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Parameter Properties

TypeScript has a neat shorthand feature called parameter properties that allows you to declare and initialize class properties directly in the constructor parameters. This eliminates the need to separately declare properties and then assign them in the constructor body.

Without Parameter Properties

Normally, you'd write a class like this:

class Hero {
  name: string;
  health: number;
  private level: number;

  constructor(name: string, health: number, level: number) {
    this.name = name;
    this.health = health;
    this.level = level;
  }
}

With Parameter Properties

With parameter properties, you can achieve the same result with much less code:

class Hero {
  constructor(
    public name: string,
    public health: number,
    private level: number,
  ) {}
}

In the example above, the constructor body {} is intentionally empty because parameter properties handle declaration and initialization. Other class methods should be defined in the class body, outside the constructor.

By adding an access modifier (public, private, protected, or readonly) to a constructor parameter, TypeScript automatically:

Declares a property with the same name and type
Assigns the parameter value to that property

Limitation

Parameter properties work with TypeScript's private keyword, but not with JavaScript's # private field syntax. If you need truly private fields using the # syntax, you must declare them separately:

class Hero {
  #secretPower: string;

  constructor(
    public name: string,
    secretPower: string,
  ) {
    this.#secretPower = secretPower;
  }
}

Assignment

We're building an ExecutiveMember class for our premium customers. Use parameter properties to make the code more concise.

Create the ExecutiveMember class using parameter properties in the constructor for balance and points (both should be protected)
Add an isVIP method that returns true if the balance is at least 10_000.
Add a redeemPoints method that accepts a number of points to redeem. If the member has enough points, reduce their points by that amount and return true. Otherwise, return false.