[Proposal] Initializer syntax for function types

[AlCalzone]

Using function types, it's possible to define types for functions with additional properties tacked on, e.g.

interface fnWithProp {
    (abc: string): string;
    foo: number;
}

In order to initialize variables of those types, one currently has to jump through hoops because the naive attempt does not work:

// Error: 
// Type '(str: any) => any' is not assignable to type 'fnWithProp'.
// Property 'foo' is missing in type '(str: any) => any'.
var abc: fnWithProp = (str) => str;
abc.foo = 7;

By explicitly casting the function to the correct type, one can avoid the error, but now TypeScript never checks if foo is assigned (even with strictPropertyInitialization = true):

var abc = ((str) => str) as fnWithProp;
console.log(abc.foo); // undefined

A working version looks like this

var abc = Object.assign(((str) => str), {
    foo: 7,
});

but abc is now being inferred as the rather cryptic type ((str: any) => any) & { foo: number }.

As an attempt to solve this problem, I propose an initializer syntax for these kinds of function types, which is similar to the object literal syntax:

var abc: fnWithProp = {
    (str) => {
        /* do something */
        return str;
    },
    foo: 7,
    /* optional: more properties */
};

or an alternative using the this keyword for the function body:

var abc: fnWithProp = {
    this: (str) => {
        /* do something */
        return str;
    },
    foo: 7,
    /* optional: more properties */
};

A usage example:

interface fnWithProp {
    (abc: number): number;
    foo: number;
}

function doSomething(transform: fnWithProp) { 
    return transform(transform.foo + 1);
}

doSomething({ this: (abc) => abc, foo: 7 });

This would fall out of scope of the design goals since it would add new expression-level syntax. Besides, const obj = { this: str => str }; is already valid syntax.

Anyway, you can combine the interface declaration with Object.assign with no problem:

interface fnWithProp {
    (abc: string): string;
    foo: number;
}
const o: fnWithProp = Object.assign((abc: string) => abc.toUpperCase(), { foo: 1 });

EDIT: You will need to explicitly type abc: string since we don't get a contextual type there.

[mhegazy]

The underlying issue is tracked by #15868. there is not an easy/ergonomic way in the language to create one of these functions that have properties. the best we have today is a function and namespace combo:

interface fnWithProp {
    (abc: string): string;
    foo: number;
}

function f(a: string) { return a; }
namespace f {
    export var foo: number;
}

var abc: fnWithProp = f;

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.