[JavaScript] An example of how to use an optional parameter to detect if a variable is define

reference-error-not-defined.js

function acmeReferenceError( valueExists ) {
  'use strict';

  if ( undefined === valueExists ) {
  
    /* If this is hit, then the function is being invoked
     * before the variable in question has been defined.
     */

  } else {

    /* If this particular conditional is hit, then the function
     * is being invoked after the value in question has
     * been defined.
     */

  }

}