Generic Classes

A generic class stores and operates on values of a selected type, such as a typed stack.

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

The parameter T links a class’s fields, method inputs, and method outputs. Instances can be created for different types, but the emitted JavaScript class itself is not cloned per type argument.

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 Box<T> {
  constructor(private value: T) {}
  get(): T { return this.value; }
  set(value: T): void { this.value = value; }
}
const box = new Box<number>(5);
box.set(12);
console.log(box.get() * 2);
const textBox = new Box("ready");
console.log(textBox.get().toUpperCase());

Verified output

24
READY

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.

box is configured for number, so set(12) is accepted and get returns 12; doubling gives 24. textBox infers string from ready and returns READY after uppercasing.

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 expect new Box<number>(5) to perform automatic runtime rejection when a JavaScript caller later passes a string.

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 Store<T> that maintains a list of values and returns the last value or undefined.

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.

Generic Classes: A generic class stores and operates on values of a selected type, such as a typed stack.

box is configured for number, so set(12) is accepted and get returns 12; doubling gives 24. textBox infers string from ready and returns READY after uppercasing.

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