Calling function by value of variable and keeping scope of this pointer - javascript

I have been fiddling with code to call a function with the name of the value of a variable and then keep the this scope when called, but the this pointer seems to be in context of the element I have used jQuery's bind method on, rather than the object the function I might be calling is within. To clarify here´s some code to illustrate the problem:
classname.prototype = {
bindElementToFunction: function(element, functionToCall){
$(element).bind("click",
{realThis: this, functionToCall: functionToCall},
this.callFunction);
},
// I had hoped I could change the this pointer back to the object by running
// it through this function, I have tried using .apply and .call but I can't
// seem to get them to work with function pointers
callFunction: function(event){
var realThis = event.data.realThis;
var functionToCall = event.data.functionToCall;
functionToCall = realThis[functionToCall];
// I have tried .apply and .call in several different ways but can't seem
// to get them to work in this context
functionToCall();
},
abitraryFunction: function(){
this.test();
},
};
The problem here is then that everything works fine up until abitraryFunction where this is still referring to the element from the bind function. I have tried doing .apply() with the appropriate this pointers, but they do not seem to work.
So here's the question how do I change the context of the "this" pointer in combination with function pointers?
Feel free to scrap all the code I have written, as long as I am able to do a bind function to an element that then runs a method within a object where "this" is refferring to the object the method is within.
Thanks

I think the jQuery bind is making your code way more complicated than it needs to be. The JavaScript bind() function works perfectly:
http://jsfiddle.net/bQGWS/
By simply assigning a function to the onclick (or any other event hook) of an element, this is evaluated from the element's point of view and so points to the element itself.
When you use bind, you end up with a copy of the function where this is effectively replaced with the var you passed into bind().
classname = function(){}
classname.prototype = {
method: function(){
try {
alert( this.othermethod() );
} catch(e) {
// Method doesn't exist in scope
alert( 'Wrong scope :(');
}
},
othermethod: function(){
return 'hello desired scope!';
},
referenceToElement: function(elementId, functionname){
var el = document.getElementById(elementId);
// Just assigning the function as is
el.onclick = this[functionname];
},
bindToElement: function(elementId, functionname){
var el = document.getElementById(elementId);
// Using the bind function to create a copy in the
// scope of this (within the prototype)
el.onclick = this[functionname].bind(this);
}
}
var instance = new classname();
instance.referenceToElement('reference', 'method');
instance.bindToElement('bound', 'method');

