play video when scrolled to w/ wechat browser support

player-wechat.js

(function($, document, window) {
  var ua = navigator.userAgent.toLowerCase();
  var isWechat = ua.indexOf("micromessenger") >= 0;
  
  var Player = function(videos) {
    this.init(videos);
  };

  Player.prototype.init = function(videos) {
    this.videos = [];

    videos.each($.proxy(function(i, el) {
      var $el = $(el);
      $el.attr("playsinline", "true");
      $el.attr("muted", "true");
      
      if (isWechat) {
        // must play for wechat to enable this video.
        $el.get(0).play();
        $el.get(0).pause();
      }

      this.add($el);
    }, this));
  };

  Player.prototype.add = function($el) {
    var top = $el.offset().top;
    var bottom = top + $el.height();

    this.videos.push({
      top: top,
      bottom: bottom,
      $el: $el,
    });
  };

  Player.prototype.scrolled = function() {
    var $window = $(window);
    var top = $window.scrollTop();
    var bottom = top + $window.height();

    $.each($.map(this.videos, function(obj, i) {
      var percent = 0;
      if (obj.top <= bottom && obj.bottom >= top) {
        var h = obj.bottom - obj.top;

        if (obj.bottom > bottom) {
          percent = (bottom - obj.top) / h;
        } else if (obj.top < top) {
          percent = (obj.bottom - top) / h;
        } else {
          percent = 1;
        }
      }

      return {
        percent: percent,
        $el: obj.$el,
      };
    }), function(i, obj) {
      if (obj.percent > 0.75) {
        obj.$el.get(0).play();
      } else {
        obj.$el.get(0).pause();
      }
    });
  };

  Player.prototype.resized = function() {
    $.each(this.videos, function(i, obj) {
      obj.top = obj.$el.offset().top;
      obj.bottom = obj.top + obj.$el.height();
    });

    this.scrolled();
  };

  Player.prototype.listen = function() {
    var $window = $(window);
    $window.on('scroll', $.proxy(function() {
      if (this.videos.length) {
        this.scrolled();
      }
    }, this));

    $window.on('resize', $.proxy(function() {
      if (this.videos.length) {
        this.resized();
      }
    }, this));
  };

  $(document).ready(function() {
    if (isWechat) {
      document.addEventListener("WeixinJSBridgeReady", function () {
        var player = new Player($("video"));
        player.listen();
      }, false);
    } else {
      var player = new Player($("video"));
      player.listen();
    }
  });
})(jQuery, document, window);