Readonly parameter

Learn to use readonly in constructor parameters.

readonly parameter

readonly can also be written on a constructor parameter; this is a parameter property. The field is created automatically, becomes readonly, and is assigned the parameter value. You do not need to write this.x = x in the body.

class Point {
  constructor(readonly x: number, readonly y: number) {}
}
const p = new Point(3, 4);
console.log(p.x);

Long form equivalent

When not using a parameter property, you achieve the same result with the long form: declare the field with readonly and assign it with this in the constructor body. Both forms provide the same immutability.

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

Exercise Point class with readonly fields

Define a Point class with readonly x: number and readonly y: number fields. Add a constructor that takes x and y parameters and assigns them with this. Create a new Point(3, 4) and print the x field.

Show example solution

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

const p = new Point(3, 4);
console.log(p.x);

Test yourself

What happens when you write constructor(readonly apiKey: string)?
- Only the parameter becomes readonly
- The class gives an error
- apiKey becomes static
- The apiKey field is auto-created and becomes readonly
Show answer

The apiKey field is auto-created and becomes readonly — A readonly parameter property auto-defines the field and makes it readonly. It cannot be changed outside the constructor; you do not need to write this.apiKey = apiKey.

Complete the readonly field

class Point {
  ____ x: number;
  constructor(x: number) {
    this.x = x;
  }
}

Show answer

class Point {
  readonly x: number;
  constructor(x: number) {
    this.x = x;
  }
}

Readonly parameter parts

constructor(readonly x: number)

Shorthand (parameter property)
readonly x: number

Long form (field declaration)
this.x = x

Constructor assignment
p.x

External read

Next lesson: Readonly vs const

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