Develop Reference

How to assign a function to a object method in javascript?

I'd like to 'proxy' (not sure if that's the term at all) a function inside a function object for easy calling.
Given the following code
function Soldier() {
this.el = $("<div></div>").addClass('soldier');
this.pos = this.el.position; // $(".soldier").position(), or so I thought
}
In the console:
s = new Soldier();
$("#gamemap").append(s.el); // Add the soldier to the game field
s.pos === s.el.position // this returns true
s.el.position() // Returns Object {top: 0, left: 0}
s.pos() // Returns 'undefined'
What am I doing wrong in this scenario and is there an easy way to achieve my goal (s.pos() to return the result of s.el.position()) ?
I thought about s.pos = function() { return s.el.position(); } but looks a bit ugly and not apropriate. Also I'd like to add more similar functions and the library will become quite big to even load.

When you're calling s.pos(), its this context is lost.
You can simulate this behavior using call():
s.pos.call(s); // same as s.pos()
s.pos.call(s.el); // same as s.el.position()
This code is actually ok:
s.pos = function() { return s.el.position(); }
An alternative is using bind():
s.pos = s.el.position.bind(el);
You can use the prototype, that way the functions will not be created separately for every object:
Soldier.prototype.pos = function(){ return this.el.position(); }

I'd recommend to use the prototype:
Soldier.prototype.pos = function() { return this.el.position(); };
Not ugly at all, and quite performant actually.
If you want to directly assign it in the constructor, you'll need to notice that the this context of a s.pos() invocation would be wrong. You therefore would need to bind it:
…
this.pos = this.el.position.bind(this.el);

It's because the context of execution for position method has changed. If you bind the method to work inside the element context it will work.
JS Fiddle
function Soldier() {
this.el = $("<div></div>").addClass('soldier');
this.pos = this.el.position.bind(this.el);
}
var s = new Soldier();
$("#gamemap").append(s.el);
console.log(s.pos());

Store state of a JavaScript Object

Im trying to store the stats of 'this' in my javscript object so that later on in my application I can return 'this' to a previous state. I thought I could accomplish using a closure but so far I haven't successful. My idea was to do something like this
function SavedFeature() {
var self = this;
this.savedItem;
this.storeState = function() {
this.savedItem = storeClosure();
}
function storeClosure() {
var closure = self;
return function() {
return closure;
};
};
//other things the user can change...
}
so later on in my application if I needed to return to the point when I called storeState I could just do
//return the object I put in my closure
var backToNormal = savedFeature.savedItem();
that doesn't work though because any changes to my savedFeature object after I call storeState() are being reflected in the item im retrieving from called savedItem(). I'm guessing this is happening because closure is being set to a reference of self instead of copied to a new instance.
Is there anyway to store the state of my entire object in a closure like this or do I need to store this some other way.

The issue you are running into is that in js objects are passed by reference. This means that all changes performed on your object will apply to your obj.savedItem property.
Fix: Store a deep clone into obj.savedItem
this.storeState = function() {
this.savedItem = _.cloneDeep(this); // or _.clone(this, true);
}
cloneDeep is a lodash method, most js libs supply one of their own, e.g. jQuery's $.extend, etc.
You could easily roll your own deep clone function, look up the options on this thread.
A complete example with jQuery:
function SavedFeature() {
this.savedItem;
this.clone = function() {
return $.extend(true, {}, this);
},
this.storeState = function() {
this.savedItem = this.clone();
}
}
Doing it this way allows you adapt to different environments by changing your clone method as it is facading the used library method.

There are dozens of ways how to implement it. I will do just simple one. saving property.
Take into account if you want to save entire object you need to do deep copy of the object.
this is your feature:
function SavedFeature() {
this.savedItem = {'isNew': true};
this.stateMachine = new StateMachine();
}
this is some kind of state machine:
function StateMachine () {
var state = { 'isNew' : null};
function set(newState) {
state.isNew = newState.isNew;
}
function get() {
return state.isNew;
}
return {
get : get,
set : set
};
}
which, know how to store isNew property
and a working sample:
var savedFeature = new SavedFeature();
console.log(savedFeature.savedItem); // true by default
savedFeature.stateMachine.set(savedFeature.savedItem); // saving state.
savedFeature.savedItem.isNew = false; // modifying state
console.log(savedFeature.savedItem); // return false, because of statement above
var restoredState = savedFeature.stateMachine.get(); // restoring state
console.log(restoredState); // true
savedFeature.savedItem.isNew = restoredState.isNew;
console.log(savedFeature.savedItem); // true
you can adjust that code, and reach functionality whatever you need. hope that helps

