Parameter Properties

A TypeScript parameter property combines a constructor parameter and a class property declaration when an access modifier or readonly appears before the parameter name.

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

The shorthand assigns the argument to the instance automatically. Ordinary unmodified constructor parameters are local inputs and do not become properties just because they are named in the constructor.

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 Product {
  constructor(public name: string, readonly price: number) {}
  describe(): string { return `${this.name}: ${this.price}`; }
}
const product = new Product("Pencil", 12);
console.log(product.describe());
console.log(product.name);

Verified output

Pencil: 12
Pencil

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 public and readonly modifiers each declare and initialize a property. The constructor stores Pencil and 12. describe interpolates these properties; the public name is readable outside the class.

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.

Without a parameter-property modifier, constructor(name: string) does not automatically establish this.name.

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 private readonly sku: string to the constructor and expose a public method that prints the SKU.

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.

Parameter Properties: A TypeScript parameter property combines a constructor parameter and a class property declaration when an access modifier or readonly appears before the parameter name.

The public and readonly modifiers each declare and initialize a property. The constructor stores Pencil and 12. describe interpolates these properties; the public name is readable outside the class.

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