What are the differences between var, let and const?

var is function-scoped, while let and const are block-scoped. const also cannot be reassigned.

var:

• It is visible in the whole function. It ignores blocks like if and for.
• It is hoisted (registered before the code runs) and starts as undefined.
• You can declare it again in the same scope without an error.
• At the top level of a script, it becomes a property of the global object, like window.x.

let and const:

• They are visible only inside their block { ... }.
• They are hoisted too, but they stay uninitialized until their declaration line runs. This period is the temporal dead zone (TDZ). Reading them during the TDZ throws a ReferenceError.
• You cannot declare them again in the same scope.

const only:

• You cannot reassign it, and you must give it a value when you declare it.
• It does not make the value immutable. You can still change a const object or array, for example list.push(4).

A good default: use const, use let when you need to reassign, and do not use var.