function xPath(x) { return document.evaluate(x, document, null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null); }

var textnodes = xPath("//text()");

for(var i=0;i<textnodes.snapshotLength;i++){

var node = textnodes.snapshotItem(i);

node.textContent = node.textContent.replace("a", "AAAA");

}