check runtime script error for Selenium WebDriver

SJTXE.java

//make sure it's run after every page load
public static void injectSJTXE(WebDriver driver) throws IOException {
  URL url;
  String js;
  url = Resources.getResource(ExtJSUtil.class, "SJTXE.js");
  js = Resources.toString(url, Charsets.UTF_8);
  ((JavascriptExecutor) driver).executeScript(js);
}
 
//code to check if there are ExtJS Ajax error
Boolean hasJSError = (Boolean) ((JavascriptExecutor) driver).executeScript("return SJTXE.hasJSError();");

//or check it in @After for each test case
@After
public void tearDown() {
  assertThat("No runtime JavaScript error", 
	(Boolean) ((JavascriptExecutor) driver).executeScript("return SJTXE.hasJSError();"),
    	equalTo(false));
}