Question:

How can I specify a monkey-patched class in the typescript definitions file in JavaScript?

Problem:

Say I have a class A:

class A {

constructor();

someFunction(): void;

}

and a class B where I expect an instance of class A as a property:

class B {

aProperty: A;

}

How can I define typescript definitions (.d.ts) for a class C with a property of class A that is monkey patched to also include a function someOtherFunction(): void;?

Additional information

In class C, class A effectively looks like this:

class A {

constructor();

someFunction(): void;

someOtherFunction(): void;

}

I tried to define class C as follows without any luck:

class C {

aProperty: A | { someOtherFunction(): void; };

}

Solution:

If you've monkey-patched the class, then it has the new method wherever it's used. You can make the types reflect that by using >declaration merging:

interface A {

someOtherFunction(): void;

}

>Playground link

That will merge with the types for class A to make someOtherFunction available on instances of A (from a type perspective; the monkey-patching is necessary for it to be true from a runtime perspective).

I don't know that you can scope it, though. As >Konrad pointed out, you can use an intersection rather than a union for the property (aProperty: A & {some other function (): void; };). But again, if it's monkey-patched, it's monkey-patched everywhere. :-)

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