Function taking a function returning a generic type is not typesafe - typescript-generics

Question

I experience this when using React hooks, but it is a general TypeScript question.
You can see it in the playground
When I use the generic parameter which I finds most intuitive the function I pass in isn't completely typesafe. As long as it returns an object with the props in T the compiler is happy, but I can also add props that are not part of T. This becomes a problem when T has some optional props, because I can miss spell them and not know it.
If I instead set the return type explicit on the function I pass in, everything works as expected, but this is not an intuitive usage when the function I call have a generic parameter.
Can someone please explain why the compiler is allowing this?
const foo = <T>(f: ()=>T) => {
return f();
};
type Result = {
bar: number;
foo?:number;
}
const strange = foo<Result>(() => {
return {
bar: 42,
baz: 12, // why is this prop allowd?
FOO: 13 // ups I might think I set the foo prop, but I spelled it wrong
};
});
const expected = foo((): Result => {
return {
bar:42,
FOO: 12 // this is not allowd, which is what I wxpected
};
});

Answer 1

Excess property check is triggered only for "fresh" object literal.
The freshness of an object literal is lost in case of type assertion and type widening.
in strange function, there is a type widening on the return type (see issue #241). So excess properties are valid.
const strange = foo<Result>(() => {
// type widening occurs here
// freshness is lost
// No excess property check triggered
return {
bar: 42,
baz: 12, // excess property allowed
FOO: 13 // excess property allowed
};
});
in the expected function, there is no type widening since the return type is explicitly specified. the freshness of the object literal is maintained, and the excess property check is triggered
const expected = foo((): Result => {
// No type assertion
// object literal is fresh
// Excess property check is triggered
return {
bar:42,
FOO: 12 // this is not allowd, which is what I wxpected
};
});