"Pick" or "Exclude" constructor from class type

Search Terms

intersect, type, intersect constructor, pick new, pick constructor

Suggestion

class C1 {
  static S1 = 'foo';
  bar = 'bar';
  constructor(p1, p2, p3) {}
}
type T1 = Pick<typeof C1, new>;
// could simply be also:
type Only<T, K extends keyof T> = Pick<T, { [P in keyof T]: P extends K ? P : never }[keyof T]>;
type T2 = Only<typeof C1, 'constructor'>;

const clazz: T1 = null; // `null` could be anything
new clazz(); // should throw error because of missing params
new clazz(null, null, null); // should work
new clazz(null, null, null).bar; // should throw error because `bar` doesn't exist
clazz.S1; // should throw error because `S1` doesn't exist

Use Cases

It could be useful in class type intersection and/or for multiple inheritance (like PHP Trait, or Java default methods in interface). And make mixins more “clean” imho. One of my goal is to make a “type mixin” without a “value mixin”.

Examples

For example:

class C1 {
    public bar: string;
    constructor(p1: string) {}
    public foo() {}
}

class C2 {
    public constructor() {}
    public baz() {}
}

type T1 = typeof C1 & typeof C2;

const a: T1 = null;
a.prototype.foo(); // ok
a.prototype.bar; // ok
a.prototype.baz(); // ok
new a(); // ok (return C2)
new a(''); // ok (return C1)
new a().foo() // error
new a('').baz(); // error

type T2 = typeof C1 & Exclude<typeof C2, new>

const b: T2 = null;
b.prototype.foo(); // ok
b.prototype.bar; // ok
b.prototype.baz(); // ok
new a(); // error <<<
new a(''); // ok (return C1 with C2 prototype)
new a('').foo() // ok (from C1)
new a('').baz(); // ok (from C2) <<<

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

[weswigham]

Ahhh so you want to be able to pull the signatures out of a type.

[kiprasmel]

bump - need this.

I’d use the never type, but the auto-completions still show up for the property, and I don’t want that.

Also, once I want to create a new class & the parameter I want gone is not provided, typescript errors, because the parameter is required, even though it’s never…

I’d also like to be able to do something like:

export class UniqueLesson extends NonUniqueLesson implements Omit<NonUniqueLesson, "nonUniqueId"> {
	uniqueId: string;
}

to then explicitly create a unique id for the unique lesson.

BUT I still have the nonUniqueId auto-completion shenaningans etc.