Is it possible to find the name of an anonymous function?
e.g. trying to find a way to alert either anonyFu or findMe in this code http://jsfiddle.net/L5F5N/1/
function namedFu(){
alert(arguments.callee);
alert(arguments.callee.name);
alert(arguments.callee.caller);
alert(arguments.caller);
alert(arguments.name);
}
var anonyFu = function() {
alert(arguments.callee);
alert(arguments.callee.name);
alert(arguments.callee.caller);
alert(arguments.caller);
alert(arguments.name);
}
var findMe= function(){
namedFu();
anonyFu();
}
findMe();
This is for some internal testing, so it doesn't need to be cross-browser. In fact, I'd be happy even if I had to install a plugin.
You can identify any property of a function from inside it, programmatically, even an unnamed anonymous function, by using arguments.callee. So you can identify the function with this simple trick:
Whenever you're making a function, assign it some property that you can use to identify it later.
For example, always make a property called id:
var fubar = function() {
this.id = "fubar";
//the stuff the function normally does, here
console.log(arguments.callee.id);
}
arguments.callee is the function, itself, so any property of that function can be accessed like id above, even one you assign yourself.
Callee is officially deprecated, but still works in almost all browsers, and there are certain circumstances in which there is still no substitute. You just can't use it in "strict mode".
You can alternatively, of course, name the anonymous function, like:
var fubar = function foobar() {
//the stuff the function normally does, here
console.log(arguments.callee.name);
}
But that's less elegant, obviously, since you can't (in this case) name it fubar in both spots; I had to make the actual name foobar.
If all of your functions have comments describing them, you can even grab that, like this:
var fubar = function() {
/*
fubar is effed up beyond all recognition
this returns some value or other that is described here
*/
//the stuff the function normally does, here
console.log(arguments.callee.toString().substr(0, 128);
}
Note that you can also use argument.callee.caller to access the function that called the current function. This lets you access the name (or properties, like id or the comment in the text) of the function from outside of it.
The reason you would do this is that you want to find out what called the function in question. This is a likely reason for you to be wanting to find this info programmatically, in the first place.
So if one of the fubar() examples above called this following function:
var kludge = function() {
console.log(arguments.callee.caller.id); // return "fubar" with the first version above
console.log(arguments.callee.caller.name); // return "foobar" in the second version above
console.log(arguments.callee.caller.toString().substr(0, 128);
/* that last one would return the first 128 characters in the third example,
which would happen to include the name in the comment.
Obviously, this is to be used only in a desperate case,
as it doesn't give you a concise value you can count on using)
*/
}
Doubt it's possible the way you've got it. For starters, if you added a line
var referenceFu = anonyFu;
which of those names would you expect to be able to log? They're both just references.
However – assuming you have the ability to change the code – this is valid javascript:
var anonyFu = function notActuallyAnonymous() {
console.log(arguments.callee.name);
}
which would log "notActuallyAnonymous". So you could just add names to all the anonymous functions you're interested in checking, without breaking your code.
Not sure that's helpful, but it's all I got.
I will add that if you know in which object that function is then you can add code - to that object or generally to objects prototype - that will get a key name basing on value.
Object.prototype.getKeyByValue = function( value ) {
for( var prop in this ) {
if( this.hasOwnProperty( prop ) ) {
if( this[ prop ] === value )
return prop;
}
}
}
And then you can use
THAT.getKeyByValue(arguments.callee.caller);
Used this approach once for debugging with performance testing involved in project where most of functions are in one object.
Didn't want to name all functions nor double names in code by any other mean, needed to calculate time of each function running - so did this plus pushing times on stack on function start and popping on end.
Why? To add very little code to each function and same for each of them to make measurements and calls list on console. It's temporary ofc.
THAT._TT = [];
THAT._TS = function () {
THAT._TT.push(performance.now());
}
THAT._TE = function () {
var tt = performance.now() - THAT._TT.pop();
var txt = THAT.getKeyByValue(arguments.callee.caller);
console.log('['+tt+'] -> '+txt);
};
THAT.some_function = function (x,y,z) {
THAT._TS();
// ... normal function job
THAT._TE();
}
THAT.some_other_function = function (a,b,c) {
THAT._TS();
// ... normal function job
THAT._TE();
}
Not very useful but maybe it will help someone with similar problem in similar circumstances.
arguments.callee it's deprecated, as MDN states:
You should avoid using arguments.callee() and just give every function
(expression) a name.
