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πŸ““ Constructor and Prototype Methods

Now that we have an understanding of JavaScript objects, constructors, and prototypes, let's put them to work! In the next few lessons, we'll build an address book app to store contact info for our friends.

Take note that you do not have to code along with these lessons, but you can if you want to. The first classwork of this section will prompt you to recreate this project.

Since each contact will have multiple properties, we will use Contact objects to encapsulate their data. And because all Contacts should have the same combination of properties (name, phone number, etc.) we'll create a constructor that can quickly craft many different Contact objects with this same structure.

We'll focus only on the business logic for now. Then we'll build code for the user interface together in upcoming lessons.

Project Setup​

First, let's create a project directory called address-book. It will contain a js subdirectory to house JavaScript logic, with a single JavaScript file called scripts.js inside. Like all projects, we'll also include a

Our project structure should look like this:

β”œβ”€β”€ js
β”‚Β Β  └── scripts.js

Again, we'll wait to add user interface logic (that is, the HTML and JavaScript that creates the user-facing portion of the app) until after we've written our business logic.

Adding the Contact Constructor​

We'll start by building a simple Contact constructor in scripts.js:

function Contact(firstName, lastName, phoneNumber) {
this.firstName = firstName;
this.lastName = lastName;
this.phoneNumber = phoneNumber;

Let's test this code in the DevTools console! Open the DevTools console on any webpage, and then copy/paste the constructor above into the console. Then, recreate our instance of Contact by running the following:

> let testContact = new Contact("Ada", "Lovelace", "808-555-1111");

If we check the value of testContact (by typing testContact; into the console, and hitting Enter), we'll see the following response appear:

> testContact;
ContactΒ {firstName: "Ada", lastName: "Lovelace", phoneNumber: "503-555-1111"}

This means that we've successfully created a Contact object type with the Contact constructor function, and we've used it to create one Contact instance.

Adding a method to the Contact Prototype​

Next, let's create a simple prototype method to call on Contact objects. Let's say we want a Contact.prototype.fullName() method to return the contact's first and last name concatenated together.

We can define a new prototype method in our scripts.js file like this:

function Contact(firstName, lastName, phoneNumber) {
this.firstName = firstName;
this.lastName = lastName;
this.phoneNumber = phoneNumber;

Contact.prototype.fullName = function() {
return this.firstName + " " + this.lastName;

By the way, if you still have any confusion about semicolons, let's go over this again one more time because the example above illustrates the difference with defining functions. In the first example, we begin with the function keyword. Functions don't have semicolons after the closing curly brace.

However, in the second function, we are assigning a function expression to a property of the Contact.prototype. It's like assigning a value to a variable. That means we add a semicolon to the end.

It's all a bit arbitrary, really, since JavaScript doesn't really care. As we mentioned in the Introduction to Programming course, the JavaScript interpreter will automatically insert semicolons where needed so our machines can properly read our code. However, the interpreter isn't perfect and there are some fairly obscure situations where JavaScript can get things wrong. Beginners can definitely run into these situations and it's difficult to debug if you don't know what's happening. That's why we are adding semicolons instead of having JavaScript do it automatically β€” because it's hard for beginners to know those weird situations when JavaScript won't do it automatically for us.

By the way, many companies (one example is Airbnb) require semicolons in their consistency standards while many others don't. While we require semicolons and consistency at Epicodus, you may not be using semicolons at a future job.

We can copy/paste the contents of scripts.js into the DevTools console again and create another sample Contact by invoking our constructor with the following code:

> let testContact = new Contact("Ada", "Lovelace", "503-555-1111");

After that, we can call our new method in the console like this:

> testContact.fullName();
"Ada Lovelace"

And as we can see, it returns the object's firstName and lastName properties concatenated together. Now any existing or future Contact instances will have access to the Contact.prototype.fullName() method.

Now that we've created a constructor for Contact and a simple prototype method that can be called on Contact instances, let's move on to constructing the address book itself.

Example GitHub Repo for the Address Book