Related

trying to pass `this` through a function

I have a function localised to the main function and i want to use this to call it but it doesn't seem to work.
My code has:
function option(room,slot){
var div_id = document.getElementById(room);
var opacity = window.getComputedStyle(div_id).opacity
transition_opacity(div_id,opacity,0,function(){this.load});
function load(){
console.log('test'); //does not happen
}
}
Have i misunderstood the use of this or is the scope lost when i use function(){} to call load?
From your code it is not obvious, what object this could refer to. It depends on how option is called. However, if you define the load function inside of the option function anyway, it is best to just reference it directly. You will have to move the declaration of test above the transition_opacity call though:
function option(room,slot){
var div_id = document.getElementById(room);
var opacity = window.getComputedStyle(div_id).opacity;
function load() {
console.log('test');
}
transition_opacity(div_id,opacity,0,load);
}
As you can see, I just reference load directly. You could make another function which calls the load function inside (i.e. function() { load(); } – note the parentheses which calls the function) but that would give you no benefit but would just add another unneeded function to the stack. So just refer to the actual function itself.
For more information on the this keyword, check out this question. Spoiler: It’s more complicated than you would expect.
The scope of this is lost in this instance, probably pointing to the document. You can capture this to a variable in the outer scope to make this work as intended.
var context = this;
transition_opacity(div_id,opacity,0,function(){context.load();})
The above will not work however. This is because load does not exist on the context of this. You would need to define the load function as such:
context.load = function(){
console.log('test');
}
Both.
First, your load function is not a member/property of any this, the way you have it coded. Your load function is simply a nested function that exists within your option function, as has been sort of implicitly noted in other responses.
In your option function, if you want 'load' to become a member of 'this', you'd need to say so, like this:
function option(){
this.load = function(){}; // now load is actually a property of whatever this is
}
Second, you and the other poster are correct that 'this' is no longer the same 'this' by the time your anonymous function is called.
Whenever you call a function, a brand new 'this' is created and exists within the scope of that function. If you just call a function like this:
transition_opacity(args);
.. then within transition_opacity, 'this' just refers to the window object, or maybe window.document. For 'this' to refer to anything other than window or window.document, you need to (in effect) do one of the following:
myObject.transition_opacity(args);
transition_opacity.call(myObject, arg1, arg2, ..);
transition_opacity.apply(myObject, argArray);
or
var myObject = new transition_opacity(args);
In each of those cases, within transition_opacity, 'this' refers to myObject (or, well, in the last case, it refers to a new object that is being created and assigned to myObject).
Here is a way to do what it looks like you're trying to do:
var MyNamespace = {
option: function(room,slot){
var div_id = document.getElementById(room);
var opacity = window.getComputedStyle(div_id).opacity;
var _this = this;
transition_opacity(div_id,opacity,0,function(){
// Careful! Inside here, 'this' is just window or window.document,
// unless transition_opacity sets it to something using call or apply,
// in which case that 'this' is probably not the 'this' you want.
// So carefully refer to the saved instance of 'this':
_this.load();
});
},
load: function(){
console.log('test'); // now it should happen
}
}
.
.
MyNamespace.option(room, slot); // inside option, 'this' is MyNamespace.
Here's another way to do it:
function MyClass(){};
MyClass.prototype = {
// all the same stuff that is in MyNamespace above..
};
.
.
var myObject = new MyClass();
myObject.option(room, slot);
Clear as mud?
Just use
transition_opacity(div_id,opacity,0,load);
You have defined a 'load' within another function as an 'Function Declaration', so now it is only accessible within 'option' function and in other functions defined in this one by name 'load'. You can't access it by using 'this.load' no matter what 'this' is. If you want to access 'load' function as 'this.load' you can try this example to understand how 'this' keywoard works
// Function Declaration
function f1(callback){
callback();
};
// Function Declaration
function f2(){
// Function Expression
this.load = function(){
console.log("test");
};
f1(this.load);
};
var obj = new f2(); // test, this == obj, so obj.load() now exists
obj.load(); //test, this == obj
f2(); //test, this == window, so window.load() now exists
load(); //test, window is the global scope

Should I avoid creating this JavaScript closure?

