table.js

// Первый способ
var Table = function(names) {
  this.names = names;
  this.tbody = document.querySelector('.tbody');
}

Table.prototype.render = function() {
  var self = this;
  setInterval(function() {
    var x = self.tbody.children.length;
    }
  }, 1000);
}

// Второй способ
var Table = function(names) {
  this.names = names;
  this.tbody = document.querySelector('.tbody');
  this.fn = function() {
    var x = this.tbody.children.length;
  };
};

Table.prototype.render = function() {
  setInterval(this.fn.bind(this), 1000);
};