refactor(toc_obj): simplify the code (#181)

lib/toc_obj.js

@@ -8,28 +8,26 @@ const parseHtml = html => {
   return handler.dom;
 };
 
-const getId = ele => {
-  const { id } = ele.attribs;
-  const { parent } = ele;
-  return id || (!parent ? null : getId(parent));
+const getId = ({ attribs, parent }) => {
+  return attribs.id || (!parent ? null : getId(parent));
 };
 
 function tocObj(str, options = {}) {
-  options = Object.assign({
+  const { min_depth, max_depth } = Object.assign({
     min_depth: 1,
     max_depth: 6
   }, options);
 
-  const headingsSelector = ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'].slice(options.min_depth - 1, options.max_depth).join(',');
+  const headingsSelector = ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'].slice(min_depth - 1, max_depth);
+  const headings = DomUtils.find(({ tagName }) => headingsSelector.includes(tagName), parseHtml(str), true);
+  const headingsLen = headings.length;
 
-  const dom = parseHtml(str);
-  const headings = DomUtils.find(el => headingsSelector.includes(el.tagName), dom, true);
+  if (!headingsLen) return [];
 
   const result = [];
 
-  if (!headings.length) return result;
-
-  for (const el of headings) {
+  for (let i = 0; i < headingsLen; i++) {
+    const el = headings[i];
     const level = +el.name[1];
     const id = getId(el);
     const text = escapeHTML(DomUtils.getText(el));