Function Scope(lexical scope) vs Block Scope

JavaScript has function scope (lexical scope) every time we create a function we create a new execution context which has its own variable environment. However, most other programming languages have something called block scope.

Function Scope

var isValid = false

if(!isValid){
 var function_scope = true
}

console.log(function_scope)

true

• no error

Block Scope

var isValid = false

if(!isValid){
 let block_scope = true
}

console.log(block_scope)

ReferenceError: block_scope is not defined

JavaScript with After ES6 let, const keywords they allow us to use block scoping.

Actually, the variable itself let, const still in memory, but the JavaScript engine do not allow you to access it.