Best practice for obtaining 'this' context - javascript

Being a long time C++/C# developer, I find myself moving a lot of of my JS code into "classes" to group functions and data together. As those classes handle event though, I'm finding myself having to write "stub" handlers that serve only to route the calls into a class method to provide the proper this context. So I'm doing things like this:
var Manager = {
foo: 'x',
bar: 1,
onClickStub: function(evt) {
// 'this' refers to HTMLElement event source
Manager.onClick(evt);
},
onClick: function(evt) {
// 'this' now refers to Manager.
// real work goes here.
}
}
Is this the normal way of doing things or is there a better way to structure my event handlers while keeping my class organization?

As Joseph Silber said in the comments above, I think bind would be perfect in this case. If you need to support older browsers, you can always add a shim to Function.prototype.bind (see an example implementation in the MDN docs). Then your code could just be:
var Manager = {
var foo: 'x',
var bar: 1,
// no more stub!
onClick: function(evt) {
// 'this' will refer to Manager.
// real work goes here.
}
}
// And when you bind the event handler:
var el = document.getElementById('something');
el.addEventListener('click', Manager.onClick.bind(Manager));

The best way that I know to do this is to assign this to another variable at the top of your class and then refer to that one throughout the class. But of course this only works if you are not using an anonymous object as your class.
For instance:
var Manager = function(){
var self = this,
var foo = 'x',
var bar = 1;
var onClick = function(evt) {
console.log(self); // refers to the manager
console.log(this); // refers to the element the onclick is assigned to
// If you want this to equal the manager then just do: this = self;
}
}() // edit: to make this an immediate function
In response to a comment below you could attach the onclick like this
element.onclick = Manager.onClick;
Then in this case the this variable in the onclick function is indeed the html element, and the self variable is the Manager function.

Related

How adding event handler inside a class with a class-method as the callback?

How do I add an event handler inside a class with a class-method as the callback?
<div id="test">move over here</div>
<script>
oClass = new CClass();
function CClass()
{
this.m_s = "hello :-/";
this.OnEvent = OnEvent;
with(this)
{
var r = document.getElementById("test");
r.addEventListener('mouseover', this.OnEvent); // this does NOT work :-/
}
function OnEvent()
{
alert(this); // this will be the HTML div-element
alert(this.m_s); // will be undefined :-()
}
}
</script>
Yes I know some quirks to make it work but what would be the intended way when these event handlers were introduced ??? I again have the bitter feeling, that no-one truly lives OOP :-(
Here for you to play: https://jsfiddle.net/sepfsvyo/1/
The this inside the event listener callback will be the element that fired the event. If you want the this to be the instance of your class, then either:
Bind the function to the class instance:
Using Function.prototype.bind, will create a new function that its this value will always be what you specify it to be (the class instance):
r.addEventListener('mouseover', this.OnEvent.bind(this));
// ^^^^^^^^^^^
Wrap the function inside an anonymous function:
var that = this;
r.addEventListener('mouseover', function(ev) { that.OnEvent(ev); });
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
or use an arrow function (so no need for that):
r.addEventListener('mouseover', ev => this.OnEvent(ev));
// ^^^^^^^^^^^^^^^^^^^^^^
Note: As mentioned in a comment bellow, both of the above methods pass a different function to addEventListener (the one with bind create a new function, and the anounimous function is obviously !== this.OnEvent). If you are going to remove the event listener later, you'll have to store a reference to the function:
var reference;
r.addEventListener('mouseover', reference = this.OnEvent.bind(this));
// ^^^^^^^^^^^^
or:
var reference;
var that = this;
r.addEventListener('mouseover', reference = function(ev) { that.OnEvent(ev); });
// ^^^^^^^^^^^^
then you can remove the event listener like:
r.removeEventListener('mouseover', reference);
You can actually return the object as an EventListener callback, this way JS will search for an handleEvent method in the class and execute accordingly :
var myInstance = new myClass;
myInstance.addEventListener("mousedown",myInstance);
// To remove the event you can follow the same pattern
myInstance.removeEventListener("mousedown",myInstance);
You have to construct your class this way :
class myClass {
constructor(){
// Whatever this is supposed to do.
// You can also add events listener within the class this way :
this.addEventListener("mousedown",this);
}
mouseDownEvent(e)(){
// Some action related to the mouse down event (e)
console.log(e.target);
}
mouseMoveEvent(e)(){
// Some action related to the mouse move event (e)
}
mouseUpEvent(e)(){
// Some action related to the mouse up event (e)
}
handleEvent(e) {
switch(e.type) {
case "mousedown":
this.mouseDownEvent(e);
break;
case "mousemove":
this.mouseMoveEvent(e);
break;
case "mouseup":
this.mouseUpEvent(e);
break;
}
}
}
Sources :
https://medium.com/#WebReflection/dom-handleevent-a-cross-platform-standard-since-year-2000-5bf17287fd38
https://www.thecssninja.com/javascript/handleevent
https://metafizzy.co/blog/this-in-event-listeners/
I find this method clearer, also while declaring events inside the class this is pretty explicit.
Hope I helped someone.
The answer from #ibrahimmahrir does the job, but I wanted to consolidate a few points.
As many JavaScript developers struggle to understand, the this keyword is a moving target. In traditional OOP languages, an object method is exclusive to the object, so this is defined as the object to which it is attached.
JavaScript functions are more promiscuous, and can be attached to multiple objects. In JavaScript, this refers to the object which is currently invoking the function, not necessarily the one to which it was originally attached.
For an Event Handler function, the invoking object is the element to which it is attached, not the original object; thus this refers to the element. The usual safe method is to store a reference to the original object in a different variable, often called that:
oClass = new CClass();
function CClass() {
var that = this; // a reference to the original object
this.m_s = "hello :-/";
this.OnEvent = OnEvent;
var r = document.getElementById("test");
r.addEventListener('click', this.OnEvent);
function OnEvent() {
alert(that); // this is now the object
alert(that.m_s); // this works
}
}
The comments above are my updated comments. I have also removed the with statement which wasn’t contributing much and which is seriously discouraged.
Oh, and I have changed the event to click to make it easier to test.
While we’re on the confusion with this, it is not necessarily the element which started things off. Suppose we now include a span:
<div id="test">click <span>over</span> here</div>
Clicking on the span will trigger the event listener, even though the you didn’t actually click on the div to which it is attached. In this case the event is bubbled from the span to the div.
Here this refers only to the div element with the event listener. If you want to reference the span, you will need event.target:
function OnEvent(event) { // include event parameter
alert(this); // the element attached
alert(event.target); // the element clicked
alert(that); // this is now the object
alert(that.m_s); // this works
}
Here is an updated fiddle: https://jsfiddle.net/osk083xv/

