It can be anoying that jQuery event handlers always execute in the order they were bound. For example:
$('span').click(doStuff1);
$('span').click(doStuff2);
clicking on the span will cause doStuff1() to fire, followed by doStuff2().
At the time I bind doStuff2(), I would like the option to bind it before doStuff1(), but there doesn't appear to be any easy way to do this.
I suppose most people would say, just write the code like this:
$('span').click(function (){
doStuff2();
doStuff1();
});
But this is just a simple example - in practise it is not always convienient to do that.
There are situations when you want to bind an event, and the object you are binding to already has events. And in this case you may simply want the new event to fire before any other existing events.
So what is the best way to achieve this in jQuery?
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A very good question ... I was intrigued so I did a little digging; for those who are interested, here's where I went, and what I came up with.
Looking at the source code for jQuery 1.4.2 I saw this block between lines 2361 and 2392:
jQuery.each(["bind", "one"], function( i, name ) {
jQuery.fn[ name ] = function( type, data, fn ) {
// Handle object literals
if ( typeof type === "object" ) {
for ( var key in type ) {
this[ name ](key, data, type[key], fn);
}
return this;
}
if ( jQuery.isFunction( data ) ) {
fn = data;
data = undefined;
}
var handler = name === "one" ? jQuery.proxy( fn, function( event ) {
jQuery( this ).unbind( event, handler );
return fn.apply( this, arguments );
}) : fn;
if ( type === "unload" && name !== "one" ) {
this.one( type, data, fn );
} else {
for ( var i = 0, l = this.length; i < l; i++ ) {
jQuery.event.add( this[i], type, handler, data );
}
}
return this;
};
});
There is a lot of interesting stuff going on here, but the part we are interested in is between lines 2384 and 2388:
else {
for ( var i = 0, l = this.length; i < l; i++ ) {
jQuery.event.add( this[i], type, handler, data );
}
}
Every time we call bind() or one() we are actually making a call to jQuery.event.add() ... so let's take a look at that (lines 1557 to 1672, if you are interested)
add: function( elem, types, handler, data ) {
// ... snip ...
var handleObjIn, handleObj;
if ( handler.handler ) {
handleObjIn = handler;
handler = handleObjIn.handler;
}
// ... snip ...
// Init the element's event structure
var elemData = jQuery.data( elem );
// ... snip ...
var events = elemData.events = elemData.events || {},
eventHandle = elemData.handle, eventHandle;
if ( !eventHandle ) {
elemData.handle = eventHandle = function() {
// Handle the second event of a trigger and when
// an event is called after a page has unloaded
return typeof jQuery !== "undefined" && !jQuery.event.triggered ?
jQuery.event.handle.apply( eventHandle.elem, arguments ) :
undefined;
};
}
// ... snip ...
// Handle multiple events separated by a space
// jQuery(...).bind("mouseover mouseout", fn);
types = types.split(" ");
var type, i = 0, namespaces;
while ( (type = types[ i++ ]) ) {
handleObj = handleObjIn ?
jQuery.extend({}, handleObjIn) :
{ handler: handler, data: data };
// Namespaced event handlers
^
|
// There is is! Even marked with a nice handy comment so you couldn't miss it
// (Unless of course you are not looking for it ... as I wasn't)
if ( type.indexOf(".") > -1 ) {
namespaces = type.split(".");
type = namespaces.shift();
handleObj.namespace = namespaces.slice(0).sort().join(".");
} else {
namespaces = [];
handleObj.namespace = "";
}
handleObj.type = type;
handleObj.guid = handler.guid;
// Get the current list of functions bound to this event
var handlers = events[ type ],
special = jQuery.event.special[ type ] || {};
// Init the event handler queue
if ( !handlers ) {
handlers = events[ type ] = [];
// ... snip ...
}
// ... snip ...
// Add the function to the element's handler list
handlers.push( handleObj );
// Keep track of which events have been used, for global triggering
jQuery.event.global[ type ] = true;
}
// ... snip ...
}
At this point I realized that understanding this was going to take more than 30 minutes ... so I searched Stackoverflow for
jquery get a list of all event handlers bound to an element
and found this answer for iterating over bound events:
//log them to the console (firebug, ie8)
console.dir( $('#someElementId').data('events') );
//or iterate them
jQuery.each($('#someElementId').data('events'), function(i, event){
jQuery.each(event, function(i, handler){
console.log( handler.toString() );
});
});
Testing that in Firefox I see that the events object in the data attribute of every element has a [some_event_name] attribute (click in our case) to which is attatched an array of handler objects, each of which has a guid, a namespace, a type, and a handler. "So", I think, "we should theoretically be able to add objects built in the same manner to the [element].data.events.[some_event_name].push([our_handler_object); ... "
And then I go to finish writing up my findings ... and find a much better answer posted by RusselUresti ... which introduces me to something new that I didn't know about jQuery (even though I was staring it right in the face.)
