Constructor functions in javascript

Constructor functions are functions that are used to construct new objects. The new operator is used to create new instances based off a constructor function. There are some built-in JavaScript constructors, such as new Array() and new Date(), but we can also create our own custom templates from which to build new objects.

To begin, a constructor function is just a regular function. It becomes a constructor when it is called on by an instance with the new keyword. In JavaScript, we capitalize the first letter of a constructor function by convention.

function Hero(name, level) {
this.name = name;
this.level = level;
}

The this keyword will refer to the new instance that is created, so setting this.name to the name parameter ensures the new object will have a name property set.

Now we can create a new instance with new.

let hero1 = new Hero('Bjorn', 1);

Now if we get the [[Prototype]] of hero1, we will be able to see the constructor as Hero(). (Remember, this has the same input as hero1.__proto__, but is the proper method to use.)

Object.getPrototypeOf(hero1);

// Output

constructor: ƒ Hero(name, level)