Creating Classes and Objects

Instantiate a class with new to create separate objects that share behavior but can hold different values.

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

Class methods ordinarily live on the prototype and are shared rather than copied into every instance. Instance fields belong to each object. Two variables can still refer to the same instance when you assign one reference to the other.

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 Student {
  constructor(public name: string, public score: number) {}
  passed(): boolean { return this.score >= 50; }
}
const asha = new Student("Asha", 81);
const ravi = new Student("Ravi", 44);
console.log(asha.name, asha.passed());
console.log(ravi.name, ravi.passed());

Verified output

Asha true
Ravi false

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.

The constructor runs twice, creating two distinct student instances. Their own names and scores differ, while both call the same passed method logic.

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 class variable does not magically copy another instance. Assigning const other = asha creates a second reference to the same object.

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 two Book instances with different titles and page counts and compare their method output.

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.

Creating Classes and Objects: Instantiate a class with new to create separate objects that share behavior but can hold different values.

The constructor runs twice, creating two distinct student instances. Their own names and scores differ, while both call the same passed method logic.

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