What is a difference between var, let and cost?

var, let, and const are all used to declare variables in JavaScript. However, they differ in their scoping and the value they hold:

1. var: Variables declared with var are function scoped, meaning they are accessible within the entire function they were declared in. If a variable declared with var is declared inside a block (e.g. if statement), it will be accessible outside of the block. var variables are also hoisted, meaning they are moved to the top of the function, and can be used before they are declared.
2. let: Variables declared with let are block scoped, meaning they are accessible within the block they were declared in. let variables are not accessible outside the block and are not hoisted.
3. const: Variables declared with const are also block scoped, just like let. The main difference between const and let is that const variables cannot be reassigned after they have been declared, making them useful for declaring constants or values that shouldn’t change.

In general, it is recommended to use const by default, and only use let when you need to reassign a variable within the block it was declared in. It is generally not recommended to use var anymore, as it can lead to confusion and unexpected behavior in larger codebases.