In JavaScript almost everything is an object, null and undefined are exception. When you try to access an undefined variable it always returns undefined and we cannot get or set any property of undefined. In that case, an application will throw Uncaught TypeError cannot set property of undefined. 

In most cases, you are missing the initialization . So, you need to initialize the variable first. Moreover, if you are not sure a variable that will always have some value, the best practice is to check the value of variables for null or undefined before using them. If you just want to check whether there's any value, you can do:

if( value ) {

  //do something

}

Or, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator .

if ( typeof(some_variable) !== "undefined" && some_variable !== null ) {

  //deal with value

}

http://net-informations.com/js/err/set.htm