Related
I've been using JavaScript for a few years now in web design/development, and everything I know has been self-taught (I was a design major and hobbyist front-ender turned full web developer in pursuit of my career). With that background, I discovered something that I want to learn more about and have no idea what it is called, how it works, or that it may even be something extremely simple.
I've tried to search for more information about this (to prevent myself from needing to ask this), but it's difficult when I'm not sure what it's called that I'm even looking for...
I noticed in a few JavaScript libraries that I use that a variable name can be both a function and an object. For example, the jQuery library uses the name "jQuery". When logged using typeof jQuery it is a function, and typeof jQuery() is an object. What is interesting to me is that initial thought would suggest that jQuery() would be a function but it's actually an object.
//jQuery:
ƒ (a,b){return new r.fn.init(a,b)}
//jQuery():
r.fn.init {} //Expanding this reveals functions inside of __proto__ (which is obviously a clue, but I need more info)
When I try to do something like this I end up overwriting the name (as I would expect):
//Declare an object:
var planetEarth = {
'sky': 'blue',
'grass': 'green'
}
//Now overwrite the object as a function:
function planetEarth(){
console.log('inside of a function now');
}
This is understandable within the realm of JavaScript, so my question is how does a library like jQuery pull off having both at the same time?
Ultimately (using the above example) I would like to be able to do something like this:
planetEarth().sky; //"blue"
So my roundabout question is simply what is this called?
And a follow-up of where can I learn the basics of accomplishing this?
I've found resources on object-oriented JavaScript and classes, as well as prototypes, but I'm not sure if either (or both) of those concepts are what this is. All of the articles I've found aren't starting at the beginning and seem to always jump into unnecessarily complex examples. I'm not looking to make a giant library- I just want to get some more experience at the very basic level. Again, this could be embarrassingly simple and I've just never come across the concept before, so I appreciate being pointed in the right direction, thanks.
Every JavaScript function is also an object, just as every array is also an object. I don't think there is a special name for this, it's just how the language works.
You may be confusing two different things: what a function returns when you call it, vs. the function itself. Take your example:
planetEarth().sky; // "blue"
This code does not rely on the function planetEarth being an object and having any properties. You're calling the function, so to make this code work the function would need to return an object.
Because a function is an object, you can also set properties directly on the function itself. The code below uses both of these features. This version of planetEarth() returns an object with sky and grass properties, so it works as in your example: you call the function to get an object with those properties.
The code also sets an exists property directly on the function, and you can access that property without calling the function:
function planetEarth() {
// Return an object when this function is called
return {
sky: 'blue',
grass: 'green'
}
}
// Set a property directly on the function itself
planetEarth.exists = true;
// Test accessing the function's property
console.log( planetEarth.exists );
// Test accessing a property in the object that the function returns when called
console.log( planetEarth().sky );
jQuery makes use of both of these facilities. jQuery is a function that you can call. It returns a value commonly called a "jQuery object". That object has properties and methods, such as jQuery('#foo').show(). It also has "array-like" properties that you can access as you would any array, e.g. jQuery('#foo').length and jQuery('#foo')[0]. The jQuery function adds those properties to the value it returns.
At the same time, the jQuery library adds other properties and methods to the jQuery function itself. You access without calling the function, e.g. jQuery.ajax({...}).
A couple of other notes to help you understand the jQuery code. First, download the uncompressed version of jQuery. It looks like you are studying the compressed version, which has shortened names that won't make any sense.
Also, if you are wondering what jQuery.fn is, it is simply an alias for jQuery.prototype. Whenever you see jQuery.fn you can mentally substitute jQuery.prototype; learn how JavaScript prototypes work and you will understand jQuery.fn.
And now you may wonder why jQuery.fn exists at all. Why not use jQuery.prototype directly like other JavaScript code that uses prototypical inheritance? In fact, you could, and it would work the same as jQuery.fn. But the very first version of jQuery, back in 2006, didn't work this way. jQuery.fn was its own separate object, and every time you called the jQuery function, it copied all of the methods and properties from this object into the value it returned.
As you can guess, this was rather inefficient, so jQuery was changed to use .prototype so it could return an object that inherits all the methods such as .show() and .hide() instead of copying them all one by one. At the same time, jQuery.fn was kept around as an alias for jQuery.prototype to avoid breaking existing code.
