Even a task as simple as this can be complicated without jQuery at our disposal. In plain JavaScript, we could add the highlightedclass as shown in the following code snippet:

window.onload = function() {
var divs = document.getElementsByTagName('div');
for (var i = 0; i < divs.length; i++) {
if (hasClass(divs[i], 'poem-stanza') && !hasClass(divs[i], 'highlight')) { 
divs[i].className += ' highlight';

}
}
function hasClass( elem, cls ) {
var reClass = new RegExp(' ' + cls + ' ');
return reClass.test(' ' + elem.className + ' ');
}
};

在我们自己处理的时候，甚至是这么简单的任务在不使用jquery的时候都会变得复杂。用原始的js，我们可以使用下面的代码片段添加highlighted类：

Despite its length, this solution does not handle many of the situations that jQuery takes care of for us in Listing 1.2, such as the following:
•  Properly respecting other window.onloadevent handlers
•  Acting as soon as the DOM is ready
•  Optimizing element retrieval and other tasks with modern DOM methods

We can see that our jQuery-driven code is easier to write, simpler to read, and faster to execute than its plain JavaScript equivalent.

我们可以清晰的看到我们的使用query的代码比原生js代码写起来更容易，读起来更简单，运行起来更快。