I'm looking for some help because I don't quite think I understand the Javascript scoping rules. What I'm trying to do in the below example is to push a button on a page that then starts listening for keyboard input. Once the keyboard input has started if there is a break in input for two seconds I want to stop capturing the input and pop an alert with the full contents of the input collected to that point. This is an example I made purely for this question.
What I see is that I click the button and start entering input. On each keypress I am alerted to the string collected to that point. After the two second, no-action timeout takes place I see an alert with the contents "undefined". The first alerts listed above come from startLog(). The second alert comes from stopLog(). What am I doing wrong when I call stopLog that it is telling me that this.message is undefined?
function Logger() {
this.message = '';
this.listenTimer;
this.startLog = function() {
this.message = '';
$(document).bind('keypress', {this_obj:this}, function(event) {
event.preventDefault();
var data = event.data;
clearTimeout(data.this_obj.listenTimer);
data.this_obj.message += String.fromCharCode(event.which);
alert(data.this_obj.message);
data.this_obj.listenTimer = setTimeout(data.this_obj.stopLog, 2000);
});
};
this.stopLog = function() {
$(document).unbind("keypress");
alert(this.message);
};
}
var k = new Logger();
$('.logging-button').click(function() {
k.startLog();
});
The issue is this. When you pass an object method as an event handler, it loses its object context; this will refer to the window object.
There are various ways to fix this, but the main issue is that you need to pass setTimeout a closure that will still refer to the correct context:
setTimeout(function() { data.this_obj.stopLog() }, 2000);
On a separate note, you can save yourself some unnecessary code by just using a closure to refer to the object, rather than binding it as event.data:
this.startLog = function() {
this.message = '';
var this_obj = this;
$(document).bind('keypress', function(event) {
event.preventDefault();
clearTimeout(this_obj.listenTimer);
// etc
});
};
var k = new Logger();
$('.logging-button').click(function() {
k.startLog.apply(this);
//Setting context of "this" so that it refers to element even in startLog()
});
Related
I've learned that for scope reasons the this keyword inside an event listener, which is embedded in an object, doesn't refer to the global object but rather to the element which triggered the event.
Now, I understand that if I want to fetch a property I can save it to a variable before the event handler is called. But what can I do if I want to manipulate the property's value?
In the following piece of code I am trying to manipulate the drugCount property within the removeDrug event listener.
var Drugs = {
drugs: $("#drugs_table"),
drugRow: $("#drug").html(),
drugCount: 0,
init: function() {
this.addDrugRow();
this.removeDrugRowHandler();
},
addDrugRow: function() {
this.drugCount++;
this.drugs.append(this.drugRow.replace(/{{id}}/,this.drugCount));
$(".drugsSelect").select2();
},
removeDrugRowHandler: function() {
drugCount = this.drugCount;
// also a problematic solution, because it only retains the inital drugCount.
// i.e I need a way to access the "live" count from within the event
$(document).on("click",".removeDrug",function(){
if (drugCount>0) {
$(this).parents("tr").remove();
this.drugCount--; // how should I approach this?
}
});
}
}
Try This
var Drugs = function() {
var me = this;
me.drugs = $("#drugs_table");
me.drugRow = $("#drug").html();
me.drugCount = 0;
me.init = function() {
this.addDrugRow();
this.removeDrugRowHandler();
};
me.addDrugRow = function() {
this.drugCount++;
this.drugs.append(this.drugRow.replace(/{{id}}/,this.drugCount));
$(".drugsSelect").select2();
};
me.removeDrugRowHandler= function() {
var drugCount = me.drugCount;
$(document).on("click",".removeDrug",function(){
if (drugCount>0) {
$(this).parents("tr").remove();
me.drugCount--;
}
});
}
}
As it turns out the easy solution is to use the object name instead of the contextual this.
So instead of this.drugCount I used Drugs.drugCount.
However, this solution only works if I am in the context of a single object. If I were to write a "class" (i.e var Drugs = function(){ ... }) this won't work.
This question already has answers here:
JavaScript: Class.method vs. Class.prototype.method
(5 answers)
Closed 9 years ago.
