When a non-existent variable is referenced a ``++ReferenceError++`` is raised.

Due to the dynamic nature of JavaScript this can happen in a number of scenarios:

* When typo was made in a symbol's name.
* When using variable declared with ``++let++`` or ``++const++`` before declaration (unlike ``++var++``-declarations, they are not hoisted to the top of the scope).
* Due to confusion with scopes of ``++let++``- and ``++const++``-declarations (they have block scope, unlike ``++var++``-declarations, having function scope).
* When accessing a property in the wrong scope (e.g. forgetting to specify ``++this.++``).

This rule does not raise issues on global variables which are defined with ``++sonar.javascript.globals++`` and ``++sonar.javascript.environments++`` properties.

== Noncompliant Code Example

----
var john = { 
  firstName: "john", 
  show: function() { console.log(firstName); } // Noncompliant: firstName is not defined
}
john.show(); 
----

== Compliant Solution

----
var john = { 
  firstName: "john", 
  show: function() { console.log(this.firstName); }
}
john.show(); 
----