How To Use Interfaces in TypeScript

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Introduction

TypeScript is an extension of the JavaScript language that uses JavaScript’s runtime with a compile-time type checker.

TypeScript offers multiple ways to represent objects in your code, one of which is using interfaces. Interfaces in TypeScript have two usage scenarios: you can create a contract that classes must follow, such as the members that those classes must implement, and you can also represent types in your application, just like the normal type declaration. (For more about types, check out How to Use Basic Types in TypeScript and How to Create Custom Types in TypeScript.)

You may notice that interfaces and types share a similar set of features; in fact, one can almost always replace the other. The main difference is that interfaces may have more than one declaration for the same interface, which TypeScript will merge, while types can only be declared once. You can also use types to create aliases of primitive types (such as string and boolean), which interfaces cannot do.

Interfaces in TypeScript are a powerful way to represent type structures. They allow you to make the usage of those structures type-safe and document them simultaneously, directly improving the developer experience.

In this tutorial, you will create interfaces in TypeScript, learn how to use them, and understand the differences between normal types and interfaces. You will try out different code samples, which you can follow in your own TypeScript environment or the TypeScript Playground, an online environment that allows you to write TypeScript directly in the browser.