need explanation of the _.bindAll() function from Underscore.js

Question

I\'ve been learning some backbone.js and I\'ve seen plenty of instances where _.bindAll() is used. I have read through the entire backbone.js and underscore.js docu

Answer 1

The simplest explanation as for me is the next:

initialize:function () { //backbone initialize function
    this.model.on("change",this.render); //doesn't work because of the wrong context - in such a way we are searching for a render method in the window object  
    this.model.on("change",this.render,this); //works fine
    //or 
    _.bindAll(this,'render');
    this.model.on("change",this.render); //now works fine
    //after  _.bindAll we can use short callback names in model event bindings
}

Answer 2

try this

<input type="button" value="submit" id="underscore_button"/>

<script>
var buttonView = {
    id     : 'underscore',
    onClick: function () {console.log('clicked: ' + this.id)},
    onHover: function () {console.log('hovering: ' + this.id)}
}
_.bindAll(buttonView, 'onClick')
$('#underscore_button').click(buttonView.onClick)
$('#underscore_button').hover(buttonView.onHover)
</script>

Answer 3

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()).