I am trying to understand interface topic in Typescript when I came across Class type, I got this code from official docs

interface ClockConstructor {
    new (hour: number, minute: number);
}

class Clock implements ClockConstructor {
    currentTime: Date;
    constructor(h: number, m: number) { }
}

I can understand that Clock has no match for the signature new (hour: number, minute: number); that's why we get an error there.

But in docs the explaination is something which I am unable to understand. It goes in this way :

This is because when a class implements an interface, only the instance side of the class is checked. Since the constructor sits in the static side, it is not included in this check.

Any explanation would be appreciated.

I am trying to understand interface topic in Typescript when I came across Class type, I got this code from official docs

interface ClockConstructor {
    new (hour: number, minute: number);
}

class Clock implements ClockConstructor {
    currentTime: Date;
    constructor(h: number, m: number) { }
}

I can understand that Clock has no match for the signature new (hour: number, minute: number); that's why we get an error there.

But in docs the explaination is something which I am unable to understand. It goes in this way :

This is because when a class implements an interface, only the instance side of the class is checked. Since the constructor sits in the static side, it is not included in this check.

Any explanation would be appreciated.

• javascript
• typescript

• I would remend reading this first: stackoverflow./a/58400909/5290519. – NeoZoom.lua Commented May 14, 2024 at 4:04

2 Answers 2

The interface declares the method/members the instances must have, not including what the implementing class has.

For example check the Array and ArrayConstructor declarations:

interface Array<T> {
    length: number;
    toString(): string;
    toLocaleString(): string;
    push(...items: T[]): number;
    pop(): T | undefined;
    ...
    [n: number]: T;
}

interface ArrayConstructor {
    new (arrayLength?: number): any[];
    new <T>(arrayLength: number): T[];
    new <T>(...items: T[]): T[];
    (arrayLength?: number): any[];
    <T>(arrayLength: number): T[];
    <T>(...items: T[]): T[];
    isArray(arg: any): arg is Array<any>;
    readonly prototype: Array<any>;
}

As you can see, the Array has method/members which exist on any instance of array:

let a = [];
a.push(1, 2, 3);
console.log(a.length);

But the ArrayConstructor has the members/methods which exist on the Array itself:

console.log(Array. prototype);
console.log(Array.isArray(9));

The constructors are part of the "static" part which is why they are declared in the ArrayConstructor.
If you declare a constructor on an interface for example you'll have a problem implementing that interface:

interface MyInterface {
    constructor();
    getName(): string;
}

class MyClass implements MyInterface {
    constructor() {}
    
    getName() { return "name" };
}

Error:

Class 'MyClass' incorrectly implements interface 'MyInterface'. Types of property 'constructor' are inpatible. Type 'Function' is not assignable to type '() => void'. Type 'Function' provides no match for the signature '(): any'.

Before you can get an instance you need to use the static side, the constructor, to get an instance. You don't need new in your interface anyway, your class is typed itself so typescript knows whatever arguments it has to pass along the constructor.

You can make benefits of an interface with type new if you want to pass a function or class that has to meet certain constructor requirements before it can be instantiated.

interface IFoo {
   new(title: string);
}

function MyFunction(ctor: IFoo, title:string) {
   return new ctor(title);
}

class MyClass {
   constructor(public title: string) {}
}

class MySecondClass {
   constructor(public title: string) {}
}

var myClass = MyFunction(MyClass, 'title');
var mySecondClass = MyFunction(MySecondClass, 'title');

console.log(myClass.title, mySecondClass.title);

In fact, a TypeScript class is a regular function in JavaScript which is static when you don't use new in front of it. This is were the docs are referring to.

// static side
function Person() {

}

Person.SayHi = function () {
  return 'Hello';
}

console.log(Person.SayHi()); // static function..

var person = new Person() // instance side

See also this answer