Kotlin vs TS Classes

Classes in Kotlin Versus Classes in TypeScript

TypeScript's parameter properties look like Kotlin's primary constructor, but the runtime underneath is JavaScript. Here is the same Book, compiled with TypeScript 7.0.2 (tsc --strict) and run with Node 22:

BookNest's Book as a TypeScript class (book.ts)TypeScript

class Book {
  constructor(public readonly id: number, public readonly title: string,
              public readonly price: number) {}
}
const a = new Book(1, "The Quiet Harbor", 14.99);
const b = new Book(1, "The Quiet Harbor", 14.99);
console.log(a === b, JSON.stringify(a) === JSON.stringify(b), `${a}`);
const sale = { ...a, price: 11.99 };                       // nearest thing to copy()
const fake: Book = { id: 9, title: "Not a Book", price: 0 };  // structural typing
console.log(sale instanceof Book, fake instanceof Book);

Output

false true [object Object]
false false

JavaScript has no value equality for objects, so you compare serialized forms or hand-write equals(). The spread copy is a plain object that has lost its prototype, and tsc accepts fake as a Book because TypeScript types are structural: any object of the right shape fits. Kotlin types are nominal, so only a real Book is a Book. TypeScript's private also vanishes at run time; only #field is enforced there.

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