In other words:
[1,2,3].forEach(function foo() {
// you can call `foo` here for recursion
})
If what you want is to have a name for an anonymous function assigned to a variable, let's say you're debugging your code and you want to track the name of this function, then you can just name it twice, this is a common pattern:
var foo = function foo() { ... }
Except the evaling case specified in the MDN docs, I can't think of any other case where you'd want to use arguments.callee.
No. By definition, an anonymous function has no name. Yet, if you wanted to ask for function expressions: Yes, you can name them.
And no, it is not possible to get the name of a variable (which references the function) during runtime.
Related
I am working with a Javascript code that uses eval function.
eval(myString)
The value of myString = myFunc(arg), I want to call myFunc directly without using eval.
I dont have any control over the function to call as I am getting that function as a String (here myString).
The arguments to that function is also part of the same string.
So, is there any way through which I can call the intended function without using eval?
I'm a bit skeptical of allowing users to provide function names at all, but... Assume you have the function name in a variable and the value of arg in a variable. Boom:
var myString = window[fn](arg);
arg is already presumably in an argument, so that's simple enough. The next part is exatracting the function name. Just a bit of regex:
var fn = /^([a-z0-9_]+)\(arg\)$/i.exec(str)[1];
if (fn && typeof window[fn] === 'function') {
window[fn](arg);
}
This does of course assume that the function is always in the global scope, but if it's not, you should be able to adjust accordingly for that. Also, my regex is just the first thing I came up with. It probably doesn't cover all possible function names.
If you wanted to limit the string to just a certain set of functions (and you almost certainly should), that becomes quite easy as well once you have the function name:
var allowedFunctions = {fn1: fn1, fn2: fn2, someOtherFunction: function() {} },
fn = /^([a-z0-9_]+)\(arg\)$/i.exec(str)[1];
if (fn && allowedFunctions[fn]) {
allowedFunctions[fn](arg);
} else {
// Hah, nice try.
}
(If arg isn't actually a variable name but is some kind of literal or perhaps an arbitrary expression, this gets a little more complicated and a lot less safe.)
JavaScript does not provide any way of calling a function represented as a string, other than using eval. There's nothing wrong about using it, though. Given that you have no other option.
Possibly you may try using Function:
var sure = function(s) {
return confirm(s);
};
var str = 'sure("Are you sure?")';
var rtn = new Function('return ' + str)();
alert(rtn);
In Ruby I think you can call a method that hasn't been defined and yet capture the name of the method called and do processing of this method at runtime.
Can Javascript do the same kind of thing ?
method_missing does not fit well with JavaScript for the same reason it does not exist in Python: in both languages, methods are just attributes that happen to be functions; and objects often have public attributes that are not callable. Contrast with Ruby, where the public interface of an object is 100% methods.
What is needed in JavaScript is a hook to catch access to missing attributes, whether they are methods or not. Python has it: see the __getattr__ special method.
The __noSuchMethod__ proposal by Mozilla introduced yet another inconsistency in a language riddled with them.
The way forward for JavaScript is the Proxy mechanism (also in ECMAscript Harmony), which is closer to the Python protocol for customizing attribute access than to Ruby's method_missing.
The ruby feature that you are explaining is called "method_missing" http://rubylearning.com/satishtalim/ruby_method_missing.htm.
It's a brand new feature that is present only in some browsers like Firefox (in the spider monkey Javascript engine). In SpiderMonkey it's called "__noSuchMethod__" https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/NoSuchMethod
Please read this article from Yehuda Katz http://yehudakatz.com/2008/08/18/method_missing-in-javascript/ for more details about the upcoming implementation.
Not at the moment, no. There is a proposal for ECMAScript Harmony, called proxies, which implements a similar (actually, much more powerful) feature, but ECMAScript Harmony isn't out yet and probably won't be for a couple of years.
