`var`, `let`, and `const` are all used for variable declaration in JavaScript but they have different usage and scope.
1. `var`: It is function scoped, which means if you declare a variable inside a function using `var`, it cannot be accessed outside of the function. If you declare it outside any function, it becomes a global variable and can be accessed from anywhere in your code. The other thing about `var` is that it is hoisted to the top of its scope, which means you can use `var` variable before it’s actually declared.
1. `let`: It is block scoped, which means that it can only be accessed within the block it was declared. It is not hoisted. `let` allows you to reassign values.
1. `const`: Like `let`, `const` is also block scoped. However, once you assign a value to a `const` variable, you cannot change it (it’s read-only). It is not hoisted. It doesn’t mean the value it holds is immutable, it means that the variable identifier cannot be reassigned.
Here is a brief comparison:
```
// var
var x = 1;
if (true) {
var x = 2; // same variable, it’s value is changed
console.log(x); // 2
}
console.log(x); // 2
// let
let y = 1;
if (true) {
let y = 2; // different variable
console.log(y); // 2
}
console.log(y); // 1
// const
const z = 1;
if (true) {
const z = 2; // different variable
console.log(z); // 2
}
console.log(z); // 1
```