This may be a bit abstract but I'm trying to get my head round JavaScript closures etc. Take the following code:
function MyObj() {
var me = this;
this.foo = function(bar) {
// Do something with 'bar'
}
// Set up lots of local variables etc.
// ....
$(window).load(function() {
// Add a delegated click handler to specific <input> elements
$(document).on('click.myobj', 'input.special', function() {
// Do something with the <input> that triggered the click event
me.foo(this);
});
});
}
var myObj = new MyObj();
The anonymous function passed to that is bound to the click event creates a closure that references me. What I want to know is whether it's better to do something like this instead (to avoid the closure):
$(window).load(function() {
// Add a delegated click handler to specific <input> elements
(function(localMe) {
$(document).on('click.myobj', 'input.special', function() {
// Do something with the <input> that triggered the click event
localMe.foo(this);
});
})(me);
});
Is this a better approach, or am I being overly paranoid about creating a closure? Alternatively, is there a "third way"?
EDIT
Additionally, would it be better to do something like this:
$(window).load(function() {
// Add a delegated click handler to specific <input> elements
$(document).on('click.myobj', 'input.special', {localMe : me}, function(event) {
// Do something with the <input> that triggered the click event
event.data.localMe.foo(this);
});
});
The latter is (AFAIK) more efficient, but probably not measurably so unless used in a tight loop.
The reason is that all variable dereferencing must follow the scope chain. In the latter case, the variable localMe can be found in the anonymous function's parameter list.
In the former case, the variable isn't found there, but in the outer scope. This traversal up the scope chain takes extra time.
Anonymous functions are massively used in javascript now (as arguments and as an immediate function for scopes/closures). There's no performance problem with that.
But you can have a problem of code reading maybe. Because when you see a variable, you must check where the variable is from. But no big deal here.
And in your second example, you still have a closure "break". Because in your anonymous function in the click, you use the localMe variable. And the localMe is an argument of a fonction outside of your fonction.
// Here, 'me' is a direct local variable.
$(window).load(function() {
// Here, we are in an anonymous fonction, so 'me' is not a direct variable anymore. But you still can access it.
// Add a delegated click handler to specific <input> elements
(function(localMe) {
// Here, 'localMe' is a direct local variable.
$(document).on('click.myobj', 'input.special', function() {
// We are in an anonymous function, so 'localMe' is not a direct variable anymore.
// Do something with the <input> that triggered the click event
localMe.foo(this);
});
})(me);
});
If you really want to avoid a closure "break", you should bind your function to your object. But note that not every browser support the bind method on functions.
You will always create a closure if you bind the event from the constructor. In fact, you even need the closure to preserve the reference to your instance. However, you might do something like this:
function MyObj() {
this.foo = function(bar) {
// Do something with 'bar'
}
// Set up lots of local variables etc.
// ....
}
var myObj = new MyObj();
$(function() {
$(document).on('click.myobj', 'input.special', function() {
myObj.foo(this);
});
});
If you do only create a singleton instance of your constructor, it won't matter anyway.
I would probably do it this way:
var bind = function( fn, me ) { return function() { return fn.apply(me, arguments); }; },
Object = (function() {
function Object() {
this.handler = bind(this.handler, this);
// Add a delegated click handler to specific <input> elements.
$(document).on("click.myobj", "input.special", this.handler);
}
Object.prototype.foo = function( bar ) {
// Do something with "bar".
};
Object.prototype.handler = function( event ) {
// Do something with the <input> that triggered the click even.
return this.foo(event.currentTarget);
};
return Object;
})();
var obj = new Object();
This skips the uses of closures and iifes, using .apply instead. Not sure if it is more efficient or not, but it is another option.

Javascript callbacks losing 'this'

I have an issuer where I lose the this inside this object. The output of the following piece of JavaScript gives me "some-id" and then undefined. When I use this inside a callback function, the scope goes out of the object and it cannot use this any more. How can I get the callback to use 'this' or at least have access to the object?
Since I will make multiple objects, I won't be able to create a 'static' like storage.
Here is my test code that you can use to reproduce my problem. What I would like to have is CheckBox.doSomething() to return the value of this.id which should match some-id for this test case.
function CheckBox(input_id) {
this.id = input_id;
this.doSomething();
$('#some-element').click(this.doSomething);
}
Checkbox.prototype.doSomething = function() {
alert(this.input_id);
}
var some_box = new CheckBox('some-id');
some_box.doSomething();
$('#some-element').click();
I can't even get this to work as I want it to:
function CheckBox2(input_id) {
this.id = input_id;
alert(this.id);
}
CheckBox2.prototype.doSomething = function() {
alert(this.input_id);
}
var some_box = new CheckBox2('some-id');
some_box.doSomething();
Your problem is with this line: $('#some-element').click(this.doSomething);
Why this is a problem
JavaScript methods don't know anything about the object that should be assigned to this, it's set when the method is called either explicitly (with myFunction.call(obj)) or implicitly (when called using obj.myFunction()).
For example:
var x = {
logThis: function () {
console.log(this);
}
};
x.logThis(); // logs x
x.logThis.call(y); // logs y
var func = x.logThis;
func(); // logs window: the fallback for when no value is given for `this`
In your case, you're passing this.doSomething to jQuery, which is then explicitly calling it with the element that was clicked as the value of this. What's happening is (a slightly more complex version of) this:
var callback = this.doSomething;
callback.call(anElement, anEvent);
The solution
You need to make sure that doSomething is called with the right value of this. You can do that by wrapping it in another function:
var cb = this;
$('#some-element').click(function() {
return cb.doSomething();
});
jQuery provides a proxy function lets you do this more simply:
$('#some-element').click(jQuery.proxy(this.doSomething, this));
function CheckBox(input_id) {
this.id = input_id;
this.doSomething = $.proxy( this.doSomething, this );
$('#some-element').click(this.doSomething);
}
The "javascript equivalent" of this is Function#bind but that is not available in every browser and since it seems you are using jQuery I am using the jQuery equivalent $.proxy
Others have already explained the causes of the problem and how to fix it with jQuery. What's left is how you fix it with standard JavaScript. Instead of ...
$('#some-element').click(this.doSomething);
... you write:
document.getElementById('some-element').addEventListener('click', this.doSomething.bind(this));
This changes the context of this inside doSomething. You can also do that with anonymous functions - instead of ...
$('#some-element').click(function(event) {
console.log(this);
});
... you write:
document.getElementById('#some-element').addEventListener('click', (function(event) {
console.log(this);
}).bind(this));
That has been very useful to me in projects with lots of callbacks, e.g. in Node.js (where you don't have to care about outdated browsers).
Edit: getElementById() and addEventListener() instead of $(...).click(...).