As i read through some examples of Angularjs' UI add-on, i've stumbled over some code that showed me that my knowdledge of Javascript is quite improvable:
The following is a class inside of an Angular provider:
function Dialog(opts) {
var self = this, options = this.options = angular.extend({}, defaults, globalOptions, opts);
this._open = false;
this.backdropEl = createElement(options.backdropClass);
if(options.backdropFade){
// ...
}
this.handleLocationChange = function() {
self.close();
};
// more functions
}
Pretty straightforward. But outside of that class, there are prototype functions, e.g the above invoked close()
Dialog.prototype.open = function(templateUrl, controller){
var self = this, options = this.options;
// .. some code
};
Now i do not understand why that function is declared as a prototype, but handleLocationChange inside the class itself.
How do i decide which method to choose?
The full gist can be found here
Consider these 2 cases:
Dialog.prototype.open = function...
Dialog.open = function....
First case - every object created by calling new Dialog() will have this open function
Second case has nothing to do with dialog objects, consider it as static function.
EDIT
found a great answer here : javascript-class-method-vs-class-prototype-method
function open will be shared by all objects create using new Dialog().. and handleLocationChange will be different for different objects.
I think handleLocationChange is called from event triggering object that registers listeners but doesn't register the this context so when it's triggered you can't use this as it refers to handleLocationChange. To overcome this they have chosen to set a closure reference to this (=the self variable) and call other instance functions using self. Basically it's storing a value known at creation but not known when handleLocationChange is executing.
Here is some code showing the problem:
var eventSystem={
events:{},
add:function(eventname,fnCallback){
if(!this.events[eventname]){
this.events[eventname]=[];
}
this.events[eventname].push(fnCallback);
},
trigger:function(eventname){
if(!this.events[eventname]){
return;
}
var i=0;
for(i=0;i<this.events[eventname].length;i++){
this.events[eventname][i]();
}
}
};
var person=function(name){
this.name=name;
};
person.prototype.sayName=function(){
console.log("this is now:",this.toString());
// logs this is now: function (){ console.log("this is now:...
// so this is now the sayName function not the person instance
console.log(this.name);//undefined: sayName doesn't have a name property
}
var jon=new person("jon");
eventSystem.add("sayname",jon.sayName);//add event and listener function
eventSystem.trigger("sayname");//trigger the event
Here is how it's solved setting a closure reference
var eventSystem={
events:{},
add:function(eventname,fnCallback){
if(!this.events[eventname]){
this.events[eventname]=[];
}
this.events[eventname].push(fnCallback);
},
trigger:function(eventname){
if(!this.events[eventname]){
return;
}
var i=0;
for(i=0;i<this.events[eventname].length;i++){
this.events[eventname][i]();
}
}
};
var person=function(name){
var self=this;// set closure ref to this
this.name=name;
this.sayName=function(){
console.log(self.name);//use closure ref to get this
// logs jon
}
};
var jon=new person("jon");
eventSystem.add("sayname",jon.sayName);//add event and listener function
eventSystem.trigger("sayname");//trigger the event
Here is a fix to the event system to take care of the this context:
var eventSystem={
events:{},
add:function(eventname,fnCallback,thisRef){
if(!this.events[eventname]){
this.events[eventname]=[];
}
this.events[eventname].push({
"callback":fnCallback,//store the event handler
"thisRef":thisRef//store the this context
});
},
trigger:function(eventname){
if(!this.events[eventname]){
return;
}
var i=0;
for(i=0;i<this.events[eventname].length;i++){
this.events[eventname][i].callback.call(
this.events[eventname][i].thisRef);
}
}
};
var person=function(name){
this.name=name;
};
person.prototype.sayName=function(){
console.log("this is now:",this);//referring to person instance
// with the name jon
console.log(this.name);//logs jon
console.log(this instanceof person);//true
}
var jon=new person("jon");
eventSystem.add("sayname",jon.sayName,jon);//add extra parameter for this ref
eventSystem.trigger("sayname");//trigger the event
The pattern used above is not an event system (think it's pulisher subscriber) as an event usually get triggered on or invoked from an object (button, input, dialog) but in case of a more event system like implementation it would be easy to get the correct this context since you trigger the event on or from an instance (like myButton or myDialog).