This is a silent answer...
function f() {
return {};
}
console.log(typeof f, typeof f());
This is how jQuery does it. f is a function but when it gets called it returns an object.
Interesting part: function is also an object. f instanceof Function and f instanceof Object are both valid. So, you can call a variable as a function and / or assign some properties because it is also an object.
f.test = 123;
First-Class Objects
In Javascript, functions are first-class objects. This means that functions are just another kind of object. You can put a function in a variable, you can return a function, you can make an array of functions, and all that. Functions are objects.
Consider a slight change in your attempt:
//Declare a function:
function planetEarth(){
console.log('inside of a function now');
}
// Now add fields to it, since it is also an object
planetEarth.sky = 'blue';
planetEarth.grass = 'green';
// Test stuff
planetEarth(); // works
console.log(planetEarth.sky, planetEarth.grass); // works
You mention that you would like to use planetEarth().sky, but observe that while planetEarth is a function (and an object, as I said), planetEarth() is the result of calling planetEarth with no parameters. Therefore, whether you can or can't do planetEarth().sky does not depend on planetEarth as an object having the sky field, but rather depends on whatever you return from planetEarth having that field.
Bonus: did you know that functions can be declared very much like "normal" variables? See below:
// Both lines of code below are identical:
function myFunc() { ... }
var myFunc = function() { ... };
Perhaps looking at the code above will help you clear the confusion. The function is myFunc. myFunc() is simply the act of calling that function. If typeof myFunc() gives function, it is just a coincidence that the object that myFunc returned happened to also be a function.
jQuery is a function. Properties can be assigned to a defined function.
function planetEarth(options){
console.log('inside of a function now', options);
return window["planetEarth"];
}
var planetEarthOptions = {
'sky': 'blue',
'grass': 'green'
}
for (let prop in planetEarthOptions) {
planetEarth[prop] = planetEarthOptions[prop];
}
window["planetEarth"] = planetEarth;
planetEarth("selector");
console.log(planetEarth.sky);
console.log(planetEarth().grass);
I've done a lot of searching and some playing around, and I'm pretty sure the answer to this question is no, but I'm hoping a JavaScript expert might have a trick up his sleeve that can do this.
A JavaScript function can be referenced by multiple properties, even on completely different objects, so there's no such thing as the object or property that holds the function. But any time you actually call a function, you must have done so via a single object (at the very least, the window object for global function calls) and property on that object.
(A function can also be called via a function-local variable, but we can consider the function-local variable to be a property of the activation object of the scope, so that case is not an exception to this rule.)
My question is, is there a way to get that property name that was used to call the function, from inside the function body? I don't want to pass in the property name as an argument, or closure around a variable in an enclosing scope, or store the name as a separate property on the object that holds the function reference and have the function access that name property on the this object.
Here's an example of what I want to do:
var callName1 = function() { var callName = /* some magic */; alert(callName); };
var obj1 = {'callName2':callName1, 'callName3':callName1 };
var obj2 = {'callName4':callName1, 'callName5':callName1 };
callName1(); // should alert 'callName1'
obj1.callName2(); // should alert 'callName2'
obj1.callName3(); // should alert 'callName3'
obj2.callName4(); // should alert 'callName4'
obj2.callName5(); // should alert 'callName5'
From my searching, it looks like the closest you can get to the above is arguments.callee.name, but that won't work, because that only returns the name that was fixed to the function object when it was defined, and only if it was defined as a named function (which the function in my example is not).
I also considered that maybe you could iterate over all properties of the this object and test for equality with arguments.callee to find the property whose value is a reference to the function itself, but that won't work either (in the general case), because there could be multiple references to the function in the object's own (or inherited) property set, as in my example. (Also, that seems like it would be kind of an inefficient solution.)
Can this be done?
Short answer:
No, you cannot get "the property name" used to call your function.
There may be no name at all, or multiple names across different scopes, so "the property name" is pretty ill defined.
arguments.callee is deprecated and should not be used.
There exists no solution that does not use arguments or closure.
