Nested Objects

Nested objects model values containing other records. Their types mirror the hierarchical property structure.

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

Access nested fields with chained property expressions and use optional chaining if intermediate objects may be absent. A shallow copy of the outer object still shares nested object references; copying deeply requires explicit care.

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 Address = { city: string; postalCode: string };
type Customer = { name: string; address: Address };
const customer: Customer = {
  name: "Amir",
  address: { city: "Hyderabad", postalCode: "500001" }
};
console.log(customer.address.city);
console.log(customer.address.postalCode);

Verified output

Hyderabad
500001

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.

customer.address selects the nested Address object. Reading city produces Hyderabad and postalCode produces the string 500001.

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.

Postal codes are often better represented as strings so leading zeros are preserved. Do not assume a shallow object spread recursively copies address.

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.

Add an optional apartment field to Address and print it with a fallback. Try a shallow copy and change address.city.

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.

Nested Objects: Nested objects model values containing other records.

customer.address selects the nested Address object. Reading city produces Hyderabad and postalCode produces the string 500001.

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