Clean Code in JavaScript

Variable declarations

A variable declaration occurs via a var keyword followed by a valid identifier or an assignment of the form a = b:

var foo;var baz = 123;

We call things declared via var keyword variable declarations, but it's important to note that, in popular terminology, declarations made by both let and const are also considered variables.

Variables declared via var are scoped to the nearest function, module, or global environment—that is, they are not block-scoped. At parse time, variable declarations within a given scope will be collected and then, at the point of execution, those declared variables will be hoisted to the top of their execution context and initialized with the undefined value. This means that, within a given scope, ...