Long answer:
As thefourtheye commented, you should rethink what you are trying to do and ask that instead in a new question. But there are some common misconceptions, so I will try to explain why you cannot get the "simple property name".
The reason is because it is not simple.
Before we go ahead, let us clarify something. Activation Objects are not objects at all.
The ECMAScript 5.1 specification calls them Environment Records (10.2.1), but a more common term is Scope chain.
In a browser the global scope is (often) the window object, but all other scopes are not objects.
There may be an object that you use to call a function, and when you call a function you must be in some scope.
With few exceptions, scopes are not objects, and objects are not scopes.
Then, there are many names.
When you call a function, you need to reference it, such as through an object property. This reference may have a name.
Scope chain has declarations, which always have a name.
A Function (the real function, not reference) may also have a function name - your arguments.callee.name - which is fixed at declaration.
Not only are they different names, they are not (always) the "the property name" you are seeking.
var obj = { prop : function f(){} }, func = obj.prop;
// "obj" and "func" are declarations.
// Function name is "f" - use this name instead of arguments.callee
// Property name is "prop"
func(); // Reference name is "func"
obj.prop(); // Reference names are "obj" and "prop"
// But they are the same function!
// P.S. "this" in f is undefined (strict mode) or window (non-strict)
So, a function reference may comes from a binding (e.g. function declaration), an Object (arguments.callee), or a variable.
They are all References (8.7). And reference does have a name (so to speak).
The catch is, a function reference does not always come from an object or the scope chain, and its name is not always defined.
For example a common closure technique:
(function(i){ /* what is my name? */ })(i)
Even if the reference does have a name, a function call (11.2.3) does not pass the reference or its name to the function in any way.
Which keeps the JavaScript engine sane. Consider this example:
eval("(new Function('return function a(){}'))()")() // Calls function 'a'.
The final function call refers the eval function, which refers the result of a new global scope (in strict mode, anyway), which refers a function call statement, which refers a group, which refers an anonymous Function object, and which contains code that expresses and returns a function called 'a'.
If you want to get the "property name" from within a, which one should it get? "eval"? "Function"? "anonymous"? "a"? All of them?
Before you answer, consider complications such as function access across iframes, which has different globals as well as cross origin restriction, or interaction with native functions (Function.prototype.bind for example), and you will see how it quickly becomes hell.
This is also why arguments.caller, __caller__, and other similar techniques are now all deprecated.
The "property name" of a function is even more ill defined than the caller, almost unrealistic.
At least caller is always an execution context (not necessary a function).
So, not knowing what your real problem is, the best bet of getting the "property name" is using closure.
there is no reflection, but you can use function behavior to make adding your own fairly painless, and without resorting to try/catch, arguments.callee, Function.caller, or other strongly frowned-upon behavior, just wasteful looping:
// returning a function from inside a function always creates a new, unique function we can self-identify later:
function callName() {
return function callMe(){
for(var it in this) if(this[it]===callMe) return alert(it);
}
};
//the one ugly about this is the extra "()" at the end:
var obj1 = {'callName2':callName(), 'callName3':callName() };
var obj2 = {'callName4':callName(), 'callName5':callName() };
//test out the tattle-tale function:
obj1.callName2(); // alerts 'callName2'
obj2.callName5(); // alerts 'callName5'
if you REALLY want to make it look like an assignment and avoid the execution parens each time in the object literal, you can do this hacky routine to create an invoking alias:
function callName() {
return function callMe(){
for(var it in this) if(this[it]===callMe) return alert(it);
}
};
//make an alias to execute a function each time it's used :
Object.defineProperty(window, 'callNamer', {get: function(){ return callName() }});
//use the alias to assign a tattle-tale function (look ma, no parens!):
var obj1 = {'callName2': callNamer, 'callName3': callNamer };
var obj2 = {'callName4': callNamer, 'callName5': callNamer };
//try it out:
obj1.callName2(); // alerts 'callName2'
obj2.callName5(); // alerts 'callName5'
all that aside, you can probably accomplish what you need to do without all the looping required by this approach.
