Type Aliases vs Interfaces

Both type aliases and interfaces can describe object shapes. Type aliases also name unions, primitive types, and tuples; interfaces support declaration merging and explicit extends clauses.

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

For ordinary object contracts, choosing between an interface and a type alias is often a team-style decision. They are structurally checked, not separate nominal categories. Neither creates a runtime object or validator.

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

type Code = string | number;
interface Item { id: Code; label: string; }
const first: Item = { id: 7, label: "Book" };
const second: Item = { id: "A-1", label: "Pen" };
console.log(first.label, second.id);

Verified output

Book A-1

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.

Code allows either string or number. Item uses that alias in an interface property. Both objects satisfy the contract and print Book A-1.

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.

Do not believe interfaces can directly represent primitive union types. Also avoid suggesting that interface implementation is enforced during JavaScript execution.

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.

Rewrite Item as a type alias using an object literal. Then try to represent Code using an interface alone and explain the limitation.

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.

Type Aliases vs Interfaces: Both type aliases and interfaces can describe object shapes.

Code allows either string or number. Item uses that alias in an interface property. Both objects satisfy the contract and print Book A-1.

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