JavaScript Object Constructors

Object constructors are an important aspect of object oriented programming. At first, these constructors can be difficult to understand because they are a type of function and not their own separate thing. The first step in understanding how they work is to first learn how to use dot notation to access values in an object. Here is a simple example of dot notation on an object to access the value of the color property.

let bird = {
  colour: "brown",
};

console.log(bird.colour);

// output: brown

To add properties to this object we can use dot notation to name it then provide it with a value using the assignment operator.

bird.numLegs = 2;

Now now the bird object also contains the numLegs property.

Object constructors

Dot notation is used in object constructors as well, however it is done by reference using the this key word. Let’s take a look at a constructor function.

let Bird = function () {
  (this.colour = "brown"), (this.numLegs = 2);
};

let brownBird = new Bird();

console.log(brownBird.numLegs);

// output: 2

The brownBird variable contains the function Bird function. The this key word is referencing the variable brownBird which is then provided the key value pairs defined in the Bird function(object constructor). To visualize this, imagine the this keyword is the brownBird variable. It’s setting a new property just like it would using regular dot notation. It’s important to understand that the brownBird variable contains a function but also consider that functions are also objects. This means you can access certain properties defined inside of them. Pretty neat right!

The Bird function can also be changed to also include parameters to define dynamic values of the properties defined within the function.

let Bird = function (colour, numLegs) {
  (this.colour = colour), (this.numLegs = numLegs);
};

let brownBird = new Bird("purple", 2);

console.log(brownBird.colour);

// output: purple

There is no limit to what can be done using constructor functions. Once this is learned it is easier to understand how prototypes work but that’s for another article.