Advantages:
works on globals or object properties
requires no repetitive key/name passing
uses no proprietary or deprecated features
does not use arguments or closure
surrounding code executes faster (optimized) than
a try/catch version
is not confused by repeated uses
can handle new and deleted (renamed) properties
Caveats:
doesn't work on private vars, which have no property name
partially loops owner object each access
slower computation than a memorized property or code-time repetition
won't survive call/bind/apply
wont survive a setTimeout without bind() or a wrapper function
cannot easily be cloned
honestly, i think all the ways of accomplishing this task are "less than ideal", to be polite, and i would recommend you just bite the coding bullet and pass extra key names, or automate that by using a method to add properties to a blank object instead of coding it all in an object literal.
Yes.
Sort Of.
It depends on the browser. (Chrome=OK, Firefox=Nope)
You can use a factory to create the function, and a call stack parsing hack that will probably get me arrested.
This solution works in my version of Chrome on Windows 7, but the approach could be adapted to other browsers (if they support stack and show the property name in the call stack like Chrome does). I would not recommend doing this in production code as it is a pretty brittle hack; instead improve the architecture of your program so that you do not need to rely on knowing the name of the calling property. You didn't post details about your problem domain so this is just a fun little thought experiment; to wit:
JSFiddle demo: http://jsfiddle.net/tv9m36fr/
Runnable snippet: (scroll down and click Run code snippet)
function getCallerName(ex) {
// parse the call stack to find name of caller; assumes called from object property
// todo: replace with regex (left as exercise for the reader)
// this works in chrome on win7. other browsers may format differently(?) but not tested.
// easy enough to extend this concept to be browser-specific if rules are known.
// this is only for educational purposes; I would not do this in production code.
var stack = ex.stack.toString();
var idx = stack.indexOf('\n');
var lines = ex.stack.substring(idx + 1);
var objectSentinel = 'Object.';
idx = lines.indexOf(objectSentinel);
var line = lines.substring(idx + objectSentinel.length);
idx = line.indexOf(' ');
var callerName = line.substring(0, idx);
return callerName;
}
var Factory = {
getFunction: function () {
return function () {
var callName = "";
try {
throw up; // you don't *have* to throw to get stack trace, but it's more fun!
} catch (ex) {
callName = getCallerName(ex);
}
alert(callName);
};
}
}
var obj1 = {
'callName2': Factory.getFunction(),
'callName3': Factory.getFunction()
};
var obj2 = {
'callName4': Factory.getFunction(),
'callName5': Factory.getFunction()
};
obj1.callName2(); // should alert 'callName2'
obj1.callName3(); // should alert 'callName3'
obj2.callName4(); // should alert 'callName4'
obj2.callName5(); // should alert 'callName5'
I'm trying to get under the covers a little bit and get to understanding and not regurgitating code. I'm aware that bind, call and apply change the execution context by modifying what this points to. What I do not understand, is are arenas in which these methods are indispensable and/or lead to much shorter code. Consider the following:
var person = {
firstName: 'john',
lastName: 'doe',
fullName: function () {
console.log(this.firstName + " " + this.lastName);
}
}
//#1, binder is an html button
$('#binder').click(person.fullName.bind(person)); //john doe
//#2, anonymous is an html button
$('#anonymous').click(function () {
person.fullName(); //john doe
});
//seems strange I see this application a lot 'borrowing' a method
//why?
var o = { firstName: 'obj', lastName: 'ect' };
person.fullName.call(o); //obj ect to console
I would like to know some general paradigms about when it's good practice and/or a huge time saver to use call and apply (and bind outside of the not using anonymous functions a la function #1)
To focus on where each of these features are indispensable, I would say
apply is most useful when dealing with variadic functions. It lets you convert an array of values into an argument list.
function logwrapper(f){
return function(){
console.log("called");
return f.apply(this, arguments);
}
}
var bar = function(){ ... }
var foo = logwrapper(bar);
bind is most useful when you want to pass a method into some code that expects just a function. A common example is settimeout and other functions that expect callbacks:
setTimeout(function(){ obj.meth(); }, 100); //handwritten callback
setTimeout(obj.meth.bind(obj), 100); //Function.prototype.bind
setTimeout(bind(obj, "meth"), 100); //Some JS libraries have functions that
//let you write the object only once.
