Javascript Array defined in constructor is undefined in prototype

Question

I'm pretty new to writing OO JS, but this has stumped me. So I setup me new Call object, then define what I assume to be empty arrays. When I call AddFieldQueryToArray() I get

Uncaught TypeError: Cannot call method 'push' of undefined 

On this.fieldArray.push(field)

I don't really know why. I've tried this.fieldArray = fieldArray; in the constructor too.

    function Call() 
    {    
         var fieldArray = new Array();
         var queryArray = new Array();
    }

    Call.prototype.AddFieldQuerysToArray = function(field,query)
    {
        if(field !== 'undefined')
        {               
            this.fieldArray.push(field);
        }

        this.queryArray.push(query);

    }

Answer 1

You should reference instance properties with this. inside the constructor:

function Call() 
{    
     this.fieldArray = [];
     this.queryArray = [];
}

new Array() has a shortcut [] which I have used above.

If you use var fieldArray = []; you would create a local variable which is destroyed once your instance has been created unless you use this kind of construct:

function Call()
{
    var fieldArray = [];

    this.AddFieldQuerysToArray = function(field, query) {
        // ...
        fieldArray.push(field);
    }
}

This creates a closure for each instance in which fieldArray is "kept alive" throughout the instance's lifetime.