Parameter Properties

Add an access modifier on a constructor parameter to declare AND assign the field in one go.

2 min read Level 1/5 #typescript#classes#parameter-properties

What you'll learn

Use parameter properties to shorten constructors
Combine with `readonly` for immutable fields

Writing a class with a few fields is repetitive. Parameter properties collapse it.

The Long Way

class Point {
  x: number;
  y: number;

  constructor(x: number, y: number) {
    this.x = x;
    this.y = y;
  }
}

Each field is declared, then assigned. Lots of repetition.

The Short Way

class Point {
  constructor(public x: number, public y: number) {}
}

const p = new Point(3, 4);
p.x;   // 3
p.y;   // 4

Adding public (or private / protected / readonly) on a constructor parameter tells TS:

Declare a field with that name and modifier
Assign the parameter to it on construction

One line — done.

With `readonly`

class User {
  constructor(
    public readonly id: string,
    public name: string,
    private email: string,
  ) {}
}

const u = new User("u1", "Ada", "ada@example.com");
u.id;       // "u1"
u.id = "x"; // ✗ readonly
u.email;    // ✗ private

Mix and match modifiers. Each parameter property is independent.

When Not To

If the constructor needs to do anything beyond assignment — validation, transformation, derived state — a regular field + explicit assignment is clearer. Parameter properties are for the straight pass-through case.

Up Next

Decorators — annotations that modify classes and methods.

Decorators →