TypeScript · Senior · Question 1 of 75

With `strictFunctionTypes`, can you assign `(x: Dog) => void` to a variable of type `(x: Animal) => void`?

✓Correct answer: No: function parameters are checked contravariantly
2Yes: function parameters are checked covariantly
3Yes, but only if Dog is a class rather than an interface

Explanation

The variable promises to accept any Animal, but the function can only handle Dog - a call with Cat would break. So subtyping by parameter goes the opposite way: (x: Animal) => void can be assigned where (x: Dog) => void is expected, but not vice versa.

With `strictFunctionTypes`, can you assign `(x: Dog) => void` to a variable of type `(x: Animal) => void`?

Explanation

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Test yourself

With strictFunctionTypes, can you assign (x: Dog) => void to a variable of type (x: Animal) => void?

Explanation

Further reading

Test yourself

With `strictFunctionTypes`, can you assign `(x: Dog) => void` to a variable of type `(x: Animal) => void`?