If I desire a tagged function with an interface such as:
interface TaggedFun {
(args): void;
tag: boolean;
}
It appears that declaring a function to match this signature is not possible (since any function literal will lack the mandatory tag property);
The only recourse is to make tag optional and then remember to assign a value to it later.
Is there a method to uphold type safety (i.e. enforce the tag requirement in the type)?
To instantiate objects of TaggedFun, you can utilize a factory method:
interface TaggedFun {
(): void;
tag: boolean;
}
function createTaggedInstance(tag: boolean, fn?: () => void): TaggedFun {
let tagged = (fn || function () {
// add function logic here
}) as TaggedFun;
tagged.tag = tag;
return tagged;
}
This approach ensures the presence of the tag property in instances created through the factory, providing type safety.
Here's an example:
function performAction(tagged: TaggedFun) {}
performAction(() => { }); // error
performAction(createTaggedInstance(true)); // okay
performAction(createTaggedInstance(true, () => { })); // okay
Doing it in a more unconventional manner, the approach you mentioned seems suitable.
This is tailored for accommodating legacy javascript practices where functions are adorned with additional attributes instead of utilizing classes.
I strongly recommend opting for a class implementation instead.
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