Loads Google Tag Manager code snippet and data layer from an external .js file

footer.js

var GTM_ID = 'GTM-#####'

var orderId = '12345';
var OrderTotal = 100.25;
var OrderTax = 11.99;
var ItemSKU = ["1234567890", "0987654321", "123456789012"];
var ItemName = ["newhat", "newcoats", "newtoys"];
var ItemCategory = ["newhats", "newcoats", "newtoys"];
var ItemPrice = [3.99, 2.99, 2.88];
var ItemQuantity = [2, 1, 5];

(function() {
	var dataLayerScript = document.createElement('script');
	dataLayerScript.id = 'tag-manager-datalayer';
	
	if (pageName == "OrderSubmitted") {
		dataLayerScript.text = eccomDataLayer();
	}
	
	var x = document.body.firstChild;
	x.parentNode.insertBefore(dataLayerScript, x);	
	
	var noScript = document.createElement('script');
	noScript.text = '<iframe src="//www.googletagmanager.com/ns.html?id=' + GTM_ID + '" height="0" width="0" style="display:none;visibility:hidden"></iframe>';
	var t = document.getElementById('tag-manager-datalayer');
	insertAfter(t, noScript);
	
	var scriptTag = document.createElement('script');
	scriptTag.text = '(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({\'gtm.start\': new Date().getTime(),event:\'gtm.js\'});var f=d.getElementsByTagName(s)[0],' +
	'j=d.createElement(s),dl=l!=\'dataLayer\'?\'&l=\'+l:\'\';j.async=true;j.src=\'//www.googletagmanager.com/gtm.js?id=\'+i+dl;f.parentNode.insertBefore(j,f);' +
	'})(window,document,\'script\',\'dataLayer\',\'' + GTM_ID + '\')';
	var s = document.getElementById('tag-manager-datalayer');
	insertAfter(s, scriptTag);
})();

function eccomDataLayer() {
    dataLayer = buildTransactionArray();
    return dataLayer;
}

function buildProductArray() {
	var length = ItemSKU.length;
    var transactionProducts = [];
    for (var i = 0; i < length; i++) {
			transactionProducts[i] = 
			"{'sku': '" + ItemSKU[i] + "',"+
			"'name': '" + ItemName[i] + "',"+
			"'category': '" + ItemCategory[i] + "',"+
			"'price': " + ItemPrice[i] / ItemQuantity[i] + ","+
			"'quantity': " + ItemQuantity[i] +
			"}";
    }
    return transactionProducts;
}

function buildTransactionArray() {
    dataLayer = "dataLayer = [{ 'transactionId': '" + orderId + "'," +
        //"'transactionAffiliation': '" + attraction + "'," +
        "'transactionAffiliation': ' No Affil '," + "'transactionTotal': " + OrderTotal + "," + "'transactionTax': " + OrderTax + "," + "'transactionShipping': 0.00," + "'transactionProducts': [" + buildProductArray() + "]}]";
    console.log(dataLayer);
    return dataLayer;
}

function insertAfter(referenceNode, newNode) {
        referenceNode.parentNode.insertBefore(newNode, referenceNode.nextSibling);
    }