Typescript generic function overload with rest types

Question

Is there any way (special syntax) to apply something like rest parameters to templates in TypeScript?

If question is unclear then take a look at below example and comments.

Question is - can I apply here something like rest ...Types:

function mixIt<A, B>   (a: A, b: B): A & B;
function mixIt<A, B, C>(a: A, b: B, c: C): A & B & C;

/* 
 * ??
 * If I would like to add D, E, F, etc, do I have to overwrite it like above?
 */

function mixIt<A, B, C>(...args: any[]): any{
    let mixin = <A & B & C> {};

    args.forEach( obj => {
        for(let key in obj) {
            if( ! mixin.hasOwnProperty(key) ) {
                (<any>mixin)[key] = obj[key];
            }
        }
    });

    return mixin;
}

FYI - error detection is as expected:

class X {x: number = 7;}
class Y {y: string = 'ok';}
class Z {z: boolean = false;}

let x = new X;
let y = new Y;
let z = new Z;

let xy = mixIt(x, y);
let xyz = mixIt(x, y, z);

xyz.z; // Ok;
xy.z; // Error - as expected. VS Code editor also highlights it

github.com/Microsoft/TypeScript/issues/… OR github.com/Microsoft/TypeScript/issues/… -- The examples given in these two links seems to do the trick. — Jarrett GXZ
– Jarrett GXZ, Commented Sep 27, 2021 at 1:37

Titian Cernicova-Dragomir · Accepted Answer · 2019-01-08 08:21:39Z

9

Edit Since the original answer typescript has added support for tuples in rest parameters in 3.0. With this we can achieve the desired result without all the overloads:

type UnionToIntersection<U> = 
    (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never

function mixIt<T extends any[]>(...args: T): UnionToIntersection<T[number]>{ 
    let mixin: any =  {};

    args.forEach( obj => {
        for(let key in obj) {
            if( ! mixin.hasOwnProperty(key) ) {
                mixin[key] = obj[key];
            }
        }
    });

    return mixin;
}

Original answer

There is no way to currently have a variable number of type parameters, there is a proposal on this.

The only way to currently do this is to add as many signatures to the functions as needed, so in your case this would be:

function mixIt<A, B>   (a: A, b: B): A & B;
function mixIt<A, B, C>(a: A, b: B, c: C): A & B & C;
function mixIt<A, B, C, D>(a: A, b: B, c: C, d: D): A & B & C & D;
function mixIt<A, B, C, D, E>(a: A, b: B, c: C, d: D, e: E): A & B & C & D & E;
function mixIt<A, B, C, D, E, F>(a: A, b: B, c: C, d: D, e: E, f: F): A & B & C & D & E &F ;
// Private signature
function mixIt(...args: any[]): any{ // no need for this to be generic
    let mixin: any =  {};

    args.forEach( obj => {
        for(let key in obj) {
            if( ! mixin.hasOwnProperty(key) ) {
                mixin[key] = obj[key];
            }
        }
    });

    return mixin;
}

edited Jan 8, 2019 at 8:21

answered Feb 13, 2018 at 15:07

Titian Cernicova-Dragomir

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4 Comments

Coderer Over a year ago

You left in the comment "no need for this to be generic" on the now-generic function definition. But, hey, neat trick!

Titian Cernicova-Dragomir Over a year ago

@Coderer wops, removed it. Saw it got upvoted and the info was a bit stale, wanted to do a quick update and forgot the comment :)

N. Kudryavtsev Over a year ago

Wow, UnionToIntersection works but some magic happens there... Could you please explain why and how does it work? At first look U is inferred as I so U should be equal to I, but practically it isn't so. Also without U extends any check everything won't work, but can U not extend any?

N. Kudryavtsev Over a year ago

I've found a little more descriptive answer: stackoverflow.com/a/50375286/5598194. The key to understadting is "distributive conditional types": typescriptlang.org/docs/handbook/release-notes/…. U extends any is used to enable this language feature and split inference into few ones, each per the union member. As I is located on a contra-variant position (i. e. as (k: U) => void) extends (k: I) => void, then I extends U), the final result is an intersection of all candidate types (that is just how TS works).

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