[Javascript] ES6 Class Constructors and the Super Keyword

Posted 2020-11-24 Answer1215

When the ES6 class shipped back in 2015, a number of additional keywords came with it. Two of these are constructor and super. Both of these are specific to the class keyword and make working with classes manageable. Both are utilized when the new keyword is used to create a new instance of a class. constructors are called initially with the new keyword and super is how a subclass can utilize it‘s parent‘s methods (like it‘s parent‘s constructor function) within that child class

 

You have to call super() function when you extends one class:

class Rectangle {
  constuctor(height, width) {
    this.name = ‘Rectangle‘
    this.height = height
    this.width = width
  }
}

class Square extends Rectangle {
  constructor(length) {
  super(length, length)
  this.name = ‘Square‘
  }
}

const myShape = new Square(1)

console.log(myShape)

 

Again, even if our rectangle class did not have a constructor, we‘d still need to call super within the square‘s constructor because it is required when we‘re working with subclasses that have constructors and when the this keyword is used in the constructor.

class Rectangle {

}

class Square extends Rectangle {
  constructor(length) {
     super()
     this.name = ‘Square‘
  }
}