We all at some point in time got confused about using the appropriate keywords in redeclaring a variable. If you are or have been in this situation, congratulations!! because you are following in the same footsteps even the best JavaScript programmers of all-time followed.

Before you go on to understand how to redeclare a variable at different instances, you are supposed to know how to declare a variable.

Here are amazing website links to get you started on Declaring Variables.

W3schools

Javascript info

Programiz

Congrats on learning about the declaration of variables!

Redeclaring Variables using the const keyword

The meaning of the word 'Redeclaration' could be tricky for beginners. To redeclare a variable simply means to declare an already declared variable or identifier regardless of whether in the same block or outer scope.

You could declare and initialize a variable using the const keyword.

//declared and initialized the variable(firstName) and assigned a value of 'Patrick'.
const firstName = 'Patrick';
console.log(firstName) 
 //  Patrick

You may have tried to redeclare the variable or even try to change its value still using the const keyword.

const firstName = 'Patrick';
const firstName = 'John';
//Uncaught SyntaxError: Identifier 'firstName' has already been declared.

it signals a syntax error because const never allows us to tamper with its identifier. With these experiments, we could agree that A variable declared using the 'const' keyword cannot be redeclared and its value cannot be reassigned.

Redeclaring Variables using the let keyword

You can declare and initialize a variable using the let keyword.

//declared and initialized the variable and assigned a value of 'Okafor'.
let lastName= 'Okafor';
console.log(lastName)  // Okafor

Let's try to redeclare the identifier and change its value.

let lastName = 'Okafor';
let lastName = 'Okafor';
//Uncaught SyntaxError: Identifier 'lastName' has already been declared

It logs out an error signalling that the identifier has already been declared therefore we must use another identifier. Now with these experiments, you can see also that; A variable that is declared using the 'let' keyword cannot be redeclared again and its value cannot be changed or reassigned.

Note: We could declare a variable once and refer to it again without using the let keyword. For example

let lastName = 'Okafor';
lastName = 'Doe';
console.log(lastName)  //  Doe

This can run successfully because we declared with the let keyword. The const keyword would output an error.

const lastName = 'Okafor';
lastName = 'Doe';
console.log(lastName)  //  null

Redeclaring Variables using the var keyword

Just like using the const and let keywords, You can declare and initialize a variable using the var keyword.

//declared and initialized the variable and assigned a value of 'Cat'.
var myPet= 'Cat';
 console.log(myPet)  // Cat

Let's try to redeclare the variable and also change its value. We start with redeclaring the variable

var myPet = 'Cat';
var myPet = 'Cat';   // Cat

This time around, it doesn't display any error. It logs out the value. Now, let's change the value;

var mypet= 'Cat';
var myPet = 'Dog'; // Dog

With the experiments using the var keyword to redeclare variables, we could see that the output was the redeclared value. It overrides the declared variable, in our terms, it \" redeclared\" a variable. Therefore we say

A variable that is declared with the 'var' keyword can be redeclared and its value could be reassigned'.

Note: To learn more about redeclaring variables, it's necessary to know the basics of JavaScript scope.. It's a whole topic on its own but I tried to explain redeclaring variables involving Scopes of JavaScript Here.

Now, do more practice.