03_Variables.md

What is variable

Variable is the essential building block of JavaScript. We need operators to modify our variables.

Without variable, a piece of code would do the exact same thing every time. But with variable, a piece of code do something different every time.

What happens if there is no variable defined inside the script?

If we didn't have variables available, we would have to ask for information about something from users every time it was needed. As you can see in the below code.

buttonB.onclick = () => {
  alert(`Hello ${prompt("What is your name?")}, nice to see you!`);
  headingB.textContent = `Welcome ${prompt("What is your name?")}`;
};

Variable declaration

When we first time create a Variable, we declare it. This process is known as variable declaration.

To declare a variable, we use the let, var and const keyword followed by the variable name.

let firstName;

// firstName - Variable, that is not initialized yet. So it's default value will be "undefined"

Variable has a special value undefined if you have not assigned a value to it. But we can also assign a value to a variable, this process is known as initialization of a variable.

Once we have declared a variable, we can initialize it with a value. To initialize a variable, we specify the variable name, followed by an equals sign ( = ) and a value.

JavaScript allows us to declare two or more variables using a single statement. To separate two variable declarations, we use a comma ( , ) operator.

// Variable definition
let firstName = "Ajay";

// firstName - Variable Name/Identifier
// "Ajay" - Value

Now, we can use the above defined Variable in our entire program whenever we want.

JavaScript is a dynamically typed language. This means that we don’t need to specify the variable’s type in the declaration.

Since JavaScript is a dynamically typed language, we can re-assign a value of a varible of different data type. Although, it is not recommended.

Once we initialize a variable, we can change its value by re-assigning(=) a different value.

// Variable definition
let firstName = "Ajay";

// firstName - Variable Name/Identifier
// "Ajay" - Value

// re-assign
firstName = "Alex";

// firstName - Variable Name/Identifier
// "Alex" - New value

A variable has no any data type but a value has.

Undefined VS undeclared variables

An undefined variable is a variable that has been declared but has not been initialized with a value.

An undeclared variable is a variable that has not been declared or defined yet. If we want to accesss undeclared variable then JavaScript engine will throw an error.

let, var, & const

A variable definition consists of three parts:

1. Varible definition keyword (let, var, & const)

2. A variable name (eg: firstName)

3. A value (eg: "Ajay")

let, var and, const are used for variables that might have new value assigned somewhere in the program.

The var keyword is almost the same as let. It also declares a variable, but in a slightly different way. It is recommended to always use the let keyword.

Variables declared using const are called constants. They cannot be reassigned. An attempt to do so would cause an error.

These 3 keywords introduce a new concept known as scope, which will be discussed later in the week.

const declarations must be initialized.

Notes:👇

Keyword	Scope
var	Global
let	Block
const	Block