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.

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JavaScript Runtime Environment

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JavaScript Hoisting

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