nodejs selenium-webdriver getAttribute使用例子

如题：想取得内容，参考如下

var webdriver = require('selenium-webdriver');
var driver = new webdriver.Builder().
        withCapabilities(webdriver.Capabilities.ie()).
        build();
var usernameField = driver.findElement(webdriver.By.id('username'));
usernameField.getAttribute('class')
.then(function(classes){
    console.log(classes);
 });