promises and not callbacks
var Button = function(content) { this.content = content;};Button.prototype.click = function() { console.log(this.content + ' clicked');}var myButton = new Button('OK');myButton.click();var looseClick = myButton.click;looseClick(); // not bound, 'this' is not myButton - it is the global objectvar boundClick = myButton.click.bind(myButton);boundClick(); // bound, 'this' is myButton