Imagine you have a function type defined as follows:
type MyFunc = (x: string, y: number) => boolean;
However, you cannot use this type as an annotation for a general function declaration like this:
// where to annotate?
function myFunc(x: string, y: number) {
return x.length > y;
}
It would be convenient if there was a way to annotate the function like the hypothetical syntax below (which is currently unsupported):
// do not do this, it doesn't work:
function myFunc: MyFunc (x, y) {
return x.length > y;
}
Unfortunately, TypeScript does not currently support this. There is an existing feature request for this enhancement: microsoft/TypeScript#22063. While there hasn't been much progress on this issue, showing support by giving it a thumbs up on the GitHub page might slightly increase the chances of implementation. Alternatively, if you have a unique use case, consider leaving a comment on the GitHub issue. Realistically, this enhancement may never get implemented.
So, what are the workarounds? The most practical and efficient solution is to change your declared function to a declared const of a function type, as demonstrated below:
const myFunc: MyFunc = function (x, y) {
return x.length > y;
}
You also have the option to use the built-in utility types Parameters<T> and ReturnType<T> to convert the annotation of the function itself into annotations of the arguments and return types. However, this method can be quite complex and may not be preferable:
function myFunc(...[x, y]: Parameters<MyFunc>): ReturnType<MyFunc> {
return x.length > y;
}
Another workaround involves using the type system to verify that your declared function is assignable to the MyFunc type. If you define a helper type called Extends:
type Extends<T, U extends T> = void;
You can then check if your function aligns with the MyFunc type:
type MyFuncWorks = Extends<MyFunc, typeof myFunc>; // okay
function myFunc(x: string, y: number) {
return x.length > y;
}
If you make a change that renders the function incompatible with MyFunc, an error will be displayed:
type MyFuncBroken = Extends<MyFunc, typeof myFunc>; // error!
// number is not boolean ---------> ~~~~~~~~~~~~~
function myFunc(x: string, y: number) {
return x.length + y; // + instead of <, oops
}
In conclusion, the most elegant workaround remains using a function-typed const. Therefore, it's recommended to stick with this approach.
I hope this information helps. Good luck with your TypeScript development!
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