Typescript: object type to array type (tuple)

Question

I have this:

interface Obj {
    foo: string,
    bar: number,
    baz: boolean
}

The desired type is this tuple:

[string, number, boolean]

How can I convert the interface to the tuple?

Update:

My original problem is: I make some opinionated library with a declarative spirit, where a user should describe parameters of a function in an object literal. Like this:

let paramsDeclaration = {
  param1: {
    value: REQUIRED<string>(),
    shape: (v) => typeof v === 'string' && v.length < 10
  },
  param2: {
    value: OPTIONAL<number>(),
    ...
  },
}

Then the library takes this object and creates a function with parameters from it:

   (param1: string, param2?: number) => ...

So, making such function is not a problem, the problem is to correctly type it, so that user gets good code-completion (IntelliSense).

P.S. I know it's not solvable, but it would be interesting to know what is the closest possible workaround/hack.

Answer 1

Not really an answer to the question, but since I don't actually think its possible to do, hopefully this is at least helpful in some way:

function REQUIRED<T>(): T {
    //...
}
function OPTIONAL<T>(): T {
    //...
}

interface ParamsDeclaration {
    readonly [paramName: string]: {
        readonly value: any;
        readonly shape?: Function;
    };
}

type Func<T> = T extends {
    readonly [paramName: string]: {
        readonly value: infer U;
    };
} ? (...params: Array<U>) => void
    : never;

function create<T extends ParamsDeclaration>(paramsDeclaration: T): Func<T> {

    // ...
}

const paramsDeclaration = {
    param1: {
        value: REQUIRED<string>(),
        shape: (v: any) => typeof v === 'string' && v.length < 10
    },
    param2: {
        value: OPTIONAL<number>(),
        //...
    },
};
// Type is '(...params: (string | number)[]) => void'
const func1 = create(paramsDeclaration);
func1('1', 2); // Ok
func1(2, '1'); // Ok, but I assume not what you want
func1(Symbol()); // TS error

Answer 2

90% of the time you think something is impossible in Typescript, the real answer is that it is possible but you probably shouldn't do it.

Here's a solution using TuplifyUnion from this answer, which converts a union type into a tuple type; note that we need to start from a union of the object's keys, not its values, because the values may themselves be unions (e.g. boolean is technically true | false).

Read that linked answer for an elaboration of what the // oh boy don't do this comment means. If you want users of your API to specify the parameters of a function which your API generates, then the sane choice is to accept those parameter specifications in an array in the first place.

type ObjValueTuple<T, KS extends any[] = TuplifyUnion<keyof T>, R extends any[] = []> =
  KS extends [infer K, ...infer KT]
  ? ObjValueTuple<T, KT, [...R, T[K & keyof T]]>
  : R

// type Test = [string, number, boolean]
type Test = ObjValueTuple<Obj>

Playground Link

Answer 3

Alternate suggestions,
It needs to set orders of parameters.

interface Param {
    readonly value: any;
    readonly shape?: Function;
}
type Func<T extends Record<string, Param>, orders extends (keyof T)[]> = (...args:{
    [key in keyof orders]:orders[key] extends keyof T ? T[orders[key]]['value']: orders[key];
})=>void;

function create<T extends Record<string, Param>, ORDERS extends (keyof T)[]>(params: T, ...orders:ORDERS): Func<T, ORDERS> {
    return 0 as any;
}

const func1 = create({a:{value:0}, b:{value:''}, c:{value:true}}, 'a', 'b', 'c');
func1(0, '1', true); // ok
func1(true, 0, '1'); // error

or
ParamDeclarations with array

type Func2<T extends Param[]> = (...args:{
    [key in keyof T]:T[key] extends Param ? T[key]['value'] : T[key]
})=>void;

function create2<T extends Param[], ORDERS extends (keyof T)[]>(...params: T): Func2<T> {
    return 0 as any;
}

const func2 = create2({value:0}, {value:''}, {value:true});
func2(0, '1', true); // ok
func2(true, 0, '1'); // error