Undefined variable
[1] In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time.In other languages such a usage is considered to be sufficiently serious that a diagnostic being issued and the compilation fails.Some language definitions initially used the implicit declaration behavior and as they matured provided an option to disable it (e.g. Perl's "use warnings" or Visual Basic's "Option Explicit").The following provides some examples of how various programming language implementations respond to undefined variables.A ReferenceError only happens if the same piece of executed code has a let or a const (but not var) declaration later on, or if the code is executed in strict mode.