Difference between Javascript Var, Let, and Const

Var, let, and const are all keywords in JavaScript that are used to declare variables. However, they have different properties and use cases.

Var

Var variables are function-scoped, meaning that they can be accessed from anywhere within the function in which they are declared.
Var variables can be re-declared and updated.
Var variables are hoisted, meaning that they are initialized to undefined before they are declared.

Let (ES6)

Let variables are block-scoped, meaning that they can only be accessed from within the block in which they are declared.
Let variables can be re-declared but cannot be updated.
Let variables are not hoisted, meaning that they must be declared before they can be used.

Const (ES6)

Const variables are block-scoped, meaning that they can only be accessed from within the block in which they are declared.
Const variables cannot be re-declared or updated.
Const variables must be initialized when they are declared.

Here is a table that summarizes the key differences between var, let, and const:

Keyword	Scope	Re-declarable?	Updatable?	Hoisted?
Var	Function	Yes	Yes	Yes
Let	Block	Yes	No	No
Const	Block	No	No	No

When to use each keyword

Var: declare variables that need to be accessed from anywhere within the function in which they are declared. For example, var variables can be used to declare global variables or loop variables.
Let: declare variables that need to be accessed from within the block in which they are declared. For example, let variables can be used to declare variables in a loop or inside a function.
Const: declare variables that do not need to be changed. For example, const variables can be used to declare constants, such as mathematical constants or the value of a global variable.

It is generally recommended to use let and const whenever possible, and to avoid using var unless necessary. This is because let and const are more predictable and less likely to cause errors.