Grab the value of the text node child.
取得文本子节点的值。
It can either be a text node or an element.
它可以是文本节点，也可以是元素。
This is because there is no text node immediately under the element "book."
这是因为文本节点不是直接位于元素“book”下。
Elements don't actually have a value, they simply have text node children.
元素实际上并没有值，它们仅有文本子节点。
A text node always has at least one character of data.
一个文本节点总至少有一个字符的数据。
A text node always has at least one character of data.
一个文本节点总至少有一个字符的数据。
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