findLevel.js

// set heading level based on the closest heading before the node
const setLevel = (node, nodeSelector) => {
  // Get all relevant nodes and reverse the array (destructive)
  let allNodes = [...document.querySelectorAll(`h1, h2, h3, h4, h5, h6, ${nodeSelector}`)].reverse();
  
  // Truncate the array to make the target node the first item
  let index = allNodes.indexOf(allNodes.find(n => n.contains(node)));
  let truncated = allNodes.slice(index);
  
  // Find the next node thats a heading
  let heading = truncated.find(n => n.matches('h1, h2, h3, h4, h5, h6'));
  
  // Return the next highest level based on the matched heading's level
  return Math.min(parseInt(heading.tagName.substring(1)) + 1, 6);
}