vanilla-fitvids.js

// Vanilla version of FitVids
// Still licencened under WTFPL
//
// Not as robust and fault tolerant as the jQuery version.
// It's BYOCSS.
// And also, I don't support this at all whatsoever.

;(function(window, document, undefined) {
  'use strict';
  
  // List of Video Vendors embeds you want to support
  var players = [
    'iframe[src*="youtube.com"]',
    'iframe[src*="vimeo.com"]'
  ];
  
  // Select videos
  var fitVids = document.querySelectorAll(players.join(','));
  
  // If there are videos on the page...
  if(fitVids.length) {
  
    // Loop through videos
    for(var i=0;i<fitVids.length;i++) {
  
      // Get Video Information
      var fitVid = fitVids[i];
      var width = fitVid.getAttribute('width');
      var height = fitVid.getAttribute('height');
      var aspectRatio = height/width;
      var parentDiv = fitVid.parentNode;
  
      // Wrap it in a DIV
      var div = document.createElement('div');
      div.className = 'fitVids-wrapper';
      div.style.paddingBottom = aspectRatio * 100 + "%";
      parentDiv.insertBefore( div, fitVid );
      fitVid.remove();
      div.appendChild( fitVid );
  
      // Clear height/width from fitVid
      fitVid.removeAttribute('height')
      fitVid.removeAttribute('width');
    }
  }
})(window, document);

removeAttribute does not return anything, so chaining it like fitVid.removeAttribute('height').removeAttribute('width'); will cause an error.