Variable

Most of the time, a JavaScript application needs to work with information. Here are two examples:

1. An online shop – the information might include goods being sold.

2. A chat application – the information might include users, messages, and groups and more.

Variables are used to store this information.

There are three different ways to make a variable, which in JavaScript we refer to as declaring a variable. They are:

1. var

2. let

3. const

let

A variable is a “named storage” for data. To create a variable in JavaScript, use the let keyword.

Value = undefined

A variable declared without a value will have the value undefined.

The variable message will have the value undefined after the execution of this statement:

The statement below creates (in other words: declares) a variable with the name “message”:

let message;

Now, we can put some data into it by using the assignment operator =:

// store the string 'Hey' in the variable named message
let message; 
message = 'Hey';

The string is now saved into the memory area associated with the variable. We can access it using the variable name:

let message; 
message = 'Hey!'; 
alert(message); // shows the variable content

To be concise, we can combine the variable declaration and assignment into a single line:

let message = 'Hey!'; // define the variable and assign the value
alert(message);

‼️Note: We can also declare multiple variables in one line

---

let user = 'Diksha', age = 25, message = 'Hey';

That might seem shorter, but we don’t recommend it. For the sake of better readability, please use a single line per variable.

The multiline variant is a bit longer, but easier to read:

let user = 'Diksha'; 
let age = 20; 
let message = 'Hey';

You can also define multiple variables in this multiline style:

let user = 'Diksha', 
age = 20, 
message = 'Hey';

Technically, all of these variations accomplish the same task. So, everything comes down to personal preference.

A real-life analogy

If we think of a "variable" as a "box" holding data with a uniquely labelled sticker on it.

For instance, the variable message can be imagined as a box labeled "message" with the value "Hey!" in it:

We are free to put any value in the box and to make as many changes as we like:

let message;
message = 'Hey';
message = 'Hola'; // value changed
alert(message);

Result:

Copy data from one variable to another

We can also declare two variables and copy data from one into the other.

let greet = 'Hey there!';
let message;

// copy 'Hey there!' from greet into message
message = greet;

// now two variables hold the same data
alert(greet);
alert(message);

❌ A repeated declaration of the same variable is an error

---

let message = "one";
// repeated 'let' leads to an error
let message = "two"; 
// SyntaxError: 'message' has already been declared

Therefore, we should declare a variable only once and then refer to it without let

Variable naming

There are three limitations on variable names in JavaScript:

• Names can contain letters, digits, underscores, and dollar signs.

• Names must begin with a letter.

• Names can also begin with $ and _ .

⚠️ Note:

• Variable names are case-sensitive (y and Y are different variables).

• Certain terms such as reserved words in JavaScript shouldn't be used to name variables.

Examples of valid names:

let userName;
let word123;

When the name contains multiple words, camelCase is commonly used. For example: webDevelopment.

The dollar sign '$' and the underscore '_' can also be used in names.

Const

To declare a constant (unchanging) variable, use const instead of let:

const myName = 'Sahil';

❌ Note: Variables declared using const are cannot be reassigned. An attempt to do so would cause an error:

---

const myName = 'Sahil';
myName = 'sahilChandravanshi'; // error, can't reassign the constant!

When a programmer is sure that a variable will never change, they can declare it with const .

var

⚠️ In older scripts, you may also find another keyword: var instead of let

---

JavaScript

var message = 'Hey';

The var declaration is similar to let. It also declares a variable but in a slightly different, “old-school” way.

There are slight differences between let and var but most of the time we can replace let with var and vice-versa. For now, these differences do not matter for us yet. I'll cover these differences in detail in a later article.

Name things right

The meaning of a variable name should be unambiguous and describe the information it contains.

Variable names can quickly distinguish between code written by a newbie developer and code produced by an expert developer.

Some good-to-follow rules are:

• Use human-readable names like userName or shoppingCart.

• Stay away from abbreviations like a, b, c, unless you really know what you’re doing.

• Make names maximally descriptive and concise.

If you have any difficulties or questions, please make a comment.