Userscript - Replace a variable's property with a function

A website has the following code:
var Items = {
drop: function (a, b, d) {
if (!(typeof a == "undefined" || typeof sockets[a.id] == "undefined")) {
SSocket.send(sockets[a.id], {
action: "item_drop",
data: {
id: d
}
});
Inventory.add(a, d)
}
},
give_to_player: function (a, b) {
Items.drop(a, void 0, b)
},
take_from_player: function (a, b) {
var d = clients[a];
Inventory.remove(d, b);
Player.send_inventory(d.id)
},
};
I am trying to replace the give_to_player property with my own function using a userscript. However, I am having zero luck doing so. I am familiar with javascript injection and the variable scope.
I have tried the following:
Object.defineProperty(window.Item, 'give_to_player', {
value:
function(a,b){
console.log('Change occured');
}
});
This does not generate any errors, however the change does not take hold and the console remains empty. I have tried Object.defineProperties as well with no luck.
Finally the following code failed to produce results either:
window.Item.give_to_player = function(a,b){ console.log('Change occured');};
Does anyone have any suggestions?
I am using Chrome to run my userscripts.

The second method would work if you change the name to Items with a s and drop the window in the method to just Items.give_to_player = function(a,b){ console.log('Change occured');};.
EDIT: the var in var Items makes the method not accessible thru the window scope. if the var was dropped this window.Items.give_to_player won't throw error but since its there you'll not need to use the window in front of Items.(if that makes sense)
JSFIDDLE
side note: your error
window.Items.give_to_player = function(a,b){ console.log('Change occured');};
// Uncaught TypeError: Cannot set property 'give_to_player' of undefined

I really don't know how the rest of code looks like (if that object is in some particular scope, deeply nested or what) but if Items object is in global scope you can define AFTER that object (and its properties definition) again that property and that should override the previous one:
Items.give_to_player: function () {
//write your own function
}
But I'm not sure if this will work as long as I have so little information.

Referencing a parent object in callback functions with jQuery

I've a page that is generated dynamically, and that includes certain number (user-dynamically-defined) of advanced scatter plot charts. I intend to create a JavaScript object which defines the scatter plot itself, i.e. which takes some parameters, some data, and some container ID, and which will create the various elements needed to obtain the visualisation: canvas elements, toolbar, etc.. To do so, I started with the following (simplified) class:
(function () {
if (!this.namespace) { this.namespace = {};}
this._instances = { index: 0 };
this.namespace.ScatterPlot = function (containerId, file, options) {
_instances.index ++;
this.id = this.containerId+"-"+_instances.index ;
this.containerId = containerId ;
_instances [this.id] = this;
// ... Do stuffs with file and options ...
// Initialize elements once the DOM is ready
$(this.updateDOM);
}
namespace.ScatterPlot.prototype = {
updateDOM: function() {
$("<canvas>")
.click(clickCallback)
.appendTo("#"+this.containerId);
//(...)
},
clickCallback: function() {
alert("Some click: "+this.id);
}
}
})();
Each object can be created with:
var v1 = new namespace.ScatterPlot("container1", "foo", "foo");
var v2 = new namespace.ScatterPlot("container2", "foo", "foo");
There are two problems here: (1) in updateDOM, 'this' does not make reference to my initial ScatterPlot object, which means that this example will never work, and (2) similarly, the clickCallback will not be able reference the scatterplot with 'this' either.
I'm new to javascript, and I'm still struggeling to understand the logic of OO programming in javascript, so the question is: I'm I taking the wrong direction here ? After some digging, I could roughly achieve what I wanted by passing this to updateDOM:
$(this.updateDOM(this)); // This blows my eyes but does the trick, at least partially
updateDOM: function(that) {
$("<canvas>")
.click(that.clickCallback)
.appendTo("#"+that.containerId);
//(...)
},
clickCallback: function() {
// Not working either... Should pass 'that' to the function too
alert("Some click: "+this.id);
}
But I don't feel this patters to be very elegant... And the problem is not fixed either regarding the click callback.
Thoughts ?

