videoCover.js

this.el = document.getElementById('video');

var minW = 1024;
var minH = 661;

var onResize = function() {
  var ch, cr, cw, ph, pr, pw, css;
  if (!this.el) {
    return;
  }
  pw = this.el.videoWidth;
  ph = this.el.videoHeight;
  cw = window.innerWidth;
  ch = window.innerHeight;
  pr = pw / ph;
  cr = cw / ch;

  cw = cw > minW ? cw : minW;
  ch = ch > minH ? ch : minH;

  if (cr < pr) {
    css = {
      width: ch * pr,
      height: ch,
      top: 0,
      left: -((ch * pr) - cw) * 0.5
    };
  } else {
    css = {
      width: cw,
      height: cw / pr,
      top: -((cw / pr) - ch) * 0.5,
      left: 0
    };
  }

  this.el.style.width = css.width+'px';
  this.el.style.height = css.height+'px';
  this.el.style.top = css.top+'px';
  this.el.style.left = css.left+'px';
}.bind(this);

this.el.style.position = 'absolute';

// Listen to window resize event
window.addEventListener('resize',onResize);