See following code for event system like implementation:
var eventSystem={
add:function(eventname,fnCallback){
if(!this.events[eventname]){
this.events[eventname]=[];
}
this.events[eventname].push(fnCallback);
},
//change in trigger as it's passing the event object now
trigger:function(event){
if(!this.events[event.type]){
return;
}
var i=0;
for(i=0;i<this.events[event.type].length;i++){
this.events[event.type][i](event);
}
},
initES:function(){//set the instance variables needed
this.events=this.events||{};
}
};
function addProtos(o,protos){
for(item in protos){
o.prototype[item]=protos[item];
}
}
var person=function(name){
this.name=name;
this.initES();//needed to initialeze eventsystem
};
// make person capable of storing event handlers
// and triggering them
addProtos(person,eventSystem);
person.prototype.askQuestion=function(){
//asking a question will trigger an "answer" event
this.trigger({type:"answer",target:this});
}
// handler for when jon will fire an answer event
function answerHandler(event){
console.log("answer from:",event.target);
console.log("name of the person:",event.target.name);
}
var jon=new person("jon");
jon.add("answer",answerHandler);//add event listener
jon.askQuestion();//triggers the answer event from within jon
jon.trigger({type:"answer",target:jon});//trigger the event externally
Not sure why Angular choose to "break" prototype by using closures as the examples show there are other alternatives. Maybe someone can explain that who is more familiar with Angular.
JSFiddle: http://jsfiddle.net/M2ALY/3/
My goal is to make a module that I can use and distribute. Therefore I must not pollute the global namespace. The module I'm making is also going to be used multiple times on one web page. That's why I chose to use OOP, but this introduced a problem.
I want my object to bind a function to be run when the user clicks an element in the DOM. In this simplified example I made, I want an alert box to pop up when the user clicks a paragraph. As an example, one of the things I need in the real project I'm working on is: The user clicks a canvas, the function figures out where the user clicked and saves it to this.clientX and this.clientY.
Instead of doing
this.bind = function() {
$("p1").bind('click', function() {
// code here
});
}
I figured it would work if I did:
this.bind = function() {obj.codeMovedToThisMethod()}
The problem is that this isn't a good design. Inside the "class" you shouldn't need to know the name of the object(s) that is going to be made of this "class". This doesn't get better when I'm making multiple objects of the "class"...
So I figured I could do
$("p1").bind('click', function(this) {
// code here
});
}
But it didn't work because sending this into the function didn't work as I thought.
How should I solve this problem?
Here is a simplified sample problem. (Same as JSFiddle.)
var test = function() {
this.alert = function() {
alert("Hi");
}
this.bind = function() {
$("#p1").bind('click', function() {
obj.alert();
});
}
}
window.obj = new test();
obj.bind();
// What if I want to do this:
var test2 = function() {
// Private vars
this.variable = "This secret is hidden.";
this.alert = function() {
alert(this.variable);
}
this.bind = function() {
$("#p2").bind('click', function(this) {
obj2.alert();
this.alert();
});
}
}
window.obj2 = new test2();
obj2.bind();
Thanks!
Read MDN's introduction to the this keyword. As it's a keyword, you can't use it as a parameter name.
Use either
this.bind = function() {
var that = this;
$("#p2").on('click', function(e) {
that.alert();
// "this" is the DOM element (event target)
});
}
or $.proxy, the jQuery cross-browser equivalent to the bind() function:
this.bind = function() {
$("#p2").on('click', $.proxy(function(e) {
this.alert();
}, this));
}
Is it possible to change the state of a toggle function? Like:
myDiv.toggle ... function 1 , function 2
I click on the myDiv element, the function 1 executes
I click again, function 2
I click again, function 1
BUT
Change the state
function 1 again
etc.
But I need to be able to change the state from outside the toggle function.
