Is it possible to create a type alias for a function with multiple signatures in TypeScript?

Question

Is it possible to create a type alias for a function with multiple signatures in TypeScript?

Is it feasible to generate a type alias for an overloaded function signature?

For instance, I have a function as follows:

function whenChanged(scope: ng.IScope, fn: ()=>void): ()=>void;
function whenChanged(fn: ()=>void, truthy:any): ()=>void;
function whenChanged(a,b): ()=>void {
    //...
}

I aim to establish a type alias for that overloaded signature in order to minimize redundancy and utilize it elsewhere while describing this function's type.

I attempted:

type WC1 = (scope: ng.IScope, fn: ()=>void) => ()=>void;
type WC2 = (fn: ()=>void, truthy:any) => ()=>void;
type WhenChanged = WC1 | WC2;

const whenChanged: WhenChanged = (a,b) => {
    //...
};

However, attempting to use this function leads to an error stating "Cannot invoke an expression whose type lacks a call signature".

I am unable to find any information in the documentation regarding type aliasing function overloads.

typescript

Answer 1

Answer №1

Given the existing function whenChanged:

function whenChanged(scope: ng.IScope, fn: ()=>void): ()=>void;
function whenChanged(fn: ()=>void, truthy:any): ()=>void;
function whenChanged(a,b): ()=>void {
    //...
}

The most straightforward approach to creating a type alias for its type involves using a typeof type query:

type WhenChanged = typeof whenChanged;

An alternative method is to utilize an overloaded function signature for defining the type alias:

type WhenChanged = {
  (scope: ng.IScope, fn: () => void): () => void;
  (fn: () => void, truthy: any): () => void;
}

You should note that no interface is required in this setup.

Another option is to combine both function signatures into one type declaration, albeit with some caveats due to the nature of overloads in TypeScript:

type WC1 = (scope: ng.IScope, fn: ()=>void) => ()=>void;
type WC2 = (fn: ()=>void, truthy:any) => ()=>void;
type WhenChanged = WC1 & WC2; // intersection, not union

Remember that intersections of function signatures are not interchangeable because they represent overloads. Therefore, different orderings can yield distinct types.

Lastly, keep in mind that attempting to call a function with type WC1 | WC2 may lead to ambiguity since the compiler cannot ascertain the correct overload from the union of signatures.

I hope this clarifies things for you. Best of luck with your TypeScript endeavors!

Answer 2

Given the existing function whenChanged:

function whenChanged(scope: ng.IScope, fn: ()=>void): ()=>void;
function whenChanged(fn: ()=>void, truthy:any): ()=>void;
function whenChanged(a,b): ()=>void {
    //...
}

The most straightforward approach to creating a type alias for its type involves using a typeof type query:

type WhenChanged = typeof whenChanged;

An alternative method is to utilize an overloaded function signature for defining the type alias:

type WhenChanged = {
  (scope: ng.IScope, fn: () => void): () => void;
  (fn: () => void, truthy: any): () => void;
}

You should note that no interface is required in this setup.

Another option is to combine both function signatures into one type declaration, albeit with some caveats due to the nature of overloads in TypeScript:

type WC1 = (scope: ng.IScope, fn: ()=>void) => ()=>void;
type WC2 = (fn: ()=>void, truthy:any) => ()=>void;
type WhenChanged = WC1 & WC2; // intersection, not union

Remember that intersections of function signatures are not interchangeable because they represent overloads. Therefore, different orderings can yield distinct types.

Lastly, keep in mind that attempting to call a function with type WC1 | WC2 may lead to ambiguity since the compiler cannot ascertain the correct overload from the union of signatures.

I hope this clarifies things for you. Best of luck with your TypeScript endeavors!

Answer 3

Answer №2

Through experimentation, it appears that utilizing an interface is crucial to finding the solution. This approach seems to yield positive results:

interface OnUpdate {
    (context: ng.IScope, action: ()=>void): ()=>void
    (action: ()=>void, condition:any): ()=>void
}

const onUpdate: OnUpdate = (x,y) => {
    //...
};

Answer 4

Through experimentation, it appears that utilizing an interface is crucial to finding the solution. This approach seems to yield positive results:

interface OnUpdate {
    (context: ng.IScope, action: ()=>void): ()=>void
    (action: ()=>void, condition:any): ()=>void
}

const onUpdate: OnUpdate = (x,y) => {
    //...
};

Answer 5

Answer №3

When the error "Cannot invoke an expression whose type lacks a call signature" appears, it means that a union of types cannot be called and you must choose one type to use.

The issue in the example provided is that no types are defined for a and b, so the compiler cannot determine whether WC1 or WC2 should be used. By updating the definition of whenChanged:

const whenChanged: WhenChanged = (a: ng.IScope, b: ()=>void) => {
  return b;
};

the compiler can now identify that the type of whenChanged is restricted to WC1 allowing for safe calling.

If whenChanged was a variable and could potentially be either WC1 or WC2, requiring a type assertion to specify the correct type before calling:

(whenChanged as WC1)(some_scope, some_fn);

or:

(whenChanged as WC2)(some_fn, some_truthy);

You cannot determine the contained type within whenChanged in order to make the appropriate function call. The type information needs to be obtained from another source.

Answer 6

When the error "Cannot invoke an expression whose type lacks a call signature" appears, it means that a union of types cannot be called and you must choose one type to use.

The issue in the example provided is that no types are defined for a and b, so the compiler cannot determine whether WC1 or WC2 should be used. By updating the definition of whenChanged:

const whenChanged: WhenChanged = (a: ng.IScope, b: ()=>void) => {
  return b;
};

the compiler can now identify that the type of whenChanged is restricted to WC1 allowing for safe calling.

If whenChanged was a variable and could potentially be either WC1 or WC2, requiring a type assertion to specify the correct type before calling:

(whenChanged as WC1)(some_scope, some_fn);

or:

(whenChanged as WC2)(some_fn, some_truthy);

You cannot determine the contained type within whenChanged in order to make the appropriate function call. The type information needs to be obtained from another source.

Is it possible to create a type alias for a function with multiple signatures in TypeScript?

Answer №1

Answer №2

Answer №3

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