What is the best way to describe a type in TypeScript that serves as both an object and a function simultaneously?

Upon coming across this insightful answer to the query

How does jQuery's $ function operate as both a function and an object?
, I couldn't help but ponder. How can one define such a type in typescript?

In traditional JS, the following code is perfectly valid:

var f = function() { alert('yo'); }
f.foo = "bar";

alert(f.foo); // displays "bar"
f();          // displays "yo"

However, in typescript, calling f.foo will result in an error:

Property 'foo' does not exist on type '() => void'
.

One workaround would be using bracket notation:

var f = function() { alert('yo'); }
f['foo'] = "bar";

alert(f['foo']); // displays "bar"
f();            // displays "yo"

However, this approach completely circumvents the type system and consequently sacrifices the type safety offered by typescript.

Is there a method to integrate this functionality without compromising the type safety of typescript?

Answer №1

To specify a custom type like this, you can define an interface with an invocation signature:

interface MyCustomType {
    (): void;
    customProperty: string;
}

Then, when initializing an object with this type, you need to cast the function as any before assigning it to a variable:

let obj: MyCustomType = function() { alert('Hello'); } as any;
obj.customProperty = 'world';
someFunction();

You can play around with this code snippet in the playground here.


On a related note, the definition of the JQueryStatic type (used for the $ variable) is similar in the JQuery definitions:

interface JQueryStatic<TElement extends Node = HTMLElement> {
    ...
    Deferred: JQuery.DeferredStatic;
    ...
    (html: JQuery.htmlString, ownerDocument_attributes: Document | JQuery.PlainObject): JQuery<TElement>;
    ...
    (selector: JQuery.Selector, context: Element | Document | JQuery | undefined): JQuery<TElement>;

If you're interested, these are the relevant links:

It's worth noting that while JQuery doesn't face the same initialization challenge because it's not written in Typescript.

Answer №2

If you want to define a variable with multiple types, you can use the `&` symbol. This is known as an "Intersection Type". For more information, refer to the official documentation.

Below is an example of how to use Intersection Types:

let f: Function & {foo?: String} = function() { alert('yo'); }
f.foo = "bar";

alert(f.foo); // outputs "bar"
f();          // outputs "yo"

Playground Link

In this case, the type of f is defined as Function & {foo?: String}, meaning it must be both a Function and have a property {foo?: String}. The presence of `?` indicates that initially it's only a Function, and then later a value is assigned to foo.

You can verify in the Playground that the code compiles successfully, providing the same JavaScript code as intended.

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