You can use the Proxy class.
var myObj = {
someAttr: 'foo'
};
var p = new Proxy(myObj, {
get: function (target, methodOrAttributeName) {
// target is the first argument passed into new Proxy, aka. target is myObj
// First give the target a chance to handle it
if (Object.keys(target).indexOf(methodOrAttributeName) !== -1) {
return target[methodOrAttributeName];
}
// If the target did not have the method/attribute return whatever we want
// Explicitly handle certain cases
if (methodOrAttributeName === 'specialPants') {
return 'trousers';
}
// return our generic method_missing function
return function () {
// Use the special "arguments" object to access a variable number arguments
return 'For show, myObj.someAttr="' + target.someAttr + '" and "'
+ methodOrAttributeName + '" called with: ['
+ Array.prototype.slice.call(arguments).join(',') + ']';
}
}
});
console.log(p.specialPants);
// outputs: trousers
console.log(p.unknownMethod('hi', 'bye', 'ok'));
// outputs:
// For show, myObj.someAttr="foo" and "unknownMethod" called with: [hi,bye,ok]
About
You would use p in place of myObj.
You should be careful with get because it intercepts all attribute requests of p. So, p.specialPants() would result in an error because specialPants returns a string and not a function.
What's really going on with unknownMethod is equivalent to the following:
var unk = p.unkownMethod;
unk('hi', 'bye', 'ok');
This works because functions are objects in javascript.
Bonus
If you know the number of arguments you expect, you can declare them as normal in the returned function.
eg:
...
get: function (target, name) {
return function(expectedArg1, expectedArg2) {
...
I've created a library for javascript that let you use method_missing in javascript: https://github.com/ramadis/unmiss
It uses ES6 Proxies to work. Here is an example using ES6 Class inheritance. However you can also use decorators to achieve the same results.
import { MethodMissingClass } from 'unmiss'
class Example extends MethodMissingClass {
methodMissing(name, ...args) {
console.log(`Method ${name} was called with arguments: ${args.join(' ')}`);
}
}
const instance = new Example;
instance.what('is', 'this');
> Method what was called with arguments: is this
No, there is no metaprogramming capability in javascript directly analogous to ruby's method_missing hook. The interpreter simply raises an Error which the calling code can catch but cannot be detected by the object being accessed. There are some answers here about defining functions at run time, but that's not the same thing. You can do lots of metaprogramming, changing specific instances of objects, defining functions, doing functional things like memoizing and decorators. But there's no dynamic metaprogramming of missing functions as there is in ruby or python.
I came to this question because I was looking for a way to fall through to another object if the method wasn't present on the first object. It's not quite as flexible as what your asking - for instance if a method is missing from both then it will fail.
I was thinking of doing this for a little library I've got that helps configure extjs objects in a way that also makes them more testable. I had seperate calls to actually get hold of the objects for interaction and thought this might be a nice way of sticking those calls together by effectively returning an augmented type
I can think of two ways of doing this:
Prototypes
You can do this using prototypes - as stuff falls through to the prototype if it isn't on the actual object. It seems like this wouldn't work if the set of functions you want drop through to use the this keyword - obviously your object wont know or care about stuff that the other one knows about.
If its all your own code and you aren't using this and constructors ... which is a good idea for lots of reasons then you can do it like this:
var makeHorse = function () {
var neigh = "neigh";
return {
doTheNoise: function () {
return neigh + " is all im saying"
},
setNeigh: function (newNoise) {
neigh = newNoise;
}
}
};
var createSomething = function (fallThrough) {
var constructor = function () {};
constructor.prototype = fallThrough;
var instance = new constructor();
instance.someMethod = function () {
console.log("aaaaa");
};
instance.callTheOther = function () {
var theNoise = instance.doTheNoise();
console.log(theNoise);
};
return instance;
};
var firstHorse = makeHorse();
var secondHorse = makeHorse();
secondHorse.setNeigh("mooo");
var firstWrapper = createSomething(firstHorse);
var secondWrapper = createSomething(secondHorse);
var nothingWrapper = createSomething();
firstWrapper.someMethod();
firstWrapper.callTheOther();
console.log(firstWrapper.doTheNoise());
secondWrapper.someMethod();
secondWrapper.callTheOther();
console.log(secondWrapper.doTheNoise());
nothingWrapper.someMethod();
//this call fails as we dont have this method on the fall through object (which is undefined)
console.log(nothingWrapper.doTheNoise());
This doesn't work for my use case as the extjs guys have not only mistakenly used 'this' they've also built a whole crazy classical inheritance type system on the principal of using prototypes and 'this'.
