gistfile1.js

// JavaScriptを含んだHTMLを評価して返す
function evaluateHTML(html) {
	var iframe = document.createElement('iframe');
	document.body.appendChild(iframe);
	var d=iframe.contentWindow.document;
	d.open();
	d.write(html);
	d.close();
	var result = d.body.innerHTML;
	document.body.removeChild(iframe);
	return result;
}

evaluateHTML('<div id="id"></div><script>document.getElementById("id").textContent="bar"</script>');
// => "<div id="id">bar</div><script>document.getElementById("id").textContent="bar"</script>"

iframe削除に対応済み。