Getting selected simple product id in configurable product Magento

1) place the code right after “ var spConfig = new Product.Config(getJsonConfig() ?>);”
spConfig.getIdOfSelectedProduct = function()
{
var existingProducts = new Object();
for(var i=this.settings.length-1;i>=0;i–)
{
var selected = this.settings[i].options[this.settings[i].selectedIndex];
if(selected.config)
{
for(var iproducts=0;iproducts<selected.config.products.length;iproducts++)
{
var usedAsKey = selected.config.products[iproducts]+””;
if(existingProducts[usedAsKey]==undefined)
{
existingProducts[usedAsKey]=1;
}
else
{
existingProducts[usedAsKey]=existingProducts[usedAsKey]+1;
}
}
}
}
for (var keyValue in existingProducts)
{
for ( var keyValueInner in existingProducts)
{
if(Number(existingProducts[keyValueInner])<Number(existingProducts[keyValue]))
{
delete existingProducts[keyValueInner];
}
}
}
var sizeOfExistingProducts=0;
var currentSimpleProductId = “”;
for ( var keyValue in existingProducts)
{
currentSimpleProductId = keyValue;
sizeOfExistingProducts=sizeOfExistingProducts+1
}
if(sizeOfExistingProducts==1)
{
alert(“Selected product id  is: “+currentSimpleProductId)
}

}

2) call the function on the change event of dropdown

1) place the code right after “ var spConfig = new Product.Config(getJsonConfig() ?>);”
spConfig.getIdOfSelectedProduct = function()
{
var existingProducts = new Object();
for(var i=this.settings.length-1;i>=0;i–)
{
var selected = this.settings[i].options[this.settings[i].selectedIndex];
if(selected.config)
{
for(var iproducts=0;iproducts<selected.config.products.length;iproducts++)
{
var usedAsKey = selected.config.products[iproducts]+””;
if(existingProducts[usedAsKey]==undefined)
{
existingProducts[usedAsKey]=1;
}
else
{
existingProducts[usedAsKey]=existingProducts[usedAsKey]+1;
}
}
}
}
for (var keyValue in existingProducts)
{
for ( var keyValueInner in existingProducts)
{
if(Number(existingProducts[keyValueInner])<Number(existingProducts[keyValue]))
{
delete existingProducts[keyValueInner];
}
}
}
var sizeOfExistingProducts=0;
var currentSimpleProductId = “”;
for ( var keyValue in existingProducts)
{
currentSimpleProductId = keyValue;
sizeOfExistingProducts=sizeOfExistingProducts+1
}
if(sizeOfExistingProducts==1)
{
alert(“Selected product id  is: “+currentSimpleProductId)
}

}

2) call the function on the change event of dropdown

<select onchange=”spConfig.getIdOfSelectedProduct()”