Extending Interfaces

An interface can extend one or more other interfaces to require a larger shape while reusing shared property contracts.

What you will learn: the purpose of extending interfaces, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Extension works at the type level: it does not copy runtime properties into objects. Extended members must be mutually compatible, and extending multiple interfaces with incompatible definitions is an error.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

interface Identified { id: number; }
interface Named { name: string; }
interface Member extends Identified, Named { active: boolean; }
const member: Member = { id: 12, name: "Tara", active: true };
console.log(`${member.id} ${member.name} ${member.active}`);

Verified output

12 Tara true

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

Member includes requirements from both parents plus active. The object supplies all three fields, then console.log interpolates the stored values.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Extending an interface does not supply default values for inherited fields; object values still have to provide them.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Define Shape with color, then Rectangle extends Shape with width and height. Create a valid Rectangle value.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Extending Interfaces: An interface can extend one or more other interfaces to require a larger shape while reusing shared property contracts.

Member includes requirements from both parents plus active. The object supplies all three fields, then console.log interpolates the stored values.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.