Creating 2d array of objects in javascript with string keys

I'm trying to create a summary report for a order based on products in the order

However my summary array is always empty.

var summary = [];

_.each(this.products, function(product,counter) {
    var invoice = {};

    invoice.total = 0;
    invoice.dealer_discount = 0;
    invoice.program_discount = 0;
    invoice.adjusted_total = 0;

    _.each(product.materials, function(material) {
        _.each(material.variants, function(variant) {
            var difference = 0;

            invoice.total = parseFloat(invoice.total + variant.price.msrp);

            if(variant.price.discount_type === 'dealer') {
                difference = parseFloat(variant.price.msrp - variant.price.discount_price);

                invoice.dealer_discount = parseFloat(invoice.dealer_discount + difference);
            } else {
                difference = parseFloat(variant.price.msrp - variant.price.discount_price);

                invoice.program_discount = parseFloat(invoice.program_discount + difference);
            }   
        });
    });

    // This never seems to get populated?   
    // If I set the array key to counter like summary[counter] it works fine but I need:    
    summary[product.fulfilled_by] = invoice;
});

It is probably something simple that I'm doing wrong.

Any help is appreicated.