strictFunctionTypes

Learn strict function types.

What is strictFunctionTypes?

strictFunctionTypes checks the assignability of function parameter types using contravariant rules. This catches runtime errors in callback and handler assignments at compile time.

type Log = (msg: string) => void;
const log: Log = (msg) => console.log(msg);
log("merhaba");

The contravariance rule

You can assign a (a: Animal) => void function where a (d: Dog) => void is expected; because a function accepting Animal also accepts Dog. The reverse assignment is an error with strictFunctionTypes on; a function expecting Dog may not accept Animal.

type Animal = { name: string };
type Dog = Animal & { age: number };
type AnimalFn = (a: Animal) => void;
type DogFn = (d: Dog) => void;
let af: AnimalFn = (a) => console.log(a.name);
let df: DogFn = af;  // valid

Exercise Safe assignment with function types

Define Animal as { name: string }. Define Dog as Animal & { age: number }. Define AnimalFn = (a: Animal) => void and DogFn = (d: Dog) => void. Define af as an AnimalFn (logging a.name), assign df: DogFn = af, and call df with { name: "Rex", age: 3 }.

Show example solution

type Animal = { name: string };
type Dog = Animal & { age: number };
type AnimalFn = (a: Animal) => void;
type DogFn = (d: Dog) => void;

const af: AnimalFn = (a) => console.log(a.name);
const df: DogFn = af;
df({ name: "Rex", age: 3 });

Test yourself

For which aspect does strictFunctionTypes bring stricter checking?
- Only for array elements
- For function parameter types
- Only for object properties
- Only for return types
Show answer

For function parameter types — strictFunctionTypes makes function parameter types be checked contravariantly. Return types are already covariant.

Complete the function type

type Log = (msg: ____) => void;

Show answer

type Log = (msg: string) => void;

strictFunctionTypes parts

strictFunctionTypes

Checks parameters contravariantly
(a: Animal) => void

A function type taking Animal
Dog = Animal & { age: number }

A subtype of Animal
let df: DogFn = af

Safe assignment direction

Next lesson: Summary - Strict mode

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