Which is proof that Stackoverflow is the best question-and-answer site on the internet, at least in my humble opinion.
So I'm posting this for posterity's sake ... and marking it a community wiki, since RussellUresti already answered the question so well.
Here's a solution for jQuery 1.4.x (unfortunately, the accepted answer didn't work for jquery 1.4.1)
$.fn.bindFirst = function(name, fn) {
// bind as you normally would
// don't want to miss out on any jQuery magic
this.bind(name, fn);
// Thanks to a comment by @Martin, adding support for
// namespaced events too.
var handlers = this.data('events')[name.split('.')[0]];
// take out the handler we just inserted from the end
var copy = {1: null};
var last = 0, lastValue = null;
$.each(handlers, function(name, value) {
//console.log(name + ": " + value);
var isNumber = !isNaN(name);
if(isNumber) {last = name; lastValue = value;};
var key = isNumber ? (parseInt(name) + 1) : name;
copy[key] = value;
});
copy[1] = lastValue;
this.data('events')[name.split('.')[0]] = copy;
};
Chris Chilvers' advice should be the first course of action but sometimes we're dealing with third party libraries that makes this challenging and requires us to do naughty things... which this is. IMO it's a crime of presumption similar to using !important in CSS.
Having said that, building on Anurag's answer, here are a few additions. These methods allow for multiple events (e.g. "keydown keyup paste"), arbitrary positioning of the handler and reordering after the fact.
$.fn.bindFirst = function (name, fn) {
this.bindNth(name, fn, 0);
}
$.fn.bindNth(name, fn, index) {
// Bind event normally.
this.bind(name, fn);
// Move to nth position.
this.changeEventOrder(name, index);
};
$.fn.changeEventOrder = function (names, newIndex) {
var that = this;
// Allow for multiple events.
$.each(names.split(' '), function (idx, name) {
that.each(function () {
var handlers = $._data(this, 'events')[name.split('.')[0]];
// Validate requested position.
newIndex = Math.min(newIndex, handlers.length - 1);
handlers.splice(newIndex, 0, handlers.pop());
});
});
};
One could extrapolate on this with methods that would place a given handler before or after some other given handler.
For jQuery 1.9+ as Dunstkreis mentioned .data('events') was removed. But you can use another hack (it is not recommended to use undocumented possibilities) $._data($(this).get(0), 'events') instead and solution provided by anurag will look like:
$.fn.bindFirst = function(name, fn) {
this.bind(name, fn);
var handlers = $._data($(this).get(0), 'events')[name.split('.')[0]];
var handler = handlers.pop();
handlers.splice(0, 0, handler);
};
You can do a custom namespace of events.
$('span').bind('click.doStuff1',function(){doStuff1();});
$('span').bind('click.doStuff2',function(){doStuff2();});
Then, when you need to trigger them you can choose the order.
$('span').trigger('click.doStuff1').trigger('click.doStuff2');
or
$('span').trigger('click.doStuff2').trigger('click.doStuff1');
Also, just triggering click SHOULD trigger both in the order they were bound... so you can still do
$('span').trigger('click');
.data("events") has been removed in versions 1.9 and 2.0beta, so you cant any longer rely on those solutions.
The standard principle is separate event handlers shouldn't depend upon the order they are called. If they do depend upon the order they should not be separate.
Otherwise, you register one event handler as being 'first' and someone else then registers their event handler as 'first' and you're back in the same mess as before.
I'm assuming you are talking about the event bubbling aspect of it. It would be helpful to see your HTML for the said span elements as well. I can't see why you'd want to change the core behavior like this, I don't find it at all annoying. I suggest going with your second block of code:
$('span').click(function (){
doStuff2();
doStuff1();
});
Most importantly I think you'll find it more organized if you manage all the events for a given element in the same block like you've illustrated. Can you explain why you find this annoying?
The selected answer authored by Anurag is only partially correct. Due to some internals of jQuery's event handling, the proposed bindFirst function will not work if you have a mix of handlers with and without filters (ie: $(document).on("click", handler) vs $(document).on("click", "button", handler)).
The issue is that jQuery will place (and expect) that the first elements in the handler array will be these filtered handlers, so placing our event without a filter at the beginning breaks this logic and things start to fall apart. The updated bindFirst function should be as follows:
$.fn.bindFirst = function (name, fn) {
// bind as you normally would
// don't want to miss out on any jQuery magic
this.on(name, fn);
// Thanks to a comment by @Martin, adding support for
// namespaced events too.
this.each(function () {
var handlers = $._data(this, 'events')[name.split('.')[0]];
// take out the handler we just inserted from the end
var handler = handlers.pop();
// get the index of the first handler without a selector
var firstNonDelegate = handlers.first(function(h) { return !h.selector; });
var index = firstNonDelegate ? handlers.indexOf(firstNonDelegate)
: handlers.length; // Either all handlers are selectors or we have no handlers
// move it at the beginning
handlers.splice(index, 0, handler);
});
};
Source: Stackoverflow.com