javascript anonymous function parameter passing

I have some javascript code (within an object) :
toggle: function() {
var me = this;
var handler = function() { me.progress() };
me.intervalId = setInterval(handler, me.intervalTime);
//...More code
}
I'm kind of new to javascript, so doing the above as far as I can tell actually passes the me variable into anonymous the function. I was wanting to see if there is a more declarative way to do so? I wanted something along the line of:
var handler = (function(o) { o.progress();})(this));
but that doesn't seem to be working... Am I missing something? Is this a case where "this is the way the language works so just declare a local variable and deal with it"?
UPDATE:
The source to my problem was/is my unclear understanding of scope and closures in javascript. I found this article to help me understand a little more.
You can use ".bind()":
var handler = function() { this.progress(); }.bind(this);
New browsers have "bind()", and the Mozilla docs have a solid implementation you can use to patch older browsers.
The reason
var handler = (function(o) { o.progress();})(this));
doesn't work because it just immediately calls the anon function, therefore immediately calling o.progress() and assigns the return value of the anon function (undefined) to handler. You need to return an actual function from the outer function:
handler = (function(me){
return function(){
return me.progress();
}
}(this));
On the flip side this is equivalent and just as bad looking as bad looking as the variable assignment (but can still be useful, particularly if this needs to be done in a loop, with the changing i rather than the fixed this).
BTW, if the progress function doesn't have any calls to this inside it , just doing handler = this.progress (without the parens) might suffice.
The anonymous function has access to me because it is declared inside of the outer function (the toggle function); it is closed over by the outer function.
Your handler function will be called by setInterval, which passes exactly zero arguments. This means you can't use parameters in the handler function itself.
I you really want to pass me explicitly, you could write a function accepting an parameter, and have that function return an anonymous function without parameters, but which could access the creator function's parameter:
toggle: function() {
var me = this;
var handler = (function (o) { return function() { o.progress() }; })(me);
me.intervalId = setInterval(handler, me.intervalTime);
//...More code
}
But this basically adds a layer of redirection without really making it more legible. Unless you pull that creating function outside:
function createProgressHandler(o) {
return function() {
o.progress();
};
}
// ...
toggle: function() {
var me = this;
var handler = createProgressHandler(me);
me.intervalId = setInterval(handler, me.intervalTime);
//...More code
}
What you have there is a closure. The function that is created and assigned to handler keeps a reference to the me object. This is normal, everyday JavaScript, and that's the way that closures work generally.
Have you tried to return the function like this?
var handler = function(o){
return function(){
o.progress();
}
}(me);
Now you can call:
handler();

