What is the difference between let, const, and var?

Hey Alita Edmonds , it's a very good question . Let's have a look -
Var
The JavaScript variables statement is used to declare a variable and, optionally, we can initialize the value of that variable.

Example: var a =10;

Variable declarations are processed before the execution of the code. The scope of a JavaScript variable declared with var is its current execution context. The scope of a JavaScript variable declared outside the function is global.

Consider the code-

function nodeSimplified(){
  var a =10;
  console.log(a);  // output 10
  if(true){
   var a=20;
   console.log(a); // output 20
  }
  console.log(a);  // output 20
}

In the above code, you can find, when the variable is updated inside the if loop, that the value of variable "a" updated 20 globally, hence outside the if loop the value persists. It is similar to the Global variable present in other languages. But, be sure to use this functionality with great care because there is the possibility of overriding an existing value.

let
The let statement declares a local variable in a block scope. It is similar to var, in that we can optionally initialize the variable.

Example: let a =10;

The let statement allows you to create a variable with the scope limited to the block on which it is used. It is similar to the variable we declare in other languages like Java, .NET, etc.

Consider the code-

function nodeSimplified(){
  let a =10;
  console.log(a);  // output 10
  if(true){
   let a=20;
   console.log(a); // output 20
  }
  console.log(a);  // output 10
}

It is almost the same behavior we see in most language.
The scope will be well maintained with a let statement and when using an inner function the let statement makes your code clean and clear.

const
const statement values can be assigned once and they cannot be reassigned. The scope of const statement works similar to let statements.
Example: const a =10;
Question: What will happen when we try to reassign the const variable? Consider the following code-

function nodeSimplified(){
  const MY_VARIABLE =10;
  console.log(MY_VARIABLE);  //output 10
  MY_VARIABLE =20;           //throws type error
  console.log(MY_VARIABLE); 
}

Error Message : Uncaught TypeError: Assignment to constant variable. The code will throw an error when we try to reassign the existing const variable.

Hope this helps :)