When to create constructor in Angular2 typescript?

Constructors define which parameters to provide when instantiate your objects. In TypeScript, you can also add modifiers like private or public to define in the same time class properties and set their values with the provided ones.

For example:

class Car {
  constructor(private engine:string, private tires:string, private doors:number){
  }
}

is similar to:

class Car {
  constructor(engine:string, tires:string, doors:number){
    this.engine = engine;
    this.tires = tires;
    this.doors = doors;
  }
}

In Angular2 constructors are used for dependency injection. The associated decorator (@Component or Injectable) collects metadata (types or hints within @Inject) to determine what to provide to the object to instantiate.

Be aware that constructors aren't part of the component lifecycle. Properties can be set later by Angular2 at this level