I use this snippet in Javascript like 100 times a day to have a closure on the enclosing object:
Class.prototype.Method = function(arg){
var Ta = this;
var e = function(){
Ta.doSomething(arg);
};
};
it there a way to avoid the Ta variable and still refere to the "outer" (is this word correct?) object?
I don't know that I'd advocate this as superior, but you could use ".bind()":
var e = function() {
this.doSomething(arg);
}.bind(this);
That ensures that the this value inside function "e" will always be the this value of the surrounding context. The .bind() function is available in newer browsers, or via a polyfill like the one on the MDC site.
I rather like keeping those local variables around, especially in complicated functions that set up event handlers and stuff like that; it helps clarify the relationships between layers of code.
a) You could continue using this approach with more meaningful variable names. Using that is a common convention -- it's indicative that your variable is just another "this" value, but for another function scope.
b) You can use a function bind utility. Some JavaScript libraries come with one. Or you can simply roll your own:
function bind(fn, scope) {
return function () {
fn.apply(scope, arguments);
};
}
// for your example:
Class.prototype.Method = function(arg) {
var e = bind(function() {
this.doSomething(arg);
}, this);
};
// Alternatively, extend the Function prototype (may raise some eyebrows):
Function.prototype.bind = function (scope) {
var fn = this;
return function () {
fn.apply(scope, arguments);
};
};
// for your example:
Class.prototype.Method = function(arg) {
var e = function() {
this.doSomething(arg);
}.bind(this);
};
Update:
As #Pointy noted, bind is actually part of a new version of the JavaScript spec, getting picked up by modern browsers already: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/bind
I don't believe there is. I do the same thing all the time.
I use a small home-made framework to easily use prototype inheritance, and in this framework I have about the same piece of code. I think there's no way to do without this.
Now the question is : Why not doing this ? do you think it's a bad practice, and why ?
The piece of code I use :
function getCallback(obj, methodName) {
var method = obj[methodName];
function callback() {
if (obj[methodName] === callback) {
return method.apply(obj, arguments);
}
return obj[methodName].apply(obj, arguments);
}
return callback;
}
Related
Background
I want a function keeping track of its own state:
var myObject = {
myFunction: function () {
var myself = this.myFunction;
var firstTime = Boolean(!myself.lastRetry);
if (firstTime) {
myself.lastRetry = Date.now();
return true;
}
// some more code
}
}
The problem with the above code is that the value of this will depend on the site of the function call. I want the function to be able to refer to itself without using:
myObject.myFunction
.bind()
.apply()
.call()
Question
Is it possible to give a function this kind of self awareness independent of its call site and without any help from external references to it?
If you want to store that state on the function instance, give the function a name, and use that name within it:
var myObject = {
myFunction: function theFunctionName() {
// ^^^^^^^^^^^^^^^--------------------- name
var firstTime = Boolean(!theFunctionName.lastRetry);
// ^--------------------------- using it
if (firstTime) {
theFunctionName.lastRetry = Date.now();
// ^------------------------------------------------ using it
return true;
}
// some more code
}
};
You'd do that whenever you want to use a function recursively as well. When you give a name to a function that way (putting the name after function and before (), that name is in-scope within the function's own code. (It's not in-scope for the code containing the function if it's a function expression, but it is if it's a function declaration. Yours is an expression.)
That's a named function expression (where previously you had an anonymous function expression). You may hear warnings about NFEs, but the issues various JavaScript implementations had with them are essentially in the past. (IE8 still handles them incorrectly, though: More in this post on my blog.)
You might consider keeping that state somewhere private, though, via an IIFE:
var myObject = (function(){
var lastRetry = null;
return {
myFunction: function() {
var firstTime = Boolean(!lastRetry);
if (firstTime) {
lastRetry = Date.now();
return true;
}
// some more code
}
};
})();
Now, nothing outside that outer anonymous function can see lastRetry at all. (And you don't have to worry about IE8, if you're supporting stubborn XP users. :-) )
Side note: The unary ! operator always returns a boolean, so your
var firstTime = Boolean(!theFunctionName.lastRetry);
...is exactly equivalent to:
var firstTime = !theFunctionName.lastRetry;
...but with an extra unnecessary function call. (Not that it hurts anything.)
