Google Docs: Numbering Document headings (titles) Script

Code.js

/**!
* NUMBER THE HEADINGS
* This script can't be used for commercial purpose.
*
* Script improved by: Geoffrey Crofte / GeoffreyCrofte.com / 2022
* Inspired by: Antoine Martin / ThierrryVanoffe.com / 2020
*/

// Constants will be used later on.
const ui      = DocumentApp.getUi();
const thisDoc = DocumentApp.getActiveDocument();
const body    = thisDoc.getBody();
const blocks  = body.getParagraphs();

// Regular expression to use for the removing function.
// Uses pipes to consider conditions "or".
const pattern = "(([0-9]{1,3}\.){1}|([0-9]{1,3}\.){1}([0-9]{1,3}\.){1}|([0-9]{1,3}\.){1}([0-9]{1,3}\.){1}([a-z]{1}\.{1})|([0-9]{1,3}\.){1}([0-9]{1,3}\.){1}([0-9]{1,3}\.){1}|([0-9]{1,3}\.){1}([0-9]{1,3}\.){1}([0-9]{1,3}\.){1}([a-z]{1}\.){1})[ ]{1}";

/**
 * When the document is opened by the user, set the menu into the UI.
 * @see: https://developers.google.com/apps-script/guides/menus
 */
function onOpen() {
  let ui = DocumentApp.getUi();

  ui.createMenu("🔧Tools")
  .addSubMenu(ui.createMenu("Number the headings")
              .addItem("Number headings 1 to 2", "numberHeadings12")
              .addItem("Number headings 1 to 3", "numberHeadings13")
              .addItem("Number headings 1 to 4", "numberHeadings14")
              .addSeparator()
              .addItem("Remove numbering","removeNumbering")
             )
  .addToUi();
}

/**
 * Default aliased function triggered to number the headings.
 * @param: to (int) the maximum level numbered. (2 to 4)
 */
function numberHeadings(to) {
  //Supprimer tous les numéros d'une numérotation précédente
  removeNumbering();
  
  // Counters start at 0 since we will increment them firts. 
  let nh1 = 0;
  let nh2 = 0;
  let nh3 = 0;
  let nh4 = 0;  
  let alpha = ["","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"];
  
  // if to (max heading level) isn't set, take 3 by default.
  to = to ? parseInt(to) : 3;
  
  // Loop on all the block types (p, hn, ul, li)
  for (let i = 0 ; i < blocks.length ; i++) {
    let thisBlock = blocks[i]; 
    let heading = thisBlock.getHeading();

    switch ( heading ) {
      // Heading 1
      case DocumentApp.ParagraphHeading.HEADING1:
        // Increment H1 and re-init subsequent headings.
        nh1++;
        nh2 = 0;
        nh3 = 0;
        nh4 = 0;
        
        // Insert the number before the text with a ". " between.
        thisBlock.editAsText().insertText(0 , nh1+". ");
      break;

      // Heading 2
      case DocumentApp.ParagraphHeading.HEADING2:
        nh2++;
        nh3 = 0;
        nh4 = 0;

        thisBlock.editAsText().insertText(0 , nh1+"."+nh2+". ");
      break;

      // Heading 3
      case DocumentApp.ParagraphHeading.HEADING3:
        if ( to === 2 ) break;
        nh3++;
        nh4 = 0;

        // Condition here, alpha if heading til 3, else it's a number.
        thisBlock.editAsText().insertText(0, nh1+"."+nh2+"." + (to === 3 ? alpha[nh3] : nh3) + ". ");
      break;

      // Heading 4
      case DocumentApp.ParagraphHeading.HEADING4:
        if ( to <= 3 ) break;
        nh4++;

        thisBlock.editAsText().insertText(0, nh1+"."+nh2+"."+nh3+"."+alpha[nh4]+". ");
        break;
    }

  }
}

/**
 * Function that removes the numbering regarding the pattern declared.
 */
function removeNumbering() {
  for (let i = 0 ; i < blocks.length ; i++) {
    let thisBlock = blocks[i]; 
    let heading = thisBlock.getHeading();
    
    if ( 
      heading == DocumentApp.ParagraphHeading.HEADING4 ||
      heading == DocumentApp.ParagraphHeading.HEADING3 ||
      heading == DocumentApp.ParagraphHeading.HEADING2 ||
      heading == DocumentApp.ParagraphHeading.HEADING1 ) { 
        thisBlock.editAsText().replaceText(pattern, '');
    }
  }
}

/**
 * Alias functions for numbering from 1 to N
 */
function numberHeadings12() {
  numberHeadings(2);
}

function numberHeadings13() {
  numberHeadings(3);
}

function numberHeadings14() {
  numberHeadings(4);
}

How to use it.txt

HOW TO USE IT

1. Go to Extensions > Apps Script menu.
2. Copy/Paste the code from Code.js above.
3. Give it a name
4. Deploy the script (button Deploy > New Deployment)

That's it, now you have a new menu Item in the toolbar of your document.