Understanding the scope of variables in Javascript

Scope determines the accessibility of variables from different parts of your code.

JavaScript has two types of scopes: Global scope and Local scope.

Global scope:

When a variable is ''globally scoped'', that variable is available from anywhere in your program. If the variable is declared outside a function or a block outside a function, leads to it being a globally scoped.

Local scope :

When variables are declared within a function or a block, they are locally scoped. It means that they can only be accessed inside the function or the block they were declared in.

Local scope variables are divided into:

• Function scoped variables: a function scoped variable means that the variable defined within a function will not be accessible from outside the function.
• Block scoped variables: a block scoped variable means that the variable defined within a block will not be accessible from outside the block. A block can reside inside a function, and a block scoped variable will not be available outside the block even if the block is inside a function.

There are three different keywords used to declare a variable in JavaScript. These are: var, let and const.

Var is function scoped, let and const are block scoped.

Here are some examples about var and let:

Function example:

Using Var:

function test() {
	var a = 4;
	console.log(a); //a=4
}
console.log(a); //a is not defined

‘var’ value is not available outside a function because it's function scoped.

Using let:

In case of a function, the braces identify a block.

function test() {
	let b = 6;
	console.log(b); //b=4
}
console.log(b); //b is not defined

‘let’ value is not available outside a function because it's block scoped.

Code Block example:

Using Var:

{
	var a = 8;
	console.log(a); //a=8
}
console.log(a); //a=8

‘var’ value is also avaiable outside a block because it's function scoped not block scoped.

Using let:

In case of a function, the braces identify a block.

{
	let b = 10;
	console.log(b); //b=10
}
console.log(b); //b is not defined

‘let’ value is only avaiable inside a block because it's block scoped.