The difference between common undefined and defined in JavaScript

In JavaScript believe that "undefined" and "defined" for everyone is certainly not unfamiliar, but not very clear of their differences, first look at two demo we say,

Example 1.

Console.log (parms); The console appears uncaught referenceerror:parms is not defined

Example 2.

VAR flag; Console.log (flag); undefined appears on the console

By the above two examples can be clearly see the difference between the two, not defined is that there is no definition, the program run error, and undefined is defined as a variable, but no assignment, so the program run error.

The difference between common undefined and defined in JavaScript