Structural Typing

TypeScript primarily uses structural compatibility: a value matches a target type when it has the required members with compatible types.

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

The name used for the type is generally not what determines compatibility. This enables flexible object-oriented APIs, but it also means a value with extra properties can be assigned when all required properties exist. Structural typing is still static checking, not runtime inspection.

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 Named = { name: string };
function showName(item: Named): string {
  return item.name.toUpperCase();
}
const employee = { name: "Dev", department: "Sales" };
console.log(showName(employee));

Verified output

DEV

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.

employee has name: string, satisfying Named despite its extra department field. showName reads name and returns DEV.

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 assume a value must be declared with the exact target type name. Equally, do not treat a successful static check as proof that parsed external JSON has the right fields.

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 an interface HasId and pass two differently named object types with compatible id properties to one function.

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.

Structural Typing: TypeScript primarily uses structural compatibility: a value matches a target type when it has the required members with compatible types.

employee has name: string, satisfying Named despite its extra department field. showName reads name and returns DEV.

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