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:

Example

x = 5;
y = 6;
z = x + y;

Note

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.

Note

Variables are containers for storing values.

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 is 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.)

Note

The "equal to" operator is written like == in JavaScript.

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 handle many types of data, but for now, just think of numbers and strings.

Strings are written inside double or single quotes. Numbers are written without quotes.

If you put a number in quotes, it will be treated as a text string.

Example

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 a value to the variable when you declare it:

let carName = "Volvo";

In the example below, we create a variable called carName