Of course you can, simply give your function an internal named representation and it can refer to itself from there. For example...
var obj = {
doThings:function doThingsInternal(arg1, arg2) {
console.log(arg1, arg2);
for (var arg in doThingsInternal.arguments) {
console.log(arg);
}
}
};
obj.doThings('John', 'Doe');
You could use a simple Closure, if you are not too bent on keeping state existence knowledge within the function. But I guess you don't want that. Another way to do this could be changing the function itself on the first call. Benefits, no/less state variables needed and no costly checks on subsequent calls! -
var myObject = {
myFunction: function () {
// Whatever you wanna do on the first call...
// ...
// And then...
this.myFunction = function(){
// Change the definition to whatever it should do
// in the subsequent calls.
}
// return the first call value.
}
};
You can extend this model to any states by changing the function definition per your state.
Here's a sample of a simple Javascript class with a public and private method (fiddle: http://jsfiddle.net/gY4mh/).
function Example() {
function privateFunction() {
// "this" is window when called.
console.log(this);
}
this.publicFunction = function() {
privateFunction();
}
}
ex = new Example;
ex.publicFunction();
Calling the private function from the public one results in "this" being the window object. How should I ensure my private methods are called with the class context and not window? Would this be undesirable?
Using closure. Basically any variable declared in function, remains available to functions inside that function :
var Example = (function() {
function Example() {
var self = this; // variable in function Example
function privateFunction() {
// The variable self is available to this function even after Example returns.
console.log(self);
}
self.publicFunction = function() {
privateFunction();
}
}
return Example;
})();
ex = new Example;
ex.publicFunction();
Another approach is to use "apply" to explicitly set what the methods "this" should be bound to.
function Test() {
this.name = 'test';
this.logName = function() {
console.log(this.name);
}
}
var foo = {name: 'foo'};
var test = new Test();
test.logName()
// => test
test.logName.apply(foo, null);
// => foo
Yet another approach is to use "call":
function Test() {
this.name = 'test';
this.logName = function() {
console.log(this.name);
}
}
var foo = {name: 'foo'};
var test = new Test();
test.logName()
// => test
test.logName.call(foo, null);
// => foo
both "apply" and "call" take the object that you want to bind "this" to as the first argument and an array of arguments to pass in to the method you are calling as the second arg.
It is worth understanding how the value of this in javascript is determined in addition to just having someone tell you a code fix. In javascript, this is determined the following ways:
If you call a function via an object property as in object.method(), then this will be set to the object inside the method.
If you call a function directly without any object reference such as function(), then this will be set to either the global object (window in a browser) or in strict mode, it will be set to undefined.
If you create a new object with the new operator, then the constructor function for that object will be called with the value of this set to the newly created object instance. You can think of this as the same as item 1 above, the object is created and then the constructor method on it is called.
If you call a function with .call() or .apply() as in function.call(xxx), then you can determine exactly what this is set to by what argument you pass to .call() or .apply(). You can read more about .call() here and .apply() here on MDN.
If you use function.bind(xxx) this creates a small stub function that makes sure your function is called with the desired value of this. Internally, this likely just uses .apply(), but it's a shortcut for when you want a single callback function that will have the right value of this when it's called (when you aren't the direct caller of the function).
In a callback function, the caller of the callback function is responsible for determining the desired value of this. For example, in an event handler callback function, the browser generally sets this to be the DOM object that is handling the event.
There's a nice summary of these various methods here on MDN.
So, in your case, you are making a normal function call when you call privateFunction(). So, as expected the value of this is set as in option 2 above.
If you want to explictly set it to the current value of this in your method, then you can do so like this:
var Example = (function() {
function Example() {
function privateFunction() {
// "this" is window when called.
console.log(this);
}
this.publicFunction = function() {
privateFunction.call(this);
}
}
return Example;
})();
ex = new Example;
ex.publicFunction();
Other methods such as using a closure and defined var that = this are best used for the case of callback functions when you are not the caller of the function and thus can't use 1-4. There is no reason to do it that way in your particular case. I would say that using .call() is a better practice. Then, your function can actually use this and can behave like a private method which appears to be the behavior you seek.