This is actually the first time I've used prototypes/constructors and I was slightly baffled that you can't just set the prototype - you also have to use a constructor. There is a magic field in objects (at least in firefox) call __proto which is basically the real prototype. it seems the actual prototype field is only used at construction time... how confusing!
Copying methods
This method is probably more expensive but seems more elegant to me and will also work on code that is using this (eg so you can use it to wrap library objects). It will also work on stuff written using the functional/closure style aswell - I've just illustrated it with this/constructors to show it works with stuff like that.
Here's the mods:
//this is now a constructor
var MakeHorse = function () {
this.neigh = "neigh";
};
MakeHorse.prototype.doTheNoise = function () {
return this.neigh + " is all im saying"
};
MakeHorse.prototype.setNeigh = function (newNoise) {
this.neigh = newNoise;
};
var createSomething = function (fallThrough) {
var instance = {
someMethod : function () {
console.log("aaaaa");
},
callTheOther : function () {
//note this has had to change to directly call the fallThrough object
var theNoise = fallThrough.doTheNoise();
console.log(theNoise);
}
};
//copy stuff over but not if it already exists
for (var propertyName in fallThrough)
if (!instance.hasOwnProperty(propertyName))
instance[propertyName] = fallThrough[propertyName];
return instance;
};
var firstHorse = new MakeHorse();
var secondHorse = new MakeHorse();
secondHorse.setNeigh("mooo");
var firstWrapper = createSomething(firstHorse);
var secondWrapper = createSomething(secondHorse);
var nothingWrapper = createSomething();
firstWrapper.someMethod();
firstWrapper.callTheOther();
console.log(firstWrapper.doTheNoise());
secondWrapper.someMethod();
secondWrapper.callTheOther();
console.log(secondWrapper.doTheNoise());
nothingWrapper.someMethod();
//this call fails as we dont have this method on the fall through object (which is undefined)
console.log(nothingWrapper.doTheNoise());
I was actually anticipating having to use bind in there somewhere but it appears not to be necessary.
Not to my knowledge, but you can simulate it by initializing the function to null at first and then replacing the implementation later.
var foo = null;
var bar = function() { alert(foo()); } // Appear to use foo before definition
// ...
foo = function() { return "ABC"; } /* Define the function */
bar(); /* Alert box pops up with "ABC" */
This trick is similar to a C# trick for implementing recursive lambdas, as described here.
The only downside is that if you do use foo before it's defined, you'll get an error for trying to call null as though it were a function, rather than a more descriptive error message. But you would expect to get some error message for using a function before it's defined.
I'm sure this has definitively been answered before, and I've tried to search for it.. maybe my search terms are wrong...
Basically I have an object myObject, and I have a set of defined properties and methods for it. What I want to do is be able to handle calls/references to properties and methods that I have not defined.
For example, let's say I have this:
var myObject = {
someProperty : 'foobar',
someFunction : function () { /* Do stuff */ }
}
Currently, if someone tries to make a call to myObject.someOtherFunction(), JavaScript yells and screams about it. What I want to do is setup a way to automatically handle that. So for example, instead of JavaScript throwing an error, my object just returns false. Is this possible?
Another way to look at it is this:
var myObject = {
someFunction : function () { /* Do stuff */ }
magicBucket : function () { /* Do stuff */ }
}
If I call myObject.someFunction(), well that is defined and does something. What I want to happen is if I were to for instance call myObject.someOtherFunction(), instead of JavaScript throwing an error, it would call myObject.magicBucket().
The reason is that I have a client that uses a third-party library on their site. They want to discontinue using it, but completely removing it is going to take a lot of time and effort. So as a short-term solution, they wanted to know if I could make a dummy file that basically does nothing. Well, this library uses several objects that has lots of methods. I could go through everything and make dummy objects, but I thought maybe there might be some easy "catch-all" method to do this.
Some have mentioned checking if the method exists first, wrapping it in a condition or try..catch, etc. Well, the point of this is that at this time I can't touch the actual calls to the methods. And since the overall goal is to eventually remove the coding altogether, it's not even applicable.
