Parameter properties

Parameter properties

In our last example, we had to declare a readonly member name and a constructor parameter theName in the Octopus class, and we then immediately set name to theName. This turns out to be a very common practice. Parameter properties let you create and initialize a member in one place. Here’s a further revision of the previous Octopus class using a parameter property:

class Octopus {
  readonly numberOfLegs: number = 8;
  constructor(readonly name: string) {
  }
}

Notice how we dropped theName altogether and just use the shortened readonly name: string parameter on the constructor to create and initialize the name member. We’ve consolidated the declarations and assignment into one location.

Parameter properties are declared by prefixing a constructor parameter with an accessibility modifier or readonly, or both. Using private for a parameter property declares and initializes a private member; likewise, the same is done for public, protected, and readonly.