JavaScript Scopes

 What is a variable scope?

The visibility and accessibility of a variable is determined by its scope. JavaScript has three scopes:

• Global scope
• Local scope and
• Block scope

Global Scope

A variable that can be accessible everywhere in the script. 

Local Scope   

If you declare a variable inside a function are local to the function. They are called local variables.

Output: 

I am from say

I am from global

Scope Chain

If interpreter cannot find a variable in its local context, then it looks for it in parent contexts until it's found. This is called scope chain.

Output: 

Hello I am message

In the above script, the JavaScript performs the following:

• It looks up for the variable message in the current context (Functional Execution Context) of the say(). But it cannot find it.
• So it goes out of the function and looks in the outer execution context which is the global execution context. It found the message variable there.
• The way that JavaScript resolves a variable is by looking at in its current scope, if it cannot find the variable then it goes to the outer scope. This process is a Scope Chain.

Block Scope

ES6 provides the let and const keywords that allow you to declare variables in a block scope.

Generally whenever you see curly braces { }, it is a block. It can be the area within the if else, switch conditions or for , do while and while loops.

let is block scoped, var is function scoped.