I guess most used way to get this done is by simply caching (storing) the value of this in a local context variable
function Example() {
var that = this;
// ...
function privateFunction() {
console.log(that);
}
this.publicFunction = function() {
privateFunction();
}
}
a more convenient way is to invoke Function.prototype.bind to bind a context to a function (forever). However, the only restriction here is that this requires a ES5-ready browser and bound functions are slightly slower.
var privateFunction = function() {
console.log(this);
}.bind(this);
I would say the proper way is to use prototyping since it was after all how Javascript was designed. So:
var Example = function(){
this.prop = 'whatever';
}
Example.prototype.fn_1 = function(){
console.log(this.prop);
return this
}
Example.prototype.fn_2 = function(){
this.prop = 'not whatever';
return this
}
var e = new Example();
e.fn_1() //whatever
e.fn_2().fn_1() //not whatever
Here's a fiddle http://jsfiddle.net/BFm2V/
If you're not using EcmaScript5, I'd recommend using Underscore's (or LoDash's) bind function.
In addition to the other answers given here, if you don't have an ES5-ready browser, you can create your own "permanently-bound function" quite simply with code like so:
function boundFn(thisobj, fn) {
return function() {
fn.apply(thisobj, arguments);
};
}
Then use it like this:
var Example = (function() {
function Example() {
var privateFunction = boundFn(this, function() {
// "this" inside here is the same "this" that was passed to boundFn.
console.log(this);
});
this.publicFunction = function() {
privateFunction();
}
}
return Example;
}()); // I prefer this order of parentheses
Voilà -- this is magically the outer context's this instead of the inner one!
You can even get ES5-like functionality if it's missing in your browser like so (this does nothing if you already have it):
if (!Function.prototype.bind) {
Function.prototype.bind = function (thisobj) {
var that = this;
return function() {
that.apply(thisobj, arguments);
};
}:
}
Then use var yourFunction = function() {}.bind(thisobj); exactly the same way.
ES5-like code that is fully compliant (as possible), checking parameter types and so on, can be found at mozilla Function.prototype.bind. There are some differences that could trip you up if you're doing a few different advanced things with functions, so read up on it at the link if you want to go that route.
I would say assigning self to this is a common technique:
function Example() {
var self = this;
function privateFunction() {
console.log(self);
}
self.publicFunction = function() {
privateFunction();
};
}
Using apply (as others have suggested) also works, though it's a bit more complex in my opinion.
It might be beyond the scope of this question, but I would also recommend considering a different approach to JavaScript where you actually don't use the this keyword at all. A former colleague of mine at ThoughtWorks, Pete Hodgson, wrote a really helpful article, Class-less JavaScript, explaining one way to do this.
I see objects in JavaScript organized most commonly in the below two fashions. Could someone please explain the difference and the benefits between the two? Are there cases where one is more appropriate to the other?
Really appreciate any clarification. Thanks a lot!
First:
var SomeObject;
SomeObject = (function() {
function SomeObject() {}
SomeObject.prototype.doSomething: function() {
},
SomeObject.prototype.doSomethingElse: function() {
}
})();
Second:
SomeObject = function() {
SomeObject.prototype.doSomething: function() {
},
SomeObject.prototype.doSomethingElse: function() {
}
}
Both of those examples are incorrect. I think you meant:
First:
var SomeObject;
SomeObject = (function() {
function SomeObject() {
}
SomeObject.prototype.doSomething = function() {
};
SomeObject.prototype.doSomethingElse = function() {
};
return SomeObject;
})();
(Note the return at the end of the anonymous function, the use of = rather than :, and the semicolons to complete the function assignments.)
Or possibly you meant:
function SomeObject() {
}
SomeObject.prototype.doSomething = function() {
};
SomeObject.prototype.doSomethingElse = function() {
};
(No anonymous enclosing function.)
Second:
function SomeObject() {
}
SomeObject.prototype = {
doSomething: function() {
},
doSomethingElse: function() {
}
};
(Note that the assignment to the prototype is outside the SomeObject function; here, we use : because we're inside an object initializer. And again we have the ; at the end to complete the assignment statement.)
If I'm correct, there's very little difference between them. Both of them create a SomeObject constructor function and add anonymous functions to its prototype. The second version replaces the SomeObject constructor function's prototype with a completely new object (which I do not recommend), where the first one just augments the prototype that the SomeObject constructor function already has.
