DocumentService (jQuery.ready substitute) 

The module TYPO3/CMS/Core/DocumentService provides native JavaScript functions to detect DOM ready-state returning a Promise<Document>.

Internally the Promise is resolved when native DOMContentLoaded event has been emitted or when document.readyState is defined already. It means the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and sub-frames to finish loading.

import $ from 'jquery';

$(document).ready(() => {
  // your application code
});

Above jQuery code can be transformed into the following using DocumentService:

import DocumentService from '@typo3/core/document-service.js';

DocumentService.ready().then(() => {
  // your application code
});