variables.md

How var, let and const keywords works in JavaScript

Earlier, pre-ES6 era, only var keyword was introduced for declaration of variable.
With ES6, the let and const keyword introduced.

How to declare variables in JavaScript

without keywords, It is same as var and not allowed in 'strict' mode.

// without keyword
name = 'Jack';

// using var
var price = 100;

// using let 
let isPermanent = false;

// using const 
const publication = 'Article';

console.log(name, price, isPermanent, publication); // Jack 100 false Article

We will discuss:

• Scope
• Reassigning new value
• When you access a variable before declaring it

Variable Scope in JavaScript

The variable may exist in a block, inside function or outside function.

A block is section of code inside { }

// It has Block Scope
{
    let name = 'John Kabir';
}

A function is a bunch of code you want to place logically together.
It is declared using function keyword.

// It has Function Scope
function test() {
    let name = 'Tahsan';
}

Everything declared outside of block and function is global Scope.

// It has Global Scope
const skyColor = 'blue';

{
    console.log(skyColor); // blue
}

function printSkyColor() {
    console.log(skyColor); // blue
}
printSkyColor();

So, there are three types of Scope:

1. Block Scope
2. Function Scope
3. Global Scope

The three keyword var, let and const work around these scopes.

How to use JavaScript Variable in Global Scope

We can use var, let and const to declare global variable. But it is recommended not to do it. By doing this, variable are accessible everywhere.
So to restrict scope of variable using var, let and const keywords, here's order of accessibility in scope starting with lowest:

• var: The functional Scope level
• let: The Block Scope level
• const: The Block Scope level

How to Reassign a new value to variable in JavaScript

You can reassign var or let variables, but you can't reassign a new value to const variable.
(const = constant)

One Tricky part

When object is declared and assigned value with const, you can still change value of its properties.

But you can not reassign any object value to same variable.

When you access a variable before declaring

With var in non-strict mode, the variable will have an undefined value.
This means variable declared but not assigned.

In strict mode, you will get ReferenceError that variable is not declared.
With let and const, you will always get ReferenceError.

Some Tips:

• Don't use var
• Use let or const
• Use const more often
• Use let, when you need to reassign