Calling a function in javascript object literal notation declaration

I'm trying to call a function within an object literal that I created, using the this keyword. But an error shows up saying this.doTheMove() is not a function:
window.onload = function(){
var animBtn = document.getElementById('startAnim');
animBtn.addEventListener('click', Animation.init, false);
}
var Animation = {
init: function(){
this.doTheMove(); // I'm calling the function here, but it gives an error.
},
doTheMove: function(){
alert('Animation!');
}
}
Why is there an error?
An explanation of what's happening. Pointy's answer is good but I want to explain it more generically. A very good research on this can be found here
An event handler is just a callback. You pass it a function and an event to listen on. Interally all it will do is call that function.
Animation.init is just a getter for that function. Think of it like this:
var callback = Animation.init
animBtn.addEventListener('click', callback, false);
...
// internal browser event handler
handler() {
// internal handler does stuff
...
// Oh click event happened. Let's call that callback
callback();
}
So all you've done is passed in
var callback = function(){
this.doTheMove(); // I'm calling the function here, but it gives an error.
}
By default in javascript this === window. This will refer to the global object if it isn't set to something. The net effect is that window.doTheMove is called. And that function doesn't exist.
In this case since callback is actaully called by an event handler the this object points at the DOM object that triggered the event so your calling node.doTheMove which still doesn't exist.
What you wanted to do is wrap it with a reference to Animation.
var callback = function() {
Animation.init();
}
This is a function execution and it executes init on Animation. When you execute it on an object like that then internally this === Animation as you would expect.
To sum up. The issue here is that Animation.init is just a reference to a function. It has no information about anything else like Pointy mentioned.
You have to change the way you set that up:
window.onload = function(){
var animBtn = document.getElementById('startAnim');
animBtn.addEventListener('click', function() { Animation.init(); }, false);
}
In JavaScript, the fact that a function happens to be defined as part of an object literal really doesn't mean very much (if anything, in fact). The reference to Animation.init does get you to the proper function, but the problem is that when the function is later invoked (in response to an actual "click"), the browser calls the function but has no idea that the object "Animation" should be the this reference. Again, the fact that the function was declared as part of the object is of no importance at all here. Therefore, if you want this to be something in particular of your own choosing, then you have to make sure it's set explicitly in code you control. The solution above is about the simplest way to do it: it handles the "click" events with an anonymous function that does nothing other than invoke the "init" function via an explicit reference through "Animation". That will ensure that this refers to the "Animation" object when "init" runs.
Another alternative would be to use the ".bind()" facility that some browsers and frameworks support:
window.onload = function(){
var animBtn = document.getElementById('startAnim');
animBtn.addEventListener('click', Animation.init.bind(Animation); }, false);
}
The net effect is almost exactly the same: that call to ".bind()" returns a function that invokes the function on which it was called (that being the "init" function in the "Animation" object), and does so with its first argument as the this reference (the "context" object). That's the same thing that we get from the first example, or effectively the same anyway.
Here's another nice approach, I think.
window.onload = function(){
var animBtn = document.getElementById('startAnim');
animBtn.addEventListener('click', Animation.init, false);
};
var Animation = {
init: function(){
Animation.doTheMove(); // This will work, but your IDE may complain...
},
doTheMove: function(){
alert('Animation!');
}
};
You might want to use the portotype base approach:
// generate a prototype object which can be instantiated
var Animation = function() { this.doTheMove(); }
Animation.prototype.doTheMove = function() {
// if the object has only one method, the whole code could be moved to
// var Animation = function() {...} above
alert('Animation!');
}
Animation.prototype.otherMethod = function(param1, param2) {
// ...
}
// run the code onload
window.onload = function(){
var animBtn = document.getElementById('startAnim');
animBtn.addEventListener('click', new Animation(), false);
}
Six and a half years later, but I'm hoping my answer can also provide some insight for current and future developers.
I tend to code using literal objects inside of self defined functions, and the original question posted works just fine if another self-executing function is added along with a try and catch statement.
It's very important to point out that it's all about scope and context.
Please correct any drawbacks or provide more effective suggestions of using this method.
(function() {
console.log(this); // window object
var animation = {
init: function() {
this.doTheMove();
},
doTheMove: function() {
alert("Animation");
console.log(animation); // animation object
}
};
(function() {
try {
console.log("animation.init"); // animation.init function
animation.init();
} catch(e) {
console.log("Error is: " + e);
}
})();
})();

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