How to extend global named functions in Typescript

Question

I've define a global named function such as:

function foo() { 
  foo._cache = {}; 
}

How do I extend a property to foo like _cache.

It seems the code below has no effact.

interface foo {
    _cache: any;
}

Answer 1

How do I extend a property to foo like _cache.

The function foo is not associated with the interface foo as they are in completely different declaration spaces (more on those).

If you want foo to be a function that also has a cache property you have to do it in two steps e.g:

const foo: {
    (): void;
    _cache: any;
} = function () {
} as any;
foo._cache = {};