index.js

var selectors = [
  'iframe[src*="player.vimeo.com"]',
  'iframe[src*="youtube.com"]',
  'iframe[src*="youtube-nocookie.com"]',
  'iframe[src*="kickstarter.com"][src*="video.html"]',
  "object"
];

var css =
  ".fluid-width-video-wrapper{width:100%;position:relative;padding:0;}.fluid-width-video-wrapper iframe,.fluid-width-video-wrapper object,.fluid-width-video-wrapper embed {position:absolute;top:0;left:0;width:100%;height:100%;}";

module.exports = function(parentSelector, opts) {
  parentSelector = parentSelector || "body";
  opts = opts || {};

  if (isObject(parentSelector)) {
    opts = parentSelector;
    parentSelector = "body";
  }

  opts.ignore = opts.ignore || "";
  opts.players = opts.players || "";

  var containers = queryAll(parentSelector);
  if (!hasLength(containers)) return;

  if (!document.getElementById("fit-vids-style")) {
    var head = document.head || document.getElementsByTagName("head")[0];
    head.appendChild(styles());
  }

  var custom = toSelectorArray(opts.players);
  var ignored = toSelectorArray(opts.ignore);
  var ignoredSelector = ignored.length > 0 ? ignored.join() : null;
  var selector = selectors.concat(custom).join();

  if (!hasLength(selector)) {
    return;
  }

  containers.forEach(function(container) {
    var videos = queryAll(container, selector);

    videos.forEach(function(video) {
      if (ignoredSelector && video.matches(ignoredSelector)) {
        return;
      }
      wrap(video);
    });
  });
}

function queryAll(el, selector) {
  if (typeof el === "string") {
    selector = el;
    el = document;
  }
  return Array.prototype.slice.call(el.querySelectorAll(selector));
}

function toSelectorArray(input) {
  if (typeof input === "string") {
    return input
      .split(",")
      .map(trim)
      .filter(hasLength);
  } else if (isArray(input)) {
    return flatten(input.map(toSelectorArray).filter(hasLength));
  }
  return input || [];
}

function wrap(el) {
  if (/fluid-width-video-wrapper/.test(el.parentNode.className)) {
    return;
  }

  var widthAttr = parseInt(el.getAttribute("width"), 10);
  var heightAttr = parseInt(el.getAttribute("height"), 10);

  var width = !isNaN(widthAttr) ? widthAttr : el.clientWidth;
  var height = !isNaN(heightAttr) ? heightAttr : el.clientHeight;
  var aspect = height / width;

  el.removeAttribute("width");
  el.removeAttribute("height");

  var wrapper = document.createElement("div");
  el.parentNode.insertBefore(wrapper, el);
  wrapper.className = "fluid-width-video-wrapper";
  wrapper.style.paddingTop = aspect * 100 + "%";
  wrapper.appendChild(el);
}

function styles() {
  var div = document.createElement("div");
  div.innerHTML = '<p>x</p><style id="fit-vids-style">' + css + "</style>";
  return div.childNodes[1];
}

function hasLength(input) {
  return input.length > 0;
}

function trim(str) {
  return str.replace(/^\s+|\s+$/g, "");
}

function flatten(input) {
  return [].concat.apply([], input);
}

function isObject(input) {
  return Object.prototype.toString.call(input) === "[object Object]";
}

function isArray(input) {
  return Object.prototype.toString.call(input) === "[object Array]";
}