Objects in TypeScript

Objects group values under named properties. TypeScript checks an object’s shape, while JavaScript stores the actual properties and values.

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

Use dot notation for simple known property names and bracket notation for computed names. Object variables hold references to objects; changing a property through one alias can be observed through another alias. A const object binding does not make its properties readonly.

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

const book = { title: "Go Basics", pages: 180 };
console.log(book.title);
book.pages += 20;
console.log(book.pages);

Verified output

Go Basics
200

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.

book begins with two properties. The first log prints the title. The property update changes pages from 180 to 200; the second log prints 200.

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.

A declared const object can still have mutable properties. TypeScript checking does not add missing runtime properties automatically.

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.

Create a student object with name, grade, and active properties; update grade and print a message from the values.

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.

Objects in TypeScript: Objects group values under named properties.

book begins with two properties. The first log prints the title. The property update changes pages from 180 to 200; the second log prints 200.

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