Introduction to Classes

A class defines a reusable blueprint for constructing objects with shared methods and per-instance state.

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

TypeScript supports JavaScript classes and adds compile-time checking for properties, constructor parameters, and method contracts. Calling new allocates an instance; class type annotations do not add extra runtime enforcement to its fields.

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 Greeter {
  constructor(public message: string) {}
  greet(): string { return `Hello, ${this.message}`; }
}
const first = new Greeter("TypeScript");
console.log(first.greet());
console.log(first instanceof Greeter);

Verified output

Hello, TypeScript
true

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.

new Greeter stores TypeScript on the instance. greet reads this.message and creates a greeting. instanceof is a genuine JavaScript runtime check against the Greeter constructor.

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 confuse a class definition with one instance. Each call to new can create separate property values.

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 Counter class with a start value and a method returning the current count.

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.

Introduction to Classes: A class defines a reusable blueprint for constructing objects with shared methods and per-instance state.

new Greeter stores TypeScript on the instance. greet reads this.message and creates a greeting. instanceof is a genuine JavaScript runtime check against the Greeter constructor.

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