Just keep in mind that IE <=8 does not support the native Function.prototype.bind. You might need to use a polyfill or helper library in that case.
call is mostly useful for borrowing methods, as you already noticed. If this is going to be useful or not is going to depend a lot on your particular use case but an important use that is very common is using array methods on the arguments. For historical reasons, arguments doesn't have any of the usual array methods (slice, map, etc) so you need to borrow them:
function myvariadic(x){
var rest = [].slice.call(x, 1);
}
Another example you might see is the hasOwnProerty method:
for(k in obj){
if(Object.prototype.hasOwnProperty.call(obj, k)){
....
}
}
This lets you call the real hasOwnProperty method even if the object shadows it with a hasOwnProperty key of its own.
Frankly I don't give a twopenny toss about good or bad practices.
If your aim is to understand anything at all, you'd better forget about them.
This being said, to understand when it is appropriate to use call/apply or bind, what you have to understand is closures, and the specific closure of this.
I use a broad definition of closure here. Not the one you will usually get in JavaScript technical chats, where we usually talk about lexical closures.
For the purpose of this little post, let's say a closure will be anything that provides a value for any variable (this included) that lives outside the current scope of the function you're focusing on.
bind, call and apply are basically there to supply a value for this (and some other arguments as an option).
It is only useful for two things:
invoke functions or methods of a given class to an object of a different class (or no class at all).
more on that later
supply a value for this in case the current closure does not suit your needs.
For instance, passing a reference to a method loses the connection to the underlying object instance. Or using a class prototype function. Or being called in the context of an event handler where JS has set this to the DOM element that caught the event.
call & apply
call will simply set this to the value of its first argument for this particular execution of the function.
Since creating objects is so easy in JS, you might want to do something like:
Worker = function ()
{
this.things_done = 0;
}
Worker.prototype = {
do_something: function (count)
{
this.things_done += count;
}
}
var worker= new Worker(); // Worker object
var wanabee_worker = { things_done: 100 }; // classless object
Et voilà! You've just created something that has no class relation with Worker but still can use Worker methods, since it has all the required properties defined.
worker.do_something.call (wanabee_worker, 10);
allows wanabee_worker to borrow unrelated object Worker's methods.
The opposite can also be used:
function reset_work_count ()
{
this.things_done = 0;
}
reset_work_count.call (worker);
Here we have a plain function that does not have anything to do with Worker, except it uses the same properties. call allows to apply it to a Worker object.
apply does exactly the same as far as this is concerned. The only difference is the way other arguments are passed.
bind
bind will create an internal closure and return a new wrapper function that will use the parameter passed to bind as a value for this.
Typical example: bind an event handler to a specific object.
$('#binder').click(person.fullName.bind(person));
Beneath the JQuery goo, what the code does eventually is
binder.addEventListener ('click', person.fullName.bind(person), false);
If the handler was simply defined as person.fullName, it would be called with this set to the DOM element that caught the event.
In that particular case, the closure of this provided by the JS engine does not suit our needs, so we provide an alternative one using bind.
Instead of person.fullName.bind(person), you could have used:
function() { person.FullName(); }
except that
the lambda function is cumbersome and obfucates the code,
bind is an internal construct that does the closure a bit more efficiently.
You could also imagine that the object used to handle the event will be dynamically allocated/computed by some proxy function. In that case, using bind would be useless since what we want is access to the methods that will allow us to use a lambda object as one of a working class.
function event_handler (param)
{
var obj = compute_lambda_obj ();
Worker.prototype.do_something.call (ojb, param);
}
I'm learning javascript right now, seems like beautiful functional language to me, it is wonderful move from PHP, I should have done this earlier. Although, I cannot figure this one out:
var v1 = (/[abc]/).test;
v1('a');
says test method called on incompatible undefined, I'm trying to store the test method of that regex into variable and invoke it later.
but it works with my own functions:
function foo(){
return 'I\'m foo';
}
var f = foo;
f(); // returns I'm foo
It should work on methods too, since functions are just methods of parent object anyway, right?
Ultimately, the reason I'm trying this is to be able to write something like this:
var a = ['a', 'b', 'c'];
a.every( (/[abc]/).test );
to check each array member against that regex.
