What is the difference between "let" and "var"?

In JavaScript, both let and var are used to declare variables, but they have different scoping behaviors and hoisting mechanisms.

var:

    Variables declared with var are function-scoped or globally-scoped, depending on where they are declared.
    They are hoisted to the top of their scope during the compilation phase, which means you can use them before they are declared, but their value will be undefined.

let:

    Variables declared with let are block-scoped, meaning they are limited to the block (enclosed by curly braces) where they are defined.
    They are also hoisted to the top of their scope, but unlike var, you can't access their value before they are declared. This is known as the "temporal dead zone."

Here's an example to illustrate the differences:

javascript

function example() {
    if (true) {
        var varVariable = "This is var";
        let letVariable = "This is let";
    }
    console.log(varVariable);  // "This is var"
    //console.log(letVariable); // ReferenceError: letVariable is not defined
}

example();

console.log(varVariable);  // "This is var"
//console.log(letVariable); // ReferenceError: letVariable is not defined

In this example:

    The varVariable declared with var is function-scoped and can be accessed outside of the block it was declared in.
    The letVariable declared with let is block-scoped and cannot be accessed outside of the block it was declared in.

Another key difference is that when using var in the global scope, it becomes a property of the global object (window in browsers). On the other hand, let doesn't create a property on the global object.

It's generally recommended to use let or const (for constants) over var due to the more predictable scoping behavior and to avoid issues related to hoisting. Use let when you need to reassign the variable's value, and const when you want to declare a variable that won't change its value.