Add word count in document info modal (#738)

Co-authored-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/components/render-page/word-counter.ts

@@ -0,0 +1,50 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import wordsCount from 'words-count'
+
+/** List of HTML tag names that should not be counted. */
+const EXCLUDED_TAGS = ['img', 'pre', 'nav']
+/** List of class names that should not be counted. */
+const EXCLUDED_CLASSES = ['katex-mathml']
+
+/**
+ * Checks whether the given node is an excluded HTML tag and therefore should be
+ * excluded from counting.
+ * @param node The node to test.
+ * @return true if the node should be excluded, false otherwise.
+ */
+const isExcludedTag = (node: Element | ChildNode): boolean => {
+  return EXCLUDED_TAGS.includes(node.nodeName.toLowerCase())
+}
+
+/**
+ * Checks whether the given node is a HTML element with an excluded class name,
+ * so that it should be excluded.
+ * @param node The node to test.
+ * @return true if the node should be excluded, false otherwise.
+ */
+const isExcludedClass = (node: Element | ChildNode): boolean => {
+  return EXCLUDED_CLASSES.some((excludedClass) => (node as HTMLElement).classList?.contains(excludedClass))
+}
+
+/**
+ * Counts the words of the given node while ignoring empty nodes and excluded
+ * nodes. Child nodes will recursively counted as well.
+ * @param node The node whose content's words should be counted.
+ * @return The number of words counted in this node and its children.
+ */
+export const countWords = (node: Element | ChildNode): number => {
+  if (!node.textContent || isExcludedTag(node) || isExcludedClass(node)) {
+    return 0
+  }
+  if (!node.hasChildNodes()) {
+    return wordsCount(node.textContent)
+  }
+  return [...node.childNodes].reduce((words, childNode) => {
+    return words + countWords(childNode)
+  }, 0)
+}