Here is a javascript object that uses closure to track it's state and toggle:
var TOGGLER = function() {
var _state = true;
var _msg = "function1";
var function1 = function() {
_msg = "function1";
}
var function2 = function() {
_msg = "function2";
}
return {
toggle: (function () {
_state = !_state;
if (_state) {
function1();
} else {
function2();
}
return _msg;
})
}
}();
Here is a jsfiddle that shows how to use it to toggle based with the following jquery: http://jsfiddle.net/yjPKH/5/
$(document).ready(function() {
$("#search").click(function() {
var message = TOGGLER.toggle();
$("#state").text(message);
});
});
The toggle function is meant for simple use cases. Changing the state externally is not "simple" anymore.
You cannot easily/safely (it's internal so it may change during minor versions) access the state variable of the toggle function easily as it's stored in the internal dataset of the element.
If you really want to do it, you can try this code though:
$._data(ELEMENT, "lastToggle" + func.guid, 0);
func is the function you passed to .toggle(), so you need to save this function in a variable. Here's a minimal example: http://jsfiddle.net/xqgrP/
However, since inside the function there's a var guid = fn.guid || jQuery.guid++ statement, I somehow think that the devs actually meant to use guid instead of func.guid for the _data key - in that case a minor update is very likely to break things. And after the fix you'd have to iterate over the data set to retrieve the correct key as there is no way to access the guid from outside.
I seem to have an issue when creating copies of a template and tying the .click() method to them properly. Take the following javascript for example:
function TestMethod() {
var test = Array();
test[0] = 0;
test[1] = 1;
test[2] = 2;
// Insert link into the page
$("#test_div").html("<br>");
var list;
for (x = 0; x < test.length; x++) {
var temp = $("#test_div").clone();
temp.find('a').html("Item #" + test[x]);
temp.click(function () { alert(x); });
if (list == undefined)
list = temp;
else
list = list.append(temp.contents());
}
$("#test_div2").append(list);
}
The problem I am seeing with this is that no matter which item the user clicks on, it always runs alert(2), even when you click on the first few items.
How can I get this to work?
Edit: I have made a very simple example that should show the problem much clearer. No matter what item you click on, it always shows an alert box with the number 2 on it.
Correct me if I'm wrong, .valueOf() in JS returns the primitive value of a Boolean object.....
this would not happen ShowObject(5,'T');... ShowObject(objectVal.valueOf(), 'T');
why not use objects[x].Value directly? ShowObject(objects[x].Value, 'T');
WOOOOOSSSHHHH!
after searching deeply... I found a solution...
because it's a closure, it won't really work that way...
here's a solution,
temp.find('a').bind('click', {testVal: x},function (e) {
alert(e.data.testVal);
return false;
});
for best explanation, please read this... in the middle part of the page where it says Passing Event Data a quick demo of above code
I think your issue arises from a misunderstanding of scopes in JavaScript. (My apologies if I'm wrong.)
function () {
for (...) {
var foo = ...;
$('<div>').click(function () { alert(foo); }).appendTo(...);
}
}
In JavaScript, only functions create a new scope (commonly referred to as a closure).
So, every round of the for loop will know the same foo, since its scope is the function, not the for. This also applies to the events being defined. By the end of looping, every click will know the same foo and know it to be the last value it was assigned.
To get around this, either create an inner closure with an immediately-executing, anonymous function:
function () {
for (...) {
(function (foo) {
$('<div>').click(function () { alert(foo); }).appendTo(...);
})(...);
}
}
Or, using a callback-based function, such as jQuery.each:
function () {
$.each(..., function (i, foo) {
$('<div>').click(function () { alert(foo); }).appendTo(...);
});
}
For your issue, I'd go with the latter (note the changes of objects[x] to just object):
var list;
jQuery.each(data.objects, function (x, object) {
// Clone the object list item template
var item = $("#object_item_list_template").clone();
// Setup the click action and inner text for the link tag in the template
var objectVal = object.Value;
item.find('a').click(function () { ShowObject(objectVal.valueOf(), 'T'); }).html(object.Text);
// add the html to the list
if (list == undefined)
list = item;
else
list.append(item.contents());
});