Constructors

A constructor runs when new creates an instance, letting you initialize required state.

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

A constructor may receive typed arguments and validate them at runtime. TypeScript enforces definite property initialization in strict mode, but those checks do not replace actual validation of untrusted JavaScript values.

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 Rectangle {
  width: number;
  height: number;
  constructor(width: number, height: number) {
    if (width <= 0 || height <= 0) throw new Error("Positive sides required");
    this.width = width;
    this.height = height;
  }
  area(): number { return this.width * this.height; }
}
console.log(new Rectangle(4, 6).area());

Verified output

24

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 verifies 4 and 6, then writes both instance properties. The area method multiplies them to produce 24. Invalid nonpositive inputs would throw before a valid instance is returned.

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 leave a required field uninitialized under strictPropertyInitialization or assume a type annotation assigns its value.

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 perimeter method, then try an invalid negative width and catch the runtime error.

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.

Constructors: A constructor runs when new creates an instance, letting you initialize required state.

The constructor verifies 4 and 6, then writes both instance properties. The area method multiplies them to produce 24. Invalid nonpositive inputs would throw before a valid instance is returned.

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