Why doesn't this work? Is it limitation in passing built-in functions around? Or am I just doing something wrong?
PS: If you grind your teeth now and muffling something about bad practices, screw good practices, I'm just playing. But I'd like to hear about them too.
it works with my own functions
You are not using this inside the function. Consider this example:
var obj = {
foo: 42,
bar: function() {
alert(this.foo);
}
};
var f = obj.bar;
f(); // will alert `undefined`, not `42`
It should work on methods too, since functions are just methods of parent object anyway, right?
"Method" is just a colloquial term for a function assigned to a property on object. And functions are standalone values. There is no connection to the object a function is assigned to. How would this even be possible, since a function could be assigned to multiple objects?
Why doesn't this work?
What this refers to inside a function is determined at run time. So if you assign the function to a variable and call it later
var v1 = (/[abc]/).test;
v1('a');
this inside the function will refer to window, not to the regular expression object.
What you can do is use .bind [MDN] to explicitly bind this to a specific value:
var a = ['a', 'b', 'c'];
var pattern = /[abc]/;
a.every(pattern.test.bind(pattern));
Note though that since .bind returns a function, the only advantage over using a function expression is that it is a tad shorter to write.
Is it limitation in passing built-in functions around?
No, the problem exists for every method/function because that's how functions work. The nice thing about built-in functions though is that they often explicitly tell you when this is referring to the wrong type of object (by throwing an error).
Learn more about this.
If you store just a method, it does not carry with it a reference to your object - it just stores a reference to the .test method, but no particular object. Remember, a method is "just" a property on an object and storing a reference to a method doesn't bind it to that object, it just stores a reference to the method.
To invoke that method on a particular object, you have to call it with that object.
You can make your own function that calls the method on the desired object like this:
var v1 = function(x) {
return /[abc]/.test(x);
}
Then, when you do this:
v1('a');
It will execute the equivalent of this in your function:
/[abc]/.test('a');
But, it isn't entirely clear why you're doing that as you could also just define the regex and call .test() on it several times:
var myRegex = /[abc]/;
console.log(myRegex.test('a'));
console.log(myRegex.test('b'));
console.log(myRegex.test('z'));
The test function expects this to be a regular expression. The expression /[abc]/.test gives an unbound function (it does not remember that it belongs to /[abc]/). When you invoke it like you do, this will be undefined and the function will fail.
You can use bind to make the function remember the object it belongs to:
var v1 = /[abc]/.test.bind(/[abc]/);
or
var v1 = RegExp.prototype.test.bind(/[abc]/);
Your reference to the method has lost its knowledge of what it was a method of.
This isn't so much good practice as just the way JS works.
You can do:
var v1 = /[abc]/;
v1.test('a');
If you must encapsulate the test method, then you could do:
var v1 = function(str){
return /[abc]/.test(str);
};
v1('a');
I don't know if this is an acceptable solution, but you can do:
v1 = function(exp) { return (/[abc]/).test(exp); }
v1('a');
I was wondering if someone could explain the difference between these two JavaScript modules. I have been trying to learn how to design javascript modules by reading some underscore.js and jQuery source code, amongst others and have noticed these two patterns.
Example 1
(function() {
var _this = this;
_this.VERSION = '0.1';
}).call(this);
Example 2
(function(_this) {
_this.VERSION = '0.1';
}(this));
call(obj[, arg[, arg[, arg[, ...]]]]) runs function in obj context, function () {}(obj) will run function in current context passing obj in arguments. In this particular example there is no difference - both examples will do same thing. For some people it is just cleaner to run anonymous function with call or apply instead of just () and I think here it is the case.
In second example line with var _this = _this; is useless, _this is already defined within scope and that line is redefining already existing variable with same value (so its even incorrect).
Example 1 explicitly sets the value of the this variable within the function.
In Example 2 the function does not care about the value of this. It rather expects an argument that it can work with. You also could say that it does not rely on the value of this. An advantage of this pattern is that by the name of the parameter it can be made clear what you want (window, myObjectContext ...).
In your specific case it does not make a difference. But if the function contained code that made actual use of this, e.g. this.myObject = {}, it could make a difference, because this could have a different value in each case. But to be true, someone using the second pattern would never reference this within the function.