Calling a method in a Javascript Constructor and Accessing Its Variables

Yes, it is possible, when your constructor function executes, the this value has already the [[Prototype]] internal property pointing to the ValidateFields.prototype object.

Now, by looking at the your edit, the errArray variable is not available in the scope of the CreateErrorList method, since it is bound only to the scope of the constructor itself.

If you need to keep this variable private and only allow the CreateErrorList method to access it, you can define it as a privileged method, within the constructor:

function ValidateFields(pFormID){
  var aForm = document.getElementById(pFormID);
  var errArray = [];

  this.CreateErrorList = function (formstatid){
    // errArray is available here
  };
  //...
  this.CreateErrorList();
}

Note that the method, since it’s bound to this, will not be shared and it will exist physically on all object instances of ValidateFields.

Another option, if you don’t mind to have the errArray variable, as a public property of your object instances, you just have to assign it to the this object:

//..
this.errArray = [];
//..

More info:

• Private Members in JavaScript
• Closures