How to get the current object reference in a class method?

Usually I develop on Java, and now I am studying JavaScript/HTML5 Canvas things. And I get a strange situation from Java developer's point of view.
There's a html5 canvas object on the html page, and I want to track the mouse click events on this canvas.
I declared class GameBoard and initialized its properties:
function GameBoard() {
// defining a property for GameBoard class instance
this.myProperty = 'some text here';
// getting canvas element
this.boardCanvas = document.getElementById("myCanvas");
// setting the mouse click event listener
this.boardCanvas.addEventListener("mousedown", this.handleMouseClick, false);
}
and there's a class method to handle mouse click events:
GameBoard.prototype.handleMouseClick = function(event) {
alert(this.myProperty);
}
handleMouseClick() will display undefined because this in handleMouseClick() method refers to the HTML5 Canvas instance (boardCanvas).
My question: how can I refer the current GameBoard class instance inside of handleMouseClick method to get myProperty field value defined in the class constructor?
What I am doing wrong here?
Thank you.
One of the common conventions is to use an alias to this, usually with a variable named self:
function GameBoard() {
// defining alias
var self = this;
this.myProperty = 'some text here';
this.boardCanvas = document.getElementById("myCanvas");
this.handleMouseClick = function()
{
// using alias
alert(self.myProperty);
};
this.boardCanvas.addEventListener("mousedown", this.handleMouseClick, false);
}
However, since you're defining the method on the prototype here, you can either use bind (as proposed by #Alexander) or try this:
var self = this;
this.boardCanvas.addEventListener("mousedown", function(e)
{
// calling the function with 'self/this' context
self.handleMouseClick(e);
}, false);
(Thanks to #Alexander for his contributions)
You can use bind in order to set this for function
this.boardCanvas.addEventListener("mousedown", this.handleMouseClick.bind(this), false);
Example: http://jsbin.com/vutugi/1/

Access private properties and methods from event handler

I've used the jQuery Boilerplate template as starting point for a jQuery plug-in. This template provides a set up where this represents the plug-in instance and gives access to properties and methods:
init: function() {
$(this.element).css({borderColor: "red"});
this.drawMarker([100, 200]);
},
drawMarker: function(coordinates) {
if (this.settings.isAbsolute) {
// ...
}
}
Now I need to handle some mouse clicks and it's all getting really confusing because callback functions redefine the this variable to represent the clicked event so, in order to access the plugin stuff, I came up with this ugly workaround:
this.container.on("click", "." + this.settings.markerClass,
{plugin: this}, this.removeMarker);
... and:
removeMarker: function(event){
var plugin = event.data.plugin;
var marker = $(this);
if (plugin.settings.isAbsolute) {
// ...
}
}
Is this actually what I'm supposed to do or I'm overlooking a most straightforward approach?
One possibility is to use the jQuery.proxy() function (added on 1.4) to force a given context inside event handlers:
this.$container.on("click", "." + this.settings.markerClass,
$.proxy(this.removeMarker, this));
Then, the stuff you need can be reached as follows:
Plugin properties/methods: this
Clicked element: event.target (on delegated events, it's the precise element the user clicked on; the one we normally want)
removeMarker: function(event){
var $marker = $(event.target);
if (this.settings.isAbsolute) {
// ...
}
}
This technique is courtesy of Patrick Evans.
If you need to access privately scoped variables (using functions that are therefore by definition not on the plugins prototype) just create an additional variable that aliases the plugin object:
var plugin = this;
this.container.on('click', function() {
// use plugin here
...
});
If the callback function in question is on the prototype, you can access the object within the callback thus:
var plugin = $(element).data('plugin_' + pluginName);

