unsafe because a generic Animal may lack bark.

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

Parameter variance is not simply interchangeable: accepting a broader input is different from requiring a narrower one.

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

Try assigning a callback requiring Dog to a variable declared to accept Animal, and study the strictFunctionTypes 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.

For the exact current options and platform behavior, consult the official documentation. These examples were verified using the tool versions described above.

strictFunctionTypes checks function parameter compatibility more safely, especially when functions are stored in variables.

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