First of all, a variable is a pointer to a data stored in a memory, we use variables to store data temporally in the computer's memory. Thera are three ways to declare a variable, by the use of the keywords; var, let and const. Before let and const, there was already var. One of the features that came with the newJavaScript edition (ES6) is the addition of let and const , which can be used for variable declaration. In this article, we'll discuss the difference between var ,let and const with respect to their scope, use, and hoisting.

var

Var has some limitations, There are issues associated with variables declared with var. That is why it was necessary for new ways to declare variables to emerge. Let's look into the features of var.

Scope of var

Scope essentially means where these variables are available for use. var declarations are globally scoped or function/locally scoped. The scope is global when a var variable is declared outside a function. This means that any variable that is declared with var outside a function block is available for use in the whole window. var is function scoped when it is declared within a function. This means that it is available and can be accessed only within that function. example :

var compliment = "hey hi"; function newFunction() { var hello = "hello"; }

Here, compliment is globally scoped because it exists outside a function while hello is function scoped. So we cannot access the variable hello outside of a function. Example:

var greet = "hey hi";

function newFunction() { var hello = "hello"; } console.log(hello); // error:

We'll get an error which is as a result of hello not being available outside the function.

var variables can be re-declared and updated

This means that we can do this within the same scope and won't get an error. var greet= "hey hi";

var greeter = "say Hello instead"

and this also means

var greeter = "hey hi"; greeter = "say Hello instead";

Hoisting of var

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. Example:

console.log (greeter);

var greeter = "say hello"

it is interpreted as this:

var greeter;

console.log(greeter);

// greet greeter = "say hello"

So var variables are hoisted to the top of their scope and initialized with a value of undefined . Problem with var There's a weakness that comes with var. Example:

var greeter = "hey hi";

var times = 4;

if (times > 3) { var greeter = "say Hello i }

console.log(greeter)

// "say H

So, since times > 3 returns true, greeter is redefined to "say Hello instead" . While this is not a problem if you knowingly want greeter to be redefined, it becomes a problem when you do not realize that a variable greeter has already been defined before. If you have used greeter in other parts of your code, you might be surprised at the output you might get. This will likely cause a lot of bugs in your code. This is why let and const are necessary.

Let

let is now preferred for variable declaration. It's no surprise as it comes as an improvement to var declarations. It also solves the problem with var that we just covered. Let's consider why this is so.

scope of let

let is block scoped A block is a chunk of code bounded by {}. A block lives in curly braces. Anything within curly braces is a block. So a variable declared in a block with let is only available for use within that block. Example:

let greeting = "say Hi";

let times = 4;

if (times > 3) { let hello = "say Hello ins console.log(hello); // "say }

console.log(hello) // hello

We see that using hello outside its block (the curly braces where it was defined) returns an error. This is because let variables are block scoped .

let can be updated but not re-declared.

Just like var , a variable declared with let can be updated within its scope. Unlike var , a let variable cannot be re-declared within its scope. Example : let greeting = "say Hi";

greeting = "say Hello instead"

this will return an error:

let greeting = "say Hi";

let greeting = "say Hello instead"

However, if the same variable is defined in different scopes, there will be no error:

let greeting = "say Hi";

if (true) { let greeting = "say Hello" console.log(greeting); // }

console.log(greeting); // "say

Why is there no error? This is because both instances are treated as different variables since they have different scopes. This fact makes let a better choice than var . When using let , you don't have to bother if you have used a name for a variable before as a variable exists only within its scope. Also, since a variable cannot be declared more than once within a scope, then the problem discussed earlier that occurs with var does not happen.

Hoisting of let

Just like var , let declarations are hoisted to the top. Unlike var which is initialized as undefined , the let keyword is not initialized. So if you try to use a let variable before declaration, you'll get a Reference Error .

Const

Variables declared with the const maintain constant values. const declarations share some similarities with let declarations. const declarations are block scoped

Scope of const

Like let declarations, const declarations can only be accessed within the block they were declared.

const cannot be updated or re-declared

This means that the value of a variable declared with const remains the same within its scope. It cannot be updated or re-declared. So if we declare a variable with const , we can neither do this. Example: const greeting = "say Hi";

greeting = "say Hello instead"

nor this:

const greeting = "say Hi";

const greeting = "say Hello instead"

Every const declaration, therefore, must be initialized at the time of declaration. This behavior is somehow different when it comes to objects declared with const . While a const object cannot be updated, the properties of this objects can be updated. Example : const greeting = { message: "say Hi", times: 4 }

while we cannot do this:

greeting = { words: "Hello", number: "five" } // error: Assignment to con

we can do this:

greeting.message = "say Hello"

This will update the value of greeting.message without returning errors.

Hoisting of const

Just like let , const declarations are hoisted to the top but are not initialized.

conclusion

So just in case you missed the differences, here they are: var declarations are globally scoped or function scoped while let and const are block scoped. var variables can be updated and re-declared within its scope; let variables can be updated but not re-declared; const variables can neither be updated nor re-declared. They are all hoisted to the top of their scope. But while var variables are initialized with undefined , let and const variables are not initialized. While var and let can be declared without being initialized, const must be initialized during declaration.