Class Properties

Class properties describe the state stored on an object, with types controlling valid assignments in checked TypeScript source.

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

An initializer such as count = 0 supplies a runtime initial value and often gives TypeScript enough information to infer number. Instance properties are different from static fields, which live on the constructor.

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

class Basket {
  items: string[] = [];
  add(item: string): void { this.items.push(item); }
  size(): number { return this.items.length; }
}
const basket = new Basket();
basket.add("apple");
basket.add("pear");
console.log(basket.size());
console.log(basket.items.join(", "));

Verified output

2
apple, pear

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.

Each Basket starts with its own empty array. add pushes apple and pear into that array. size reads length 2; join lists the two items in insertion order.

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 use a shared static array when you intend every Basket instance to keep independent item lists.

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 a removeLast method and compare two separately created baskets.

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.

Class Properties: Class properties describe the state stored on an object, with types controlling valid assignments in checked TypeScript source.

Each Basket starts with its own empty array. add pushes apple and pear into that array. size reads length 2; join lists the two items in insertion order.

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