A Google App Script to build a Table of TODOs from TODOs distributed in a Google Doc document.

updateTableOfTodos.js

/*
See https://www.calliopesounds.com/2019/10/adding-table-of-todos-to-google-document.html

This Google App Script script collects the text of all the TODO paragraphs 
and organizes them under a Table of TODOs heading. The TODO paragraph is 
normal-styled and starts with the string "TODO: ". The Table of TODOs is a 
heading-styled paragraph and contains only the text "Table of TODOs". The 
TODO texts are organized in to numbered list items under the heading. In 
addition, the script adds a Bookmark to each TODO paragraph so that it can 
be referenced from the numbered list item.
*/
function updateTableOfTodos() {
  var document = DocumentApp.getActiveDocument();
  var body = document.getBody();

  // Find Table of TODOs remove exiting TODO list items
  var tot = null;  
  for ( var c = body.getChild(0); c !== null; c = c.getNextSibling() ) {
    // Find the Table of TODOs header
    if ( c.getType() === DocumentApp.ElementType.PARAGRAPH ) {
      var p = c.asParagraph();
      if ( p.getHeading() !== DocumentApp.ParagraphHeading.NORMAL ) {
        if ( p.getText().indexOf("Table of TODOs") === 0 ) {
          // Found the Table of TODOs header
          tot = c;
          // Remove the Table of TODOs list items
          for ( c = c.getNextSibling(); c !== null && c.getType() == DocumentApp.ElementType.LIST_ITEM; /* empty */ ) {
            var t = c.getNextSibling();
            c.removeFromParent();
            c = t;
          }
          // Done
          break;
        }
      }
    }
  }
  
  if ( tot !== null ) {
    // Map each bookmark position element index to the bookmark itself.
    // We do this so we don't duplicate bookmarks unnecessarily.
    // We use element index (and not element itself) as the index is the only stable reference to a document part
    var p2b = {};  
    var bookmarks = document.getBookmarks();
    for ( var i = 0; i < bookmarks.length; i++ ) {
      var bookmark = bookmarks[i];
      if ( bookmark.getPosition().getOffset() == 0 && bookmark.getPosition().getElement().getType() === DocumentApp.ElementType.PARAGRAPH ) {
        p2b[ body.getChildIndex(bookmark.getPosition().getElement()) ] = bookmark;  
      }
    };
    
    var todos = [];
    
    // Collect the TODO items
    // These are normal-style paragraphs with leading "TODO: " text
    for ( var c = body.getChild(0); c !== null; c = c.getNextSibling() ) {
      if ( c.getType() === DocumentApp.ElementType.PARAGRAPH ) {
        var p = c.asParagraph();
        if ( p.getHeading() === DocumentApp.ParagraphHeading.NORMAL ) {
          if ( p.getText().indexOf("TODO: ") == 0 ) {
            var todo = p.getText().substring(6);
            var bookmark = p2b[ body.getChildIndex(c) ] || null;
            todos.push( { text: todo, element: p, bookmark: bookmark });
          }
        }
      }
    }
    
    // Populate the Table of TODOs header list items
    if ( todos.length > 0 ) {
      var t = body.getChildIndex(tot);
      for ( var i = 0; i < todos.length; i++ ) {
        var todo = todos[i];
        // add Table of TODOs item
        var li = body.insertListItem(++t, todo.text + " ");
        // add bookmark, if necessary
        if ( ! todo.bookmark ) {
          todo.bookmark = document.newPosition(todo.element,0).insertBookmark()
        }
        // add link to bookmark
        var link = li.appendText(">>");
        link.setLinkUrl("#bookmark="+todo.bookmark.getId());
      }
    }
  }
}

function onOpen() {
  var m = DocumentApp.getUi().createAddonMenu();
  m.addItem("Update Table of TODOs", 'updateTableOfTodos');
  m.addToUi();
}

// END