Variable as eventListener named function expression name

I'm writing an awesome IIFE and want this to be as easy as possible for my users who use it. So I was thinking since some of them don't know that to easily remove an eventlistener without it already being a function we can give that inline function a name
Example
document.addEventListener('click',function dood(){
//some function
},false);
document.removeEventListener('click',dood,false);
//instead of
function dood(){
//some function
}
document.addEventListener('click',dood,false);
document.removeEventListener('click',dood,false);
But since they shouldn't know the name exactly I was wondering if we could do
var k = "name_of_function";
document.addEventListener('click',function window[k](){
//the function
},false);
Though I know this does not work is there a way to do this? I'd like to make it so they can easily do this
object.cancel('name_of_function') //which will be the name of the instance
// they created earlier if they gave that instance a name
object={
cancel:function(nm){
document.removeEventListener(self.trigger,window[nm],false);
//self.trigger really is this.trigger which they assign as either scroll,click,mousemove,etc.
}
};
Any ideas? Or is this not possible at all?
usage is:
scrollex('element',{
max:500,
min:500,
pin:200,
offset:0.5,
name:'google',//this then would be used in multiple instances
});
scrollex.events //shows all events and their names
scrollex.listen('google'); //it'll console log all info for this event
scrollex.cancel('google');
I think you're on the right track. But you should not use window, and some local object instead. And dynamically naming function expressions (or whatever that function window[k](){} was supposed to mean) is impossible a pain - don't try this. Just let them stay anonymous, and reference them only via property names / variables.
var object = (function() {
var listeners = {
name_of_function: function dood(){…}
};
document.addEventListener('click', listeners.name_of_function, false);
return {
cancel: function(nm) {
document.removeEventListener('click', listeners[nm], false);
}
};
}());
// now, you can
object.cancel('name_of_function')

dojo using this

hello I'm just started to learn dojo, how can I use this object? I've created something like below but I thing its not right
var node = dojo.query('.verticalslider')[0];
dojo.connect(node, "onclick", function(){
var c = dojo.query(this).parent();
console.log(c);
})
Fixed code:
// eventlistener is setup on every DOM node with className 'verticalslider'
dojo.query('.verticalslider').connect("click", function(){
// this references the clicked DOM node
var c = this.parentNode
// parentNode of the clicked DOM node with class 'vertical..'
console.log(c);
})
This is more of a general js question then it is a dojo but for the .connect and .on functions following applies:
dojo.connect is a wrapper for creating eventlistener. normally if you write code like node.foo = function() {} you can only have the one function, as equal sign overrides the existing one. The standard behavior of .connect is that the same scope applies, so 'this' is referencing the object we're listening on. In this case 'node'.
dj.connect(node, "foo", function() { this == node evaluates to true and arguments[0] == event });
dojo.hitch (dojo/_base/lang) is a scope attach helper. It works for any event but a timeout/interval hook and will force the function object passed to, say .connect, to run in the given scope as such: dojo.hitch(scope, functor).
dj.connect(node, "bar", dj.hitch(dojo.doc(), function() { this == window.document evals true }));
As far as dojo.query goes, it will return you with a NodeList. A list cannot have a single parent, so your dojo.query(node).parent() is wrong. the correct use of .query is to pass a selector as your first use of it. Like so:
dj.query(
/* String */ "CSS Selector",
/* Optional DOM node, defaults to body */ contextNode
) // => dojo.NodeList
see NodeList docs
The above code mentioned is a straight way through, but if you need the context of this inside any function/ callback , use dojo.hitch (<1.7) or lang.hitch (1.7+). It passes the context of this inside the function.
For Ex:
var myObj = {
foo: "bar"
};
var func = dojo.hitch(myObj, function(){
console.log(this.foo);
});
Here this inside the function refers to the context of the object myObj.
Another Fixed code for you can be:
var node = dojo.query('.verticalslider')[0];
dojo.connect(node, "onclick", dojo.hitch(this,function(){
var c = dojo.query(this).parent(); // here this will be having the outside context .
console.log(c);
}))

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