Have a look at MDN's introduction to the this keyword.
The standard ways of dealing with that issue are using a that variable - not as an argument, but in a separate function:
var that = this;
$(function() {
that.updateDOM();
});
// or
$(this.getClickCallback());
...
namespace.ScatterPlot.prototype.getClickCallback = function() {
var that = this;
return function clickCallback(e) {
alert("Some click: "+that.id);
};
};
Alternatively, you can always use .bind() (or $.proxy for older browsers) which do quite what the second example does in a more generic way:
$(this.clickCallback.bind(this));

Creating methods on the fly

Hi I'm trying to author a jQuery plugin and I need to have methods accessible to elements after they are initialized as that kind of object, e.g.:
$('.list').list({some options}); //This initializes .list as a list
//now I want it to have certain methods like:
$('.list').find('List item'); //does some logic that I need
I tried with
$.fn.list = function (options) {
return this.each(function() {
// some code here
this.find = function(test) {
//function logic
}
}
}
and several other different attempts, I just can't figure out how to do it.
EDIT:
I'll try to explain this better.
I'm trying to turn a table into a list, basically like a list on a computer with column headers and sortable items and everything inbetween. You initiate the table with a command like
$(this).list({
data: [{id: 1, name:'My First List Item', date:'2010/06/26'}, {id:2, name:'Second', date:'2010/05/20'}]
});
.list will make the <tbody> sortable and do a few other initial tasks, then add the following methods to the element:
.findItem(condition) will allow you to find a certain item by a condition (like findItem('name == "Second"')
.list(condition) will list all items that match a given condition
.sort(key) will sort all items by a given key
etc.
What's the best way to go about doing this?

If you want these methods to be available on any jQuery object, you will have to add each one of them to jQuery's prototype. The reason is every time you call $(".list") a fresh new object is created, and any methods you attached to a previous such object will get lost.
Assign each method to jQuery's prototype as:
jQuery.fn.extend({
list: function() { .. },
findItem: function() { .. },
sort: function() { .. }
});
The list method here is special as it can be invoked on two occasions. First, when initializing the list, and second when finding particular items by a condition. You would have to differentiate between these two cases somehow - either by argument type, or some other parameter.
You can also use the data API to throw an exception if these methods are called for an object that has not been initialized with the list plugin. When ('xyz').list({ .. }) is first called, store some state variable in the data cache for that object. When any of the other methods - "list", "findItem", or "sort" are later invoked, check if the object contains that state variable in its data cache.
A better approach would be to namespace your plugin so that list() will return the extended object. The three extended methods can be called on its return value. The interface would be like:
$('selector').list({ ... });
$('selector').list().findOne(..);
$('selector').list().findAll(..);
$('selector').list().sort();
Or save a reference to the returned object the first time, and call methods on it directly.
var myList = $('selector').list({ ... });
myList.findOne(..);
myList.findAll(..);
myList.sort();

I found this solution here:
http://www.virgentech.com/blog/2009/10/building-object-oriented-jquery-plugin.html
This seems to do exactly what I need.
(function($) {
var TaskList = function(element, options)
{
var $elem = $(element);
var options = $.extend({
tasks: [],
folders: []
}, options || {});
this.changed = false;
this.selected = {};
$elem.sortable({
revert: true,
opacity: 0.5
});
this.findTask = function(test, look) {
var results = [];
for (var i = 0,l = options.tasks.length; i < l; i++)
{
var t = options['tasks'][i];
if (eval(test))
{
results.push(options.tasks[i]);
}
}
return results;
}
var debug = function(msg) {
if (window.console) {
console.log(msg);
}
}
}
$.fn.taskList = function(options)
{
return this.each(function() {
var element = $(this);
if (element.data('taskList')) { return; }
var taskList = new TaskList(this, options);
element.data('taskList', taskList);
});
}
})(jQuery);
Then I have
$('.task-list-table').taskList({
tasks: eval('(<?php echo mysql_real_escape_string(json_encode($tasks)); ?>)'),
folders: eval('(<?php echo mysql_real_escape_string(json_encode($folders)); ?>)')
});
var taskList = $('.task-list-table').data('taskList');
and I can use taskList.findTask(condition);
And since the constructor has $elem I can also edit the jQuery instance for methods like list(condition) etc. This works perfectly.

this.each isn't needed. This should do:
$.fn.list = function (options) {
this.find = function(test) {
//function logic
};
return this;
};
Note that you'd be overwriting jQuery's native find method, and doing so isn't recommended.
Also, for what it's worth, I don't think this is a good idea. jQuery instances are assumed to only have methods inherited from jQuery's prototype object, and as such I feel what you want to do would not be consistent with the generally accepted jQuery-plugin behaviour -- i.e. return the this object (the jQuery instance) unchanged.