Variables are Containers for Storing Data. JavaScript Variables can be declared in 4 ways:

• Automatically
• Using var
• Using let
• Using const

In this first example, x, y and z are undeclared variables. They are automatically declared when first used.

x = 5; 
y = 6; 
z = x+y; 

It is considered good programming practice to always declare variables before use.

From the examples you can guess:

• x stores the value 5
• y stores the value 6
• z stores the value 11

Example using var

var x = 5; 
var y = 6; 
var z = x+y; 

Note

• The var keyword was used in all JavaScript code from 1995 to 2015.
• The let and const keywords were added to JavaScript in 2015.
• The var keyword should only be used in code written for older browsers.

Example using let

let x = 5; 
let y = 6; 

let z = x+y; 

Example using const

const x = 5; 
const y = 6; 
const z = x+y; 

Mixed Example

const price1 = 5;
const price2 = 6;
let total = price1 + price2;

The two variables price1 and price2 are declared with the const keyword. These are constant values and cannot be changed. The variable total is declared with the let keyword. The value total can be changed.

When to Use var, let, or const?

1. Always declare variables
2. Always use const if the value should not be changed
3. Always use const if the type should not be changed (Arrays and Objects)
4. Only use let if you can't use const
5. Only use var if you MUST support old browsers.

Just Like Algebra

Just like in algebra, variables hold values:

let x = 5;
let y = 6;

Just like in algebra, variables are used in expressions:

let z = x+y; 

From the example above, you can guess that the total is calculated to be 11.

JavaScript Identifiers

All JavaScript variables must be identified with unique names. These unique names are called identifiers. Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume). The general rules for constructing names for variables (unique identifiers) are:

• Names can contain letters, digits, underscores, and dollar signs.
• Names must begin with a letter.
• Names can also begin with $ and _ (but we will not use it in this tutorial).
• Names are case sensitive (y and Y are different variables).
• Reserved words (like JavaScript keywords) cannot be used as names.

Note: JavaScript identifiers are case - sensitive.

The Assignment Operator

In JavaScript, the equal sign (=) is an “assignment” operator, not an “equal to” operator. This different from algebra. The following does not make sense in algebra:

x = x + 5 

In JavaScript, however, it makes perfect sense: it assigns the value of x + 5 to x. It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.

JavaScript Data Types

JavaScript variables can hold numbers like 100 and text values like “John Doe”. In programming, text values are called text strings. JavaScript can be handle many types of data, but for now, just think of numbers and strings. Strings are written inside double of single quotes. Number are written without quotes. If you put a number in quotes, it will be treated as a text string.

const pi = 3.14;
let person = "John Doe";
let answer = 'Yes I am!';

Declaring a JavaScript Variable

Creating a variable in JavaScript is called "declaring" a variable. You declare a JavaScript variable with the var or the let keyword:

var carName;

or:

let carName;

After the declaration, the variable has no value (technically it is undefined). To assign a value to the variable, use the equal sign:

carName = "Volvo"; 

You can also assign value to the variable when you declare it:

let carName = "Volvo";

In the example below, we create a variable called carName and assign the value "Volvo" to it. Then we "output" the value inside an HTML paragraph with id="demo":

<p id="demo"></p>

<script>
	let carName = "Volvo";
	document.getElementById("demo").innerHTML = carName;
</script>

One Statement, Many Variables

You can declare many variables in one statement. Start the statement with let and separate the variables by comma:

let person = "John Doe", carName = "Volvo", price = 200;

A declaration can span multiple lines:

let person = "John Doe",
carName = "Volvo",
price = 200;

Value - undefined

In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input. A variable declared without a value will have the value undefined. The variable “carName” will have the value undefined after the execution of this statement:

let carName; 

Re - Declaring JavaScript Variables

If you re - declare a JavaScript variable declared with var, it will not lost its value.

The variable carName will still have the value “Volvo” after the execution of these statements.

var carName = "volvo"; 
var carName; 

You cannot re-declare a variable declared with let or const. This will not work:

let carName = "Volvo";
let carName;

JavaScript Arithmetic

As with algebra, you can do arithmetic with JavaScript variables, using operators like “=” and “+”:

let x = 5 + 2 + 3; 

You can also add strings, but strings will be concatenated:

let x = "John" + " " + "Doe";

Also try this:

let x = "5" + 2 + 3;

If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated. Now try this:

let x = 2 + 3 + "5";

JavaScript Dollar Sign $

let $ = "Hello World";
let $$$ = 2;
let $myMoney = 5;

Using the dollar sign is not very common in JavaScript, but professional programmers often use it as an alias for the main function in a JavaScript library. In the JavaScript library jQuery, for instance, the main function $ is used to select HTML elements. In jQuery $("p"); means "select all p elements".

JavaScript Underscore()

Since JavaScript treats underscore as a letter, identifiers containing - are valid variable name:

let _lastName = "Johnson";
let _x = 2;
let _100 = 5;

Using the underscore is not very common in JavaScript, but a convention among professional programmers is to use it as an alias for "private (hidden)" variables.