There's a special property called __noSuchMethod__ which does precisely what you just described. However it's a non-standard property. It only works in Firefox. Here's how you use it:
var o = {
__noSuchMethod__: function (name, args) {
alert(name); // prints the name of the method
alert(args); // prints the array of arguments
}
};
o.abc(1, 2, 3); // OUTPUT: abc 1,2,3
The future however are proxy objects. The following is a short tutorial on proxies: Proxy Tutorial
No, you can't have arbitrary getters in JavaScript. You can test if a function exists before calling it to prevent the error though:
if (myObject.someOtherFunction)
myObject.someOtherFunction();
Or, better, if you don't know that it's necessarily a function:
if (typeof myObject.someOtherFunction == 'function')
myObject.someOtherFunction();
An update on Proxies, here is an infinite array:
$ var squares = new Proxy([], {get:(target,key) => key*key});
$ squares[2]
4
$ Array.isArray(squares)
true
Unfortunately:
$ squares.length
NaN // Want Infinity.
And a dummy object:
$ x = new Proxy({}, {get:(target,key) => console.error("The computer says no", key)})
$ x.laugh
The computer says no laugh
This latter would help the OP make a dummy object, although it would take a bit of black magic to divine what sort of dummy to return.
An up-to date reference: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Proxy
You could create a wrapper function like so:
function callFunction(fn, args) {
var funct = this[fn];
return (typeof funct == "function")
? funct.apply(this, args)
: false;
}
And call with:
callFunction("blah", [1, 2, 3]);
>>> false
An example:
this.foo = function(a, b) {
console.log(a);
return b;
}
callFunction("foo", [1, 2, 3]);
>>> 1
>>> 2 # return value
With Proxy the following works:
function createPseudoObject() {
return new Proxy(new Function(), { get() { return createPseudoObject() } })
}
Eg. createPseudoObject().foo.bar.toString() does not throw error and returns undefined. By using new Function() as a base for the dummy object I can reference not just any property, but call any function on the dummy object.
Maybe there are some edge cases that are not covered by this, but something like this should work.
This question just got upvoted so can update question with what I did
I solved it by iterating over the window object (or user specified object root) and when I found the correct instance I backtracked and got the name from the index. The final solution can be found here
https://github.com/AndersMalmgren/Knockout.BindingConventions
Update end
I'm planning on writing a convention over configuration template source engine for KnockoutJS / MVC.
I'm started with a little client side POC and ran into a show stopper right away
My plan is use this syntax or something similar
MyApp.EditCustomersViewModel = function() {
ko.templates.loadView(this);
};
When doing this it will check the tamplate cache or fetch the templates from server using the object name as key.
The problem is I cant get the name of the prototype object, i tried this
Object.prototype.getName = function() {
var funcNameRegex = /function (.{1,})\(/;
var results = (funcNameRegex).exec((this).constructor.toString());
return (results && results.length > 1) ? results[1] : "";
};
If works for objects defined like this
function MyClass() {
}
If you add a prototype to the above object it will not work, or if you define it like this
MyApp = {};
MyApp.MyClass = function() {
};
Prototype and scoping is two musts so this is a showstopper, any ideas?
Fiddle: http://jsfiddle.net/aRWLA/
edit: The background for this is like this.
On the server you have structure like this
Templates\ [ViewName]\index.html
Templates\ [ViewName]\sub-model-template.html
on the client you will do
MyApp.EditCustomersViewModel = function() {
ko.templates.loadView(this);
};
which will generate a ajax request with the objects name as key, which will fetch all the templates for the view in question
Only hoisted functions (function someFunc() {) have a retrievable name.
Assigned functions do not, because you are not technically naming the function but creating an anonymous function and assigning a reference to it (in the memory) to a named variable.
So it's the var, not the function, that is named.
This makes the very idea of retrieving function names pretty much a none-starter, since in any vaguely mature pattern you'll be writing methods, not hoisted functions - and methods of course are assigned functions.
Named expressions (see other answers) are a partial workaround but these have other issues - not least lack of support in older IEs.
(Sidenote: I've long expected browser vendors to build around this such that the names of assigned functions became retrievable, but no joy yet AFAIK.)
I think you problem in improper replacing function prototype: if you replace function prototype object then you must preserve constructor member in prototype:
function Test1() {
}
Test1.prototype={
constructor: Test1
};
MyApp={};
MyApp.MyClass=function MyClass(){
};
MyApp.MyClass.prototype={
constructor: MyApp.MyClass
};
Your example: http://jsfiddle.net/aRWLA/1/
Modified example: http://jsfiddle.net/aRWLA/2/
You can make use of named function expressions:
MyApp.MyClass = function MyClass() { ... };
But note that (suprise) they don't work correctly in all versions of IE.
