Javascript (Var vs Let vs Const)

75 Comments Javascript

VAR

Var is functioned-scoped and variables defined in the function CAN NOT be accessed from its outer function. But variables can be accessed from its inner function.

Children-scoped variables can USE its parents variables.

Parents can not use its children variables – because … well, yeah, children like to keep secrets 😀

Var is hoisted

Hoisting is when a variable declaration set upfront at the beginning of function execution while its assignment or value initialization will be done later, when the execution reaches the statement.

Example for Hoisting :

console.log(myDog) // undefined -- but no error raised

var myDog = 2; // although the assignment happens after the variable gets called

Basically that is the same like :

var myDog;

console.log(myDog); // undefined

myDog = 2; // because the assignment happens here

Var is Function-Scoped

function myFunction() {

  var myVar = "Noodles";

  console.log(myVar) // works! Same scope
}

console.log(myVar) // ReferenceError, the child is too shy!

VAR-declared variables can not be accessed outside the function

More subtle example :

function myFunction() {

  var myVar = "Noodles";

  if (true) {

    var myVar = "Woof!";

    console.log(myVar); // "Woof!"

    // actually, with myVar being function-scoped, we just erased the previous myVar 
    value "Noodles" for "Woof!"
}

console.log(myVar); // "Woof!" - notice the assignment in the if block has reassigned this value for good
}

console.log(myVar); // Throws a ReferenceError, myVar is not accessible outside the function.

Var can be redeclared

var myVar = 2;

var myVar = 3;

console.log(myVar) // 3

LET

Let declared variable are block-scoped, can not be redeclared in the same scope.

function myFunction() {

  let myVar = "Noodles";

  if (true) {

    let myVar = "Woof!";

    console.log(myVar); // "Woof!"

    // with let myVar being block-scoped, we just created a new variable called myVar
}

console.log(myVar); // "Noodles", it then accesses the same-scoped variable where console log is invoked
}

console.log(myVar); // Throws a ReferenceError, myVar is not accessible outside the function.

Let is not hoisted

console.log(myDog); // a ReferenceError!

let myDog = "Noodles"; // because the assignment happens after the variable gets called

Let can not be redeclared

let myVar = 2;

let myVar = 3;

console.log(myVar) // SyntaxError: Duplicate declaration

CONST

Const variables can not be reassigned and redeclared.

const dog = "Noodles"; // declaration with value assignment

dog = "Meow"; // error - reassignment is not allowed

const dog = "Meow"; // error - redeclaration is not allowed

BUT! const Object and Array are mutable!

For objects:

const dog = {

name: 'Noodles'

};

dog.name = 'Meow'; // this will work ! dog variable is not completely reassigned, but mutated

console.log(dog.name) // "Woof"

dog = "Noodles" // raises an error, because reassignment is not allowed with const declared variables

For arrays :

const dogs = [];

dogs.push('Pillow'); // this will work ! dogs array is not reassigned, but mutated

console.log(dog[0]) // "Pillow"

dogs = ["Noodles"] // raises an error, because reassignment is not allowed with const declared variables

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