TypeScript: Functions are assignable but produces error if target parameter type is not assignable to source parameter type

Question

Consider the following example: (In strict-null-checks) mode

const fn: (p: { x: number } | null) => void = (p: { x: number }) => console.log(p.x);
fn(null);

This code produces no errors in typescript, but has a runtime type exception.

Seems to me like Typescript should have enforced target parameter type (here {x: number } | null) to be assignable to source parameter type (here { x: number }), so that, for example null wouldn't be passed where something else was expected and cause an error.

Why is this not enforcing that? Is that a bug in typescript? Or something in the configuration that I'm missing?

I get an error in Typescript with your code typescriptlang.org/play/index.html#code/… — distante
– distante, Commented Aug 5, 2019 at 11:02

Titian Cernicova-Dragomir · Accepted Answer · 2019-08-05 11:32:53Z

2

By default function parameters relate bi-variantly. This means that if the function is assignable if either the target parameter is a subtype of the source parameter or vice-versa (which is why this succeeds since { x: number } is a subtype of { x: number } | null

If you enable strictFunctionTypes parameters will relate contravariantly, this will mean that the assignment will succeed only if the target parameter is a subtype of source parameter. This will produce an error here since { x: number } | null is not a subtype of { x: number }

Enable strictFunctionTypes and you will get an error. You can see this behavior in the playground too: with the option and without the option

answered Aug 5, 2019 at 11:32

Titian Cernicova-Dragomir

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