See: http://kangax.github.com/nfe/
THIS DOES NOT ANSWER THE QUESTION
However, the code might be useful to other people, so I'm leaving it here, just in case. I don't expect upvotes, but please don't abuse it for downvoting either. Thanks.
I don't know your use case, as such I think you've got a design issue - the problem you describe shouldn't happen in practice.
But let's say you do need to have this working. An easy way to do what you need would be something like:
function findNamed(obj, fn){
for(var p in obj)
if(obj[p] === fn)
return p;
return false;
}
var m = {};
m.MyClass = function() {};
console.log(findNamed(m, m.MyClass));
Of course, the solution could be made into a more appropriate OOP form, but this is just to give an idea.
To replicate your use case, it would look like:
m.MyClass = function() {
findNamed(this, arguments.callee);
};
So, the final code is:
Object.prototype.getNameOfCall = function(fn) {
for(var p in this)
if(this[p] === fn)
return p;
throw "Callback not in object.";
};
var m = {};
m.MyClass = function() {
console.log(this.getNameOfCall(arguments.callee)); // MyClass
};
m.MyClass(); // test it out
I have a function that takes a string object name and I need the function to create an new instance of a object that has the same name as the value of the string
For example,
function Foo(){}
function create(name){
return new name();
}
create('Foo'); //should be equivalent to new Foo();
While I know this would be possible via eval, it would be good to try and avoid using it. I am also interested if anyone has an alternative ideas to the problem (below)
I have a database and a set of (using classical OO methodology) classes, roughly one for each table that define common operations on that table. (Very similar to Zend_Db for those who use PHP). As everything is asynchronous doing tasks based on the result of the last one can lead to very indented code
var table1 = new Table1Db();
table1.doFoo({
success:function(){
var table2 = new Table2Db();
table2.doBar({
notFound:function(){
doStuff();
}
});
}
});
The obvious solution is to create helper methods that abstracts the asynchronous nature of the code.
Db.using(db) //the database object
.require('Table1', 'doFoo', 'success') //table name, function, excpected callback
.require('Table2', 'doBar', 'notFound')
.then(doStuff);
Which simplifies things. However the problem is that I need to be able to create the table classes, the names of which can be inferred from the first augment passed to require which leads me to the problem above...
Why not simply pass the constructor function into the require method? That way you sidestep the whole issue of converting from name to function. Your example would then look like:
Db.using(db) //the database object
.require(Table1Db, 'doFoo', 'success') //table constructor, function name, expected callback
.require(Table2Db, 'doBar', 'notFound')
.then(doStuff);
However, if you really want to use a string...
Why are you deadset on avoiding using eval? It is a tool in the language and every tool has its purpose (just as every tool can be misused). If you're concerned about allowing arbitrary execution, a simple regular expression test should render your usage safe.
If you're dead-set on avoiding eval and if all of your constructor functions are created in the default global scope (i.e. the window object), this would work:
function create(name) {
return new window[name]();
}
If you want to get fancy and support namespace objects (i.e. create('MyCompany.MyLibrary.MyObject'), you could do something like this:
function create(name) {
var current,
parts,
constructorName;
parts = name.split('.');
constructorName = parts[parts.length - 1];
current = window;
for (var i = 0; i < parts.length - 1; i++) {
current = current[parts[i]];
}
return new current[constructorName]();
}
You were at the gate of completeness. While Annabelle's solution let's you to do what's you've just wanted in the way you wanted (passing strings), let me offer you an alternative. (passing function references)
function Foo(){}
function create(name){
return new name();
}
create(Foo); // IS equivalent to new Foo();
And voila, it works :) I told you. You were at the doorsteps of the solution.
What happened is that you've try to do this
new 'Foo'()
Which doesn't makes much sense, does it? But now you pass the function by reference so the line return new name(); will be transformed into return new Foo(); just how you would expect.
And now the doors are opened to abstract the asynchronousness of your application. Have fun!
Appendix: Functions are first-class objects, which means that they can be stored by reference, passed as an argument by reference or returned by another function as values.