Babel is doing its magic, which makes me very confused about what's going on.
What's the difference between foo and bar in this react Component? And when should I use which?
class MyComponent extends Component {
foo() {
//...
}
bar = () => {
//...
}
}
(My own guess is foo is in the prototype, and bar is in the constructor? Anyways, I don't know what I'm talking about)
My own guess is foo is in the prototype, and bar is in the constructor?
That's exactly right.
foo() {}
in this context is a method declaration and the value gets assigned to the prototype. It's equivalent to
MyComponent.prototype.foo = function() {};
bar = ... ;
on the other hand is a class field. This is a shorthand notation for assigning properties to the instance in the constructor:
constructor() {
this.bar = ... ;
}
And because of how arrow functions work, using class fields with arrow functions basically lets you create "bound" methods.
More on arrow functions: Arrow function vs function declaration / expressions: Are they equivalent / exchangeable?
And when should I use which?
The tl;dr is: Use class field + arrow function when you need a bound function.
When do you need a bound function? Whenever you want this inside the function to refer to a specific value but you don't control how the function is called.
That's mostly the case for event handlers (i.e. functions that are passed to other functions / components) that need to access the component instance (e.g. call this.setState or access this.props/this.state).
You don't have to use it though, you could also bind the method when you need to. However, binding a method only once in the constructor is ideal for React components so that, if the method is passed as event handler, always the same function object is passed.
As noted in another answer, this is not related to React at all. Class fields are likely officially integrated into the language this year.
In the second example bar is an arrow function.
Until arrow functions, every new function defined its own this value.
For instance, this can be a new object in the case of a constructor.
function Person(age){
this.age=age;
console.log(this);
}
let person=new Person(22);
Or this can points to the base object if the function created can be accessed like obj.getAge().
let obj={
getAge:function(){
console.log(this);
return 22;
}
}
console.log(obj.getAge());
An arrow function does not create its own this, it's just used the this value of the enclosing execution context. In the other hand, arrow function uses this of parent scope.
foo is an instance method of class MyComponent. While bar is an instance property of class MyComponent (that happens to be assigned an anonymous function).
It might make more sense to see it used in the traditional sense...
class MyComponent {
// instance property
someProperty = 42;
// instance method
someMethod() {}
}
So why use an instance property instead of an instance method?
Instance methods in javascript must be called within the class scope to inherit the class context (this)...
class MyComponent {
// instance method
someMethod() {
const value = this.state.value // ERROR: `this` is not defined
}
render() {
return <div onClick={this.someMethod} />
}
}
However, since arrow functions inherit their scope, this will be available if you use an arrow function instead
class MyComponent {
// instance method
someProperty = () => {
const value = this.state.value // this works!
}
render() {
return <div onClick={this.someProperty} />
}
}
Essentially, there is no difference.
This has nothing to do with React, though.
Arrow functions
Arrow functions are a relatively new feature added to Javascript that give you a way of defining functions in a more concise way.
function foo (param1, param2, etc) {
// do something
}
// becomes
var foo = (param1, param2, etc) => {
// do something
}
If you have only 1 param, you don't need the parantheses:
function foo (param) {
// do something
}
// becomes
var foo = param => {
// do something
}
If there is only 1 expression, and the result is returned, you can omit the {} too!
function foo (param) {
returns param * 2
}
// becomes
var foo = param1 => param * 2
this is not not updated
In an arrow function, you don't get a new this - it is the same as in the parent block. This can be useful in some cases (when using setTimeout, for example)
JS Classes
In ES6 we can also use the class keyword to define "classes" in Javascript. They still use prototype, though!
function Something (foo) {
this.bar = foo;
}
Something.prototype.baz = function () {
return baz * 2
}
// becomes
class Something {
constructor(foo) {
this.bar = foo;
}
baz () {
return baz * 2
}
}
So the constructor is done in constructor() and all the following methods are added to the Prototype. It's nothing but syntactic sugar (but kinda does some of the Prototype heavy lifting for you)
You can use extends!
class SomethingElse extends Something {
constructor(foo, lol) {
this.lol = lol
// Use super() to call the parent's constructor
super(foo)
}
baz () {
return baz * 2
}
}
This question already has answers here:
Should I write methods as arrow functions in Angular's class
(3 answers)
Arrow vs classic method in ES6 class
(1 answer)
Closed 5 years ago.
class App extends Component {
constructor(props) {
...
}
onChange = (e) => this.setState({term: e.target.value})
onSubmit(e){
e.preventDefault();
const api_key = "C1hha1quJAQZf2JUlK";
const url = `http://api.giphy.com/v1/gifs/search?q=${this.state.term}&api_key=${api_key}`;
}
render() {
return (
<div>
<form onSubmit={this.onSubmit}>
<input value={this.state.term} onChange={this.onChange}/>
<button>Search!</button>
</form>
</div>
);
}
}
What is the difference between the two type of functions declared in the class (onChange and onSubmit). I get an error on referencing this.sate in const url if I declare it as an ES6 class method but changing it to arrow function fixes it.
