var Cow = function(name) {
this.name = name;
}
Cow.prototype.moo = function() {
document.getElementById('output').innerHTML += this.name + ' moos' + '<br>';
}
var cow1 = new Cow('alice');
var cow2 = new Cow('bob');
cow1.moo(); // alice moos
cow2.moo(); // bob moos
var func = cow1.moo;
func(); // not what you expect since the function is called with this===window
_.bindAll(cow1, 'moo');
func = cow1.moo;
func(); // alice moos<div id="output" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js"></script>Unfortunately the actual “bind all” functionality only works on functions right on the object. To include a function that is defined on the prototype you need to pass those function names explicitely as additional arguments to _.bindAll().
Anyway, you wanted an explanation: Basically it allows you to replace a function on an object with a function that has the same name and behaviour, but is also bound to that object, so this === theObject even without calling it as a method (theObject.method()).