A variable is a data container with a name and a value. The value is the content you want to store. The name is a key to accessing this content. Variables are stored in the data container, which is in the JS section of the template:

data: {
  userName: 'Steve Allen',
  itemsInCart: 2,
  isUserLoggedIn: true

The syntax is fairly simple:

  • The name and the value of a variable are separated by a colon and a space.
  • Variables are separated from each other by a comma and a line break.

Naming variables

You are free to choose names for variables yourself, but there are a couple rules to consider:

  • A name can include letters, numbers, and underscores.
  • A name cannot start with a number.
  • You cannot use one of the words reserved for JavaScript purposes as a name.

A good name is a balance between clarity and length so that a variable doesn’t take up too much space in your code yet allows you to understand what to expect from its contents. Also, camel case is used to name variables in this course, but you may adopt another style if you like.

data: {
  5door: true,          // Invalid name: starts with a number.
  new: 'girl',          // Invalid name: ‘new’ is a reserved word.
  new-items: 2,         // Invalid name: has a special symbol ‘-’.
  newItems: 0           // Good name: brief, clear and readable.

Also, note that name are case sensitive. This means that newItems and newitems are two different names in JavaScript.

Data types

There are different types of data that variables can store, and each type has unique abilities and behavior. This article covers a group of types called primitives, which include strings, numbers, booleans and null.


The string data type is basically a text value. It’s used to store names, states, emails, titles, even paragraphs. String values are enclosed in single or double quotation marks.

userName: 'Steve Allen',
currentScreen: 'Dashboard',
productDescription: 'Lorem ipsum dolor sit amet consectetur adipisicing elit.',
usersReply: ''

Quotation marks with nothing in between them represent an empty string. It’s used, for example, for inputs that take a text value without providing a default text value.

Single quotation marks are used for string values in this course. If a single quotation mark is part of your string, you have to add a backslash in front of the character:

callToAction: 'Let\'s go'

The backslash tells browsers that the symbol that follows doesn’t mark the end of a string and that it is a part of the string’s content. One good idea is to use proper curly quotes and apostrophes in your text unless you really need straight marks.


A number is a data type used for storing numeric values:

itemsInCart: 0,
pocketMoney: 3.5,
degreesOutside: -15

A number in JavaScript can be positive, negative, or zero. It can also be a whole number or a decimal.

Often, you need numbers to store meaningful amounts: items, currencies, weights, etc. Users expect to see values like $5, -15°C, and 15 kg in the UI, but you should store only numeric values in variables of type number if you want to perform mathematical operations on these values. If you don’t need to do that, you should use a string instead:

degreesOutside: -15,       // valid number
degreesOutside: '-15°C',   // valid string
degreesOutside: -15°C      // invalid value

The next article explains how to add symbols to numbers that store only numeric values in your UI.


Some decisions in an interactive UI rely on a straightforward answer to a yes-no question:

  • Is a user logged in? If yes, show the home page. If no, show the login page.
  • Did a user agree to the privacy policy? If no, disable the confirmation button.
  • Did a user press Delete? If yes, show confirmation dialog.

There is a special data type for storing these kinds of answers, and it’s called a boolean. It’s value can be either true or false:

isUserLoggedIn: false,
doShowConfirmationDialog: true,
didAcceptAgreement: true

Even though boolean values look like proper English words they are written without quotation marks. This allows browsers to understand that they are not strings.

The Conditions section of the course goes into the details of using the boolean data type to show and hide containers and to apply conditional styles.


Sometimes it’s useful to indicate that a variable doesn’t have any value at all. Usually you do it by defining the variable as an empty string (for text) or zero (for numbers). But there’s also a special data type, null, that serves the same purpose:

userName: '',       // empty string
itemsInCart: 0,     // zero
listOfGuests: null  // null

Just like boolean values, null is a reserved word, so it is written without quotation marks. On the surface, the difference between an empty string and null or between 0 and null is almost non-existent, and null is not used in this course at all. However, when you see null in someone else’s code or start learning about functions in detail, treat null as an absence of value and consider learning more about its behavior.