Why is it necessary to merge an interface with a function when using the new keyword?

Question

Why is it necessary to merge an interface with a function when using the new keyword?

Assuming that the setting noImplicityAny is enabled.

Consider the following:

function Bar() {
    this.f = 100;
}

The following code snippet will not function as expected:

let x = new Bar();

An error message will be displayed:

'new' expression, whose target lacks a construct signature, implicitly has an 'any' type.

While it may seem understandable, adding an interface changes things:

interface Bar { 
     f: number;
}

function Bar() {
    this.f = 100;
}

let x = new Bar();
console.log(x.f);

When inspecting the type of Bar, you'll notice:

interface Bar function Bar(): void

In this case, there seems to be some merging occurring. The reason for this change in behavior remains uncertain. Why does this merged type work now and why is the implied return type no longer any?

typescript

Answer 1

Answer №1

The concept you're encountering is known as Declaration Merging in TypeScript. Essentially, when two entities share the same name in a declaration, TypeScript combines them into a single entity. This feature allows you to create declarations and interfaces for external JavaScript libraries to provide type information.

In your initial scenario, the compiler can determine the return type of Bar() as void. However, it lacks knowledge about what the constructor returns, leading new Bar() to be inferred as any.

By introducing an interface with the same name, the compiler merges the interface Bar with function Bar(), resulting in the interface defining a structure resembling the function. As a result, the compiler recognizes this interface as the type of new Bar(), while keeping the type of Bar() as void.

Answer 2

The concept you're encountering is known as Declaration Merging in TypeScript. Essentially, when two entities share the same name in a declaration, TypeScript combines them into a single entity. This feature allows you to create declarations and interfaces for external JavaScript libraries to provide type information.

In your initial scenario, the compiler can determine the return type of Bar() as void. However, it lacks knowledge about what the constructor returns, leading new Bar() to be inferred as any.

By introducing an interface with the same name, the compiler merges the interface Bar with function Bar(), resulting in the interface defining a structure resembling the function. As a result, the compiler recognizes this interface as the type of new Bar(), while keeping the type of Bar() as void.

Answer 3

Answer №2

The TypeScript type system recognizes this as a constructor function that can be instantiated.

Instead of using separate function and interface declarations, you have the option to define it within a single class.

class Foo {
  num = 42;
} 

let instance = new Foo();
console.log(instance.num);

If your target is ES5, the resulting code will closely resemble the constructor function outlined in your query.

Answer 4

The TypeScript type system recognizes this as a constructor function that can be instantiated.

Instead of using separate function and interface declarations, you have the option to define it within a single class.

class Foo {
  num = 42;
} 

let instance = new Foo();
console.log(instance.num);

If your target is ES5, the resulting code will closely resemble the constructor function outlined in your query.

Why is it necessary to merge an interface with a function when using the new keyword?

Answer №1

Answer №2

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