How to Reopen an Interface in TypeScript

Updated: January 7, 2024 By: Guest Contributor Post a comment


TypeScript’s powerful type system includes an ability to extend interfaces. This capability is essential for creating flexible and reusable code. By understanding how to reopen interfaces, developers can incrementally shape and adapt their type definitions to various needs.

Introduction to Interfaces

In TypeScript, an interface declares the shape of data structures like objects and classes. It outlines the contract any entity should conform to if it promises to implement the said interface. Defining a basic interface is straightforward:

interface User {
    username: string;
    age: number;

But what if you later need to add new properties to this interface? TypeScript provides multiple mechanisms to extend or modify existing interfaces.

Extending Interfaces with Inheritance

One common approach to enhance an interface is by extending it. Below is how you would define a ‘Developer’ interface as a specialization of ‘User’:

interface Developer extends User {
    favoriteLanguage: string;

This not only adds favoriteLanguage but also keeps the original User properties.

Reopening Interfaces

Alternatively, TypeScript allows you to reopen an existing interface by declaring it a second time with new properties:

interface User {
    isPremiumMember: boolean;

This doesn’t overwrite the original User interface but adds isPremiumMember to it, benefiting from a feature often referred to as interface merging or declaration merging.

Merging Interfaces

When you declare an interface with the same name more than once, TypeScript merges their declarations. The end result is a single interface with combined properties. This feature is particularly useful in scenarios such as enhancing third-party type definitions without altering the original code base.

Example with Function Types

Merging isn’t limited to properties; function types in interfaces can also be extended, as you’ll see in the example below:

interface User {
    greet(message: string): void;

interface User {
    greet(message: string, date: Date): void;

After merging, User has an overloaded greet method, providing flexibility in how the function can be invoked.

Handling Conflicts in Merging

If two interfaces declare a property with the same name but different types, TypeScript will treat this as an error unless the types are compatible, in which case it performs further type inference.

interface User {
    score: number;

interface User {
    score: number | string;

The above code is valid since number is compatible with number | string. However, conflicting types would result in an error:

Error: Interface ‘User’ incorrectly extends interface. Types of property ‘score’ are incompatible.

Advanced Reopening Techniques

Beyond basic reopening and merging, one can also use generics and intersect types to create more elaborate abstractions. These techniques offer enhanced control and flexibility:

interface User<T> {
    data: T;

interface User<T> {
    logData(): T;

Now your User interface is not just reopened but is also generic. This enables a multitude of usage patterns catering to various data types.

Another advanced trick is using the intersection type:

type Admin = User & {
    hasAdminPrivileges: boolean;

An Admin, in this context, has all User properties, any additional extensions made to User, plus hasAdminPrivileges.

Best Practices

While reopening interfaces offers flexibility, it should be done judiciously to avoid maintenance nightmares. It’s generally best to:

  • Keep track of where and how you extend interfaces to maintain clarity.
  • Avoid extending third-party interfaces unless necessary—prefer wrapper patterns.
  • Use documentation comments to inform others why an extension was necessary.

Tools and Integration

Most modern TypeScript editors like Visual Studio Code provide IntelliSense support that recognizes merged interfaces and prevents misuse. Moreover, tools like TSLint can catch merging mishaps and prompt developers to fix potential errors preemptively.


Reopening TypeScript interfaces is a compelling feature that can significantly improve code adaptability. By responsibly leveraging this ability, your TypeScript codebase can evolve without becoming rigid or brittle. Always aim for interfaces that represent clear abstractions to ensure your codebase remains intuitive and maintainable.