When do we need constructors and super in Es6 classes?

constructors.js

/*
 * Each class has a .prototype property that by default contains a constructor
 * property that points back at the class itself.
 */
 
class Example {}
console.warn(Example === Example.prototype.constructor);

/*
 * We can use the constructor method when we want to set some defaults on a class.
 */
 
class Base {
  constructor() {
    // We use constructor to add a default property 'name' on the Example class.
    this.name = "A default name";
  }
}

console.warn(new Base().name);

/*
 * We need to invoke super() inside of a child class if we want to be able to use 'this'
 * inside of it's constructor method.
 */

// Example 1: child has no special properties to set

class Child1 extends Base {
  describe() {
    // We don't need a constructor, we're not setting any child class specific properties.
    // this.name comes from the parent.
    console.warn(this.name);
  }
}

new Child1().describe();

// Example 2: child wants to set own property without invoking super() in constructor
// this throws a SyntaxError: 'this' is not allowed before super()

class Child2 extends Base {
  constructor() {
    // super(); // will work!
    this.name = "I'm child 2";
  }
  describe() {
    console.warn(this.name);
  }
}

new Child2().describe();

// Example 3: we can use 'this' in a child class without a constructor.

class Child3 extends Base {
  sayName() {
    return "I'm child 3";
  }
  describe() {
    console.warn(this.sayName());
  }
}

new Child3().describe();

// All this and more, so well explained here: https://javascript.info/function-prototype