您不需要javascript。一个简单的CSS text-overflow：省略号; 可以解决问题。

You don't need javascript for this. a simple css text-overflow: ellipsis; can do the trick.

这是一种更好的/标准的截断长文本的方法，因为它将截断文本显示在确切位置，具体位置取决于字体大小，父容器的宽度，等等。。。用js不可能。这是一个演示：

It is a better/standard way to truncate the long text because it will truncate the text display at exact position which is depend on the font-size, width of parent container, etc... that is not possible simply with js. here is a demo:

var textHolder = document.querySelector('.demo');
var btn = document.querySelector('.btn');

function toggleText() {
  textHolder.classList.toggle("truncate");
}

btn.addEventListener('click', toggleText);
toggleText(); //to truncate at first time

.truncate {
  text-overflow: ellipsis;
  /*BOTH of the following are required for text-overflow (overflow: hidden; and white-space: nowrap;)*/
  overflow: hidden;
  white-space: nowrap;
}

<section class="demo" id="demo">
  Let's truncate a long line of text so that the words don't wrap when they're not supposed to. Multi-line of course!
  Let's truncate a long line of text so that the words don't wrap when they're not supposed to. Multi-line of course!
  Let's truncate a long line of text so that the words don't wrap when they're not supposed to. Multi-line of course!
</section>

<button class="readMore btn">Read More</button>

请注意，剪切 innerHTML 可能很危险，因为您可能在不适当的位置剪切并破坏了HTML代码的打开/关闭标签...

Note that cutting innerHTML can be dangerous as you may cut at improper position and corrupt the HTML code opening/closing tags...