Private parameters in Typescript

Question

I am learning Angular2 and working with classes in javascript first time.

What does the private parameter and why it couldn't be simply heroService: HeroService ?

constructor(private heroService: HeroService) { }

Answer 1

Looks like a parameter property. Basically, adding an access modifier (public/private/protected/readonly) to a constructor parameter will automatically assign that parameter to a field of the same name.

Specifically, from those docs:

TypeScript offers special syntax for turning a constructor parameter into a class property with the same name and value. These are called parameter properties and are created by prefixing a constructor argument with one of the visibility modifiers public, private, protected, or readonly. The resulting field gets those modifier(s)

So the following are equivalent:

class Foo {     private bar: string;     constructor(bar: string) {         this.bar = bar;     } }  class Foo {     constructor(private bar: string) {} }