A more useful form is this:
var SomeObject;
SomeObject = (function() {
function SomeObject() {
}
SomeObject.prototype.doSomething = doSomething;
function doSomething() {
}
SomeObject.prototype.doSomethingElse = doSomethingElse;
function doSomethingElse()
}
return SomeObject;
})();
There, the functions we assign to doSomething and doSomethingElse have names, which is useful when you're walking through code in a debugger (they're shown in call stacks, lists of breakpoints, etc.). The anonymous function wrapping everything is there so that the doSomething and doSomethingElse names don't pollute the enclosing namespace. More: Anonymouses anonymous
Some of us take it further:
var SomeObject;
SomeObject = (function() {
var p = SomeObject.prototype;
function SomeObject() {
}
p.doSomething = SomeObject$doSomething;
function SomeObject$doSomething() {
}
p.doSomethingElse = SomeObject$doSomethingElse;
function SomeObject$doSomethingElse()
}
return SomeObject;
})();
...so that not only do we see doSomething, but SomeObject$doSomething in the lists. Sometimes that can get in the way, though, it's a style choice. (Also note I used the anonymous function to enclose an alias for SomeObject.prototype, to make for less typing.)
First off, both snippets will not parse for me (Chrome) - you should use = in place of :. That said, my humble opinion follows.
The latter snippet is slightly strange, because you actually define methods on SomeObject's prototype at the time of the object construction, rather than at the parse time. Thus, if you have re-defined some method on SomeObject.prototype, it will get reverted to the original version once a new object is constructed. This may result in unexpected behavior for existing objects of this type.
The former one looks fine, except that the (function { ...} ())() wrapper may not be necessary. You can declare just:
function SomeObject() {}
SomeObject.prototype.doSomething = function() {}
SomeObject.prototype.doSomethingElse = function() {}
The actual difference between first and second in your questions is just:
var o = (function () {})(); # call this (A)
and
var o = function () {}; # call this (B)
Unfortunately, neither of the examples that you gave are written correctly and, while I don't think either will actually give an error at parse-time, both will break in interesting ways when you try to do things with the result.
To give you an answer about the difference between (A) and (B), (A) is the immediate function application pattern. The JavaScript Patterns book has a good discussion, which I recommend.
The actual problems in your code have been explained by other people while I was writing this. In particular T.J. Crowder points out important things.
I had a "class" defined and was making only one instance of it. The instance possessed a member function that would end up being passed around (it's a mouse handler, but that's not important). Since I would only ever make one instance of my "class", I decided to rewrite it as a singleton by using an object literal.
So I have
var mySingleton = {
theObjects : [];
}
mySingleton.mouseHandler = (function() {
var that = this;
return function (e) {
for (var indx = 0; indx < that.theObjects.length; indx++) {
// do something to that.theObjects[indx];
}
}
}());
mySingleton.addObject = function(newObj) {
this.theObjects.push(newObj);
}
However, when I try to use this code (after adding a few objects), I keep getting an that.theObjects is undefined error. It's referring to the line in the for loop.
Update for 2015 – Use Function.bind() to specify the value of this within the function. Then, instead of using that, you can use this.
mySingleton.mouseHandler = function (e) {
for (var indx = 0; indx < this.theObjects.length; indx++) {
// do something to this.theObjects[indx];
}
}.bind(mySingleton);
This doesn't work if you want mouseHandler to have the context of the 'moused' element. For that, use my original answer below.
If you need to support IE8 or (heaven forbid) earlier, you'll need to use a polyfill.
Since you are calling the function that creates mouseHandler immediately, it is run in the context of window, not mySingleton. So that refers to window. Instead of calling it immediately, just change it to a method so that it runs in the context of mySingleton:
mySingleton.getMouseHandler = function() {
var that = this;
return function() { ... };
};
myElement.onclick = mySingleton.getMouseHandler();
Of course, since you are already using a singleton, you can just reference it directly. In your click handler, instead of checking that.theObjects, check mySingleton.theObjects. Or, in mouseHandler change var that = this to var that = mySingleton.
