In the following backbone scripts, I tried to change a collection in a view click event.
var StudentView = Backbone.View.extend({
initialize: function() {
console.log("create student items view");
this.collection.bind('add',this.render,this);
this.collection.bind('remove',this.render,this);
},
render : function(){
},
events :{
"click ":"select_students"
},
select_students: function(){
this.collection.reset([]);
_.each(students.models, function(m) {
if(m.get('name')=="Daniel"){
this.collection.add(m);
}
});
}
});
var students_view = new StudentView({el:$("#student_table"),collection:selected_students});
I got this error
How should I call "this.collection" in the code?
You should change you select_students to
select_students: function(){
var self = this;
this.collection.reset([]);
_.each(students.models, function(m) {
if(m.get('name')=="Daniel"){
self.collection.add(m);
}
});
}
The problem is that in JavaScript, the this context is lost in inner functions (like the one you pass to _.each) so the general pattern is to save a reference outside of that (self) and then use that in the inner function.
You can avoid using a reference at all, utilizing Backbone's collection filter method.
select_students: function () {
this.collection.reset(students.filter(function (student) {
return student.get('name') == 'Daniel';
});
}
Rather than using underscore's each() function, backbone collections can be iterated on directly, and can take a context to define what the 'this' variable refers to (passed as the second argument to each below).
So the best way to do this is to:
select_students: function(){
this.collection.reset([]);
students.each(function(m) {
if(m.get('name')=="Daniel"){
this.collection.add(m);
}
}, this);
}
Related
I feel my whole understanding of this has been thrown up in the air.
I have a Quiz object which holds the necessary variables and methods required to play the quiz.
I am trying to reference a method of Quiz from another method in Quiz (getQuestion in skipQuestion()) however, I am seeing a message in the console saying that this.getQuestion is not defined. I was under the impression that this in this case refers to the object it is in, hence the function in question should be referred to as this.getQuestion().
The error message I am getting is script.js:18 Uncaught TypeError: this.getQuestion is not a function
Can anyone explain what is going wrong here?
In my init function it seems that this refers to the Quiz object, but in skip question it seems to change. Is this down to query having a different definition of this? where do you draw the line, and when is the context of this changed?
(function(window){
$(document).ready(function(){
var Quiz = {
score : 0,
question: '',
answer: '',
init: function() {
this.getQuestion();
this.checkAnswer();
this.skipQuestion();
},
skipQuestion: function() {
$('#skip').click(function(){
this.getQuestion();
})
},
getQuestion: function() {
$.get('http://jservice.io/api/random', function(data){
$('#question').html(data[0].question);
this.answer = data[0].answer.toLowerCase();
});
},
checkAnswer: function() {
if($('#answer').val() === this.answer) {
this.score += 1;
}
}
}
Quiz.init();
});
})(window);
Because you are nesting inside another function, the this context changes to that function, so the methods you look for are no longer available. You can try to solve it by either storing the this inside a variable that will be within the scope of the function you are defining, or by using Double Arrow Functions, which have no associated this context themselves (and therefor also don't support bind or call). Here are your options:
Declare a variable:
skipQuestion: function() {
var that = this;
$('#skip').click(function(){
that.getQuestion();
})
}
or a Double Arrow Function:
skipQuestion: function() {
var that = this;
$('#skip').click(() => that.getQuestion())
}
Your init function is considered a method of your Quiz object, while the anonymous function passed to the click event is not a method of your Quiz, it is a method of an anonymous object created in the background, and shares no methods or variables with your Quiz. This is important to consider!
The thing is you are using this inside the click event and it refers to the event rather than you context. To work around you need to assign this to another variable and then use that;
skipQuestion: function() {
var self = this;
$('#skip').click(function(){
self.getQuestion();
})
},
$.get and .click event create their own context and thus this refers to their context instead of the context of quiz.
JS
(function(window){
$(document).ready(function(){
var Quiz = {
score : 0,
question: '',
answer: '',
init: function() {
this.getQuestion();
this.checkAnswer();
this.skipQuestion();
},
skipQuestion: function() {
var self = this;
$('#skip').click(function(){
that.getQuestion();
})
},
getQuestion: function() {
var self = this;
$.get('http://jservice.io/api/random', function(data){
$('#question').html(data[0].question);
self.answer = data[0].answer.toLowerCase();
});
},
checkAnswer: function() {
if($('#answer').val() === this.answer) {
this.score += 1;
}
}
}
Quiz.init();
});
})(window);
I have a simple app, that triggers a boolean and sets a task to completed:
But I want to be able use a "Complete All" Button and set every task to complete. This here works fine:
completeAll: function() {
this.tasks.forEach(function(task) {
task.completed = true;
});
},
http://codepen.io/anon/pen/avzMYr
But instead of setting it directly, I would like to use a method that is called like this, because I have a lot of other code that needs to be separated.
completeTask: function(task) {
task.completed = true;
},
completeAll: function() {
this.tasks.forEach(function(task) {
this.completeTask(task);
});
},
Yet this does not work, see here:
http://codepen.io/anon/pen/EVaMLJ
Any idea how to call the "completeTask(task)" method inside of the completeAll method?
Your problem is that the value of this inside the .forEach() callback is not the same as what it is outside. You can save the outer value of this and then use that saved version to get what you want:
completeAll: function() {
var self = this;
this.tasks.forEach(function(task) {
self.completeTask(task);
});
},
You could use Bind for setting the this value in methods like this:
completeAll: function() {
this.tasks.forEach(function(task) {
this.completeTask(task);
}.bind(this));
}
I guess that's the simple question. I'm new in js, especially in Backbone.js.
All I want to know is how I can refer to my function inside jquery function.
getLanguages: function() {
...
return languages;
},
render: function() {
...
$("input[type='checkbox']").bind("change", function() {
// todo: getLanguages
});
}
I tried to get languages via this but, of course, I got checkbox in this case.
Edit:
It's so simple. Many thanks to all!!!
This is a classic problem in Javascript. You need to reference this inside a callback, but this changes to the element being bound to. A cheap way to do it:
render: function() {
var that = this;
$("input[type='checkbox']").bind("change", function() {
that.getLanguages();
});
}
that will stay defined as the this that render is defined on.
However, you’re using Backbone, and it has more suitable ways to handle this situation. I don’t know the name of your Backbone.View class, but here’s an example adapted from the documentation:
var DocumentView = Backbone.View.extend({
events: {
"change input[type='checkbox']": "doSomething"
},
doSomething: function() {
this.getLanguages(); # uses the correct this
}
});
Calling bind inside render is not The Backbone Way. Backbone views are made to handle event delegation without the unfortunate need to pass this around.
Save this object before bind change event in the scope of render function.
render: function() {
var CurrentObj = this;
$("input[type='checkbox']").bind("change", function() {
CurrentObj.getLanguages();
});
}
You can save the appropriate object into a local variable so from the event handler, you can still get to it:
getLanguages: function() {
...
return languages;
},
render: function() {
...
var self = this;
$("input[type='checkbox']").bind("change", function() {
var lang = self.getLanguages();
...
});
}
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.
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));
}