I have an interface and class defined like this:

interface Foo {
  constructor: typeof Foo;
}

class Foo {
  static bar = 'bar';

  constructor(data: Partial<Foo>) {
    Object.assign(this, data);
  }

  someMethod() {
    return this.constructor.bar;
  }

  prop1: string;
  prop2: number;
}

The interface is necessary so that this.constructor is strongly typed. However, it breaks my ability to pass a plain object into the class constructor:

const foo = new Foo({ prop1: 'asdf', prop2: 1234 });

// Argument of type '{ prop1: string; prop2: number; }' is not assignable to parameter of type 'Partial<Foo>'.
//  Types of property 'constructor' are inpatible.
//    Type 'Function' is not assignable to type 'typeof Foo'.
//      Type 'Function' provides no match for the signature 'new (data: Partial<Foo>): Foo'.

I understand the error message, but I don't know a way around it. Is there any way have a Partial<Foo> which allows me to pass a plain object? Here's a playground:

Playground

I have an interface and class defined like this:

interface Foo {
  constructor: typeof Foo;
}

class Foo {
  static bar = 'bar';

  constructor(data: Partial<Foo>) {
    Object.assign(this, data);
  }

  someMethod() {
    return this.constructor.bar;
  }

  prop1: string;
  prop2: number;
}

The interface is necessary so that this.constructor is strongly typed. However, it breaks my ability to pass a plain object into the class constructor:

const foo = new Foo({ prop1: 'asdf', prop2: 1234 });

// Argument of type '{ prop1: string; prop2: number; }' is not assignable to parameter of type 'Partial<Foo>'.
//  Types of property 'constructor' are inpatible.
//    Type 'Function' is not assignable to type 'typeof Foo'.
//      Type 'Function' provides no match for the signature 'new (data: Partial<Foo>): Foo'.

I understand the error message, but I don't know a way around it. Is there any way have a Partial<Foo> which allows me to pass a plain object? Here's a playground:

Playground

• javascript
• typescript
• derived-types

• Not sure what's your intention, seems like you trying to access static property from instance, that's not possible, also interface is redundant, checkout this – Medet Tleukabiluly Commented Oct 25, 2019 at 17:03
• Thanks Eric. You can access static properties from an instance via this.constructor, and it is not unmon to do so. The redundant interface is also necessary because you cannot type the constructor method on a class definition. This uses "interface merging", which is a feature of TypeScript and is also not unmon: github./microsoft/TypeScript/issues/… – Ryan Wheale Commented Oct 25, 2019 at 20:07

3 Answers 3

Here's the actual type creating a derived type from a class omitting the constructor (as in the question title) and keeping regular methods:

type NonConstructorKeys<T> = ({[P in keyof T]: T[P] extends new () => any ? never : P })[keyof T];
type NonConstructor<T> = Pick<T, NonConstructorKeys<T>>;

Usage with the Foo from the question:

type FooNonConstructorKeys = NonConstructorKeys<Foo>; // "prop1" | "prop2" | "someMethod"
type FooNonConstructor = NonConstructor<Foo>;

I ended up finding what I needed in this wonderful answer:

how to remove properties via mapped type in TypeScript

The code in that answer creates a derived type containing only methods. I needed to do the inverse of that. The following NonMethods<T> helper creates a derived type with all of the methods removed.

type NonMethodKeys<T> = ({[P in keyof T]: T[P] extends Function ? never : P })[keyof T];  
type NonMethods<T> = Pick<T, NonMethodKeys<T>>; 

Here's the Playground

Looks like you want to define an interface and then use it, then you have to define the properties in the interface itself and not the class.

interface Foo {
  prop1: string; // define your properties here
  prop2: number;
}

class Foo {
  static bar = 'bar';

  constructor(data: Partial<Foo>) {
    Object.assign(this, data);
  }

  someMethod() {
    return Foo.bar; // notice how I access static variables now
  }

}

const foo = new Foo({ prop1: 'asdf', prop2: 1234 });

Playground