Edit: Or, pass the context to your anonymous function when you call it:
mySingleton.mouseHandler = (function() {
var that = this;
return function (e) {
for (var indx = 0; indx < that.theObjects.length; indx++) {
// do something to that.theObjects[indx];
}
}
}).call(mySingleton);
There are a few popular ways to do this. First, super-simple solution is just reference mySingleton directly and bypass the confusion associated with this. Instead of that.theObjects just do mySingleton.theObjects and move on with your life and things will work fine.
However, there is a common pattern to do this binding. Here's how underscore.js does it
Check out the annoted source to underscore, where you will find this
_.bind = function(func, obj) {
if (func.bind === nativeBind && nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
var args = slice.call(arguments, 2);
return function() {
return func.apply(obj, args.concat(slice.call(arguments)));
};
};
The other answers here so far are also correct. Providing my viewpoint here in case it helps.
The key to understanding why the code doesn't behave as you expect requires understanding how this works in JavaScript. The problem is that this depends on how the function is called.
First, if you call the function in the method style, this is what you'd expect:
mySingleton.mouseHandler(); // this === mySingleton
If you attach the function to something esle, that works too.
var anotherSingleton = {};
anotherSingleton.foo = mySingleton.mouseHandler;
anotherSingleton.foo(); // this === anotherSingleton
If you detach the function, this becomes the global scope object (window)
var foo = mySingleton.mouseHandler;
foo(); // this === window
And finally, you can force this to be something else using call or apply:
var randomThingy = {};
mySingleton.mouseHandler.call(randomThingy); // this === randomThingy
The takeaway is that this is determined at runtime based on the context of how the function was called. Often, frameworks that allow you to make "classes" abstract these details from you by implicitly applying the bind pattern on your behalf. This is why it used to work, and no longer does.
As others have mentioned, you can change your handler to reference the variable by its scoped name (mySingleton) or otherwise bind it as discussed.
Here's an article I wrote on the subject a few years ago that goes into more detail: http://trephine.org/t/index.php?title=Understanding_JavaScript%27s_this_keyword
Hope this helps!
In my javascript objects i found myself writing this:
this_object = this;
It seems it's the only way to pass member variables to external functions...
google.maps.event.addListener(this.marker, 'click', function() {
this.info_window.setContent('Chicago marker');
this.info_window.open(this.map,this.marker);
});
That doesn't work, I have to copy the object into a member variable and pass the new object (and replace all this with this_object)
This feels ugly. Is there a "better" or "cleaner" way, or is this my only option?
Sure there is a better method. It involves creating a function which has the this context already bound to a particular object.
To have the this context refer to the current object, call the bind() method on the function and pass the required context as a parameter.
google.maps.event.addListener(this.marker, 'click', function() {
this.info_window.setContent('Chicago marker');
this.info_window.open(this.map,this.marker);
}.bind(this)); // <-- notice we're calling bind() on the function itself
This is now part of the ECMAScript standard, and if a browser does not implement it natively, it's easy to do it yourselves.
if (!Function.prototype.bind) {
Function.prototype.bind = function () {
var fn = this,
args = Array.prototype.slice.call(arguments),
object = args.shift();
return function () {
return fn.apply(
object, args.concat(Array.prototype.slice.call(arguments))
);
};
};
}
See all questions and answers on SO related to this.
It's actually a pretty common pattern when dealing with JavaScript to store a reference of this in a local variable i.e. var myThing=this;. Remember functions have access to local variables defined in their scope. Any variables defined in the containing functions are accessible.
You'll find this piece of code quite frequent in many libraries and projects :
function someFunction() {
var that = this;
//....
}
For example, consider this function :
function container(param) {
function dec() {
if (secret > 0) {
secret -= 1;
return true;
} else {
return false;
}
}
this.member = param;
var secret = 3;
var that = this;
return function () {
if (dec()) {
return that.member + " " + secret;
} else {
return null;
}
};
}
var c = container("foo");
alert( c() ); // "foo 2";
alert( c() ); // "foo 1";
alert( c() ); // "foo 0";
alert( c() ); // null;
Read more here.
I have seen the pattern before (with the variable in question being called), so I assume it is indeed a common javascript pattern that does not just have a cleaner solution.
I'm not certain this will help whatever scenario you are dealing with, but I've found YUI's custom event utility to work nicely with scoping issues with this and closures. It's an event-driven model, and a slightly different way of thinking, but it might be worth exploring at least.
http://developer.yahoo.com/yui/event/#customevent