JavaScript var vs. let (Functional Scope, Block Scope, Lexical Scope)🎉

What is Scope?

Scope determines the visibility or accessibility of a variable or other resource in the area of your code.

Global Scope

There's only one Global scope in the JavaScript document. The area outside all the functions is consider the global scope and the variables defined inside the global scope can be accessed and altered in any other scopes.

//global scope
var fruit = 'apple'
console.log(fruit);        //apple

function getFruit(){
    console.log(fruit);    //fruit is accessible here
}

getFruit();                //apple

Local Scope

Variables declared inside the functions become Local to the function and are considered in the corresponding local scope. Every Functions has its own scope. Same variable can be used in different functions because they are bound to the respective functions and are not mutual visible.
The var keyword behaves differently in function scopes and block scopes. A variable declared with var in a function scope can’t be accessed outside that function scope.

//global scope
function foo1(){
    //local scope 1
    function foo2(){
        //local scope 2
    }
}

//global scope
function foo3(){
    //local scope 3
}

//global scope

Local scope can be divided into function scope and block scope. The concept of block scope is introduced in ECMA script 6 (ES6) together with the new ways to declare variables -- const and let.

Function Scope

Whenever you declare a variable in a function, the variable is visible only within the function. You can't access it outside the function. var is the keyword to define variable for a function-scope accessibility.

function foo(){
    var fruit ='apple';
    console.log('inside function: ',fruit);
}

foo();                    //inside function: apple
console.log(fruit);       //error: fruit is not defined 

Block Scope

A block scope is the area within if, switch conditions or for and while loops. Generally speaking, whenever you see {curly brackets}, it is a block. In ES6, const and let keywords allow developers to declare variables in the block scope, which means those variables exist only within the corresponding block.

function foo(){
    if(true){
        var fruit1 = 'apple';        //exist in function scope
        const fruit2 = 'banana';     //exist in block scope
        let fruit3 = 'strawberry';   //exist in block scope

    }
    console.log(fruit1);
    console.log(fruit2);
    console.log(fruit3);
}

foo();
//result:
//apple
//error: fruit2 is not defined
//error: fruit3 is not defined

Lexical Scope

Another point to mention is the lexical scope. Lexical scope means the children scope have the access to the variables defined in the parent scope. The children functions are lexically bound to the execution context of their parents.

function foo1(){
    var fruit1 = 'apple';        
    const fruit2 = 'banana';     
    let fruit3 = 'strawberry';
    function foo2(){
        console.log(fruit1);
        console.log(fruit2);
        console.log(fruit3);
    }
    foo2();
}

foo1();

//result:
//apple
//banana
//strawberry

VAR VS LET

---

function iHaveScope() {
  var secret = 42;
}

A variable declared with var in a block scope is available outside of that block scope.

for (var i=0; i<10; i++) {
  // block scope for the for statement
}console.log(i) // => 10 (why oh why)

The i variable that we often use in a for loop will continue to exist beyond the scope of that loop, and that does not make sense, really.

Luckily we now have a different way to declare variables, using let. Variables declared with let inside a block scope are only accessible inside that scope, making let the ideal solution to the for loop index variable scope problem. If we use let to declare the i variable in a for loop, that variable will only be available inside the for loop.

for (let i=0; i<10; i++) {
  // block scope for the for statement
}console.log(i) // ReferenceError: i is not defined (D'oh!)