I want to know how exactly 'this' is handled in both the cases
Also, how do I do it the other way? Say, if I want to use the same onSubmit function (ES6 class method) but want to handle this when I call it (in the form element), how do I do it ?
Using this.onSubmit.bind(this) ?
It's important to know that this syntax:
class A {
method = () => {}
}
is just syntactic sugar for creating an instance method in the class constructor:
class A {
constructor() {
this.method = () => {}
}
}
Note: This syntax is not an official part of the JavaScript language yet (currently in stage 3) so you must use a transpiler like Babel to handle it.
The value of this within method is the class A because that is what this points to in the constructor (since arrow functions inherit the context from the scope they are defined in):
class A {
constructor() {
this.method = () => this;
}
}
const instance = new A();
console.log(instance.method() === instance); // true
Defining a regular (non-arrow function) method on the class creates a method on the class prototype (not instance) but sets no rules on what this will be (since this is dynamic in JS and depends on how a function is called, not how it's defined).
class A {
method() {}
}
console.log(new A().method === A.prototype.method); // true
If methods defined in either of these ways are called on the class instance (via the .), as per the rule of how this is bound when a function is called as a method of an object, this will point to the class instance in both cases:
class A {
constructor() {
this.methodOnInstance = () => this;
}
methodOnPrototype() { return this; }
}
const instance = new A();
console.log(
instance.methodOnInstance() === instance.methodOnPrototype(), // true
instance.methodOnPrototype() === instance // true
);
One major difference between the two method declarations above is that the instance method has this always fixed to the class instance while the class (prototype) method does not (we can change it by using Function.prototype.apply or Function.prototype.call)
class A {
constructor() {
this.methodOnInstance = () => this;
}
methodOnPrototype() { return this; }
}
const instance = new A();
console.log(
instance.methodOnInstance() === instance.methodOnPrototype(), // true
instance.methodOnPrototype.call('new this') === 'new this' // true
);
A common occurrence where the this changes is within an event handler, where the event handler calls the function passed into it and binds the context to the element on which the event happened (so overrides the value of this to be the element that was clicked or whatever the event was)
This happens in React as well for all (synthetic) DOM event handlers.
Therefore, if we want our method's context to always point to the instance of the React component, we can use the instance method.
Another way of restricting the context but not using the special instance method syntax that requires Babel is to directly create an instance method ourselves by creating a new function from the class (prototype) method with a bound context (using Function.prototype.bind):
class A {
constructor() {
this.methodOnInstance = this.methodOnPrototype.bind(this);
}
methodOnPrototype() { return this; }
}
const instance = new A();
console.log(
instance.methodOnInstance() === instance.methodOnPrototype(), // true
instance.methodOnPrototype() === instance // true
);
This allows us to arrive to the same result as using the special instance method syntax but with the currently available tools (ES2017 and under).
If for some reason we want a method that is always bound to something that is not an instance of the class, we can do that as well:
class A {
constructor() {
this.method = this.method.bind(console);
}
method() { return this; }
}
const instance = new A();
console.log(
instance.method() === console // true
);
An arrow function expression has a shorter syntax than a function
expression and does not have its own this, arguments, super, or
new.target. These function expressions are best suited for non-method
functions, and they cannot be used as constructors.
Arrow Functions lexically bind their context so this actually refers to the originating context.
In ES3/4 functions declaration you can use this by storing in some other variable.
const that = this;
onSubmit(e){
e.preventDefault();
const api_key = "***************";
const url = `http://api.giphy.com/v1/gifs/search?q=${that.state.term}&api_key=${api_key}`;
}
Also, how do I do it the other way? Say, if I want to use the same onSubmit function (ES6 class method) but want to handle this when I call it (in the form element), how do I do it ?
Using this.onSubmit.bind(this) ?
Yes you must bind the method to the component in the constructor. It's because the arrow functions get automatically binded to the class therefore the scope of this is set in the method. While onSubmit is a regular function that is not yet binded therefore the this inside the method will reference the function and not the component.
You need to use bind in your class's constructor with the ES6 class method. In essence, arrow functions do this for you automatically.
constructor(props) {
super(props);
this.onSubmit = this.onSubmit.bind(this);
}
The more important thing to note here is that I believe the arrow function here will be created on each instance of the class, where the ES6 class method will be made part of the class's prototype and shared amongst all instances.
The key difference is that in ES5 we don't have auto binding which means you have to bind your event handler function manually in order to play with state or props inside the function in react. But in ES6 it does auto binding. That's the key difference
ES5: you have to bind onSubmit preferably in constructor
//is valid
this.onSubmit = this.onSubmit.bind(this);
onSubmit(e){
e.preventDefault();
const api_key = "C1hha1quJAQZf2JUlK";
const url = `http://api.giphy.com/v1/gifs/search?q=${this.state.term}&api_key=${api_key}`;
}
ES6:
The below is valid because it does auto binding.
onChange = (e) => this.setState({term: e.target.value})
ES6 allows to extend special objects. So it's possible to inherit from the function. Such object can be called as a function, but how can I implement the logic for such call?
class Smth extends Function {
constructor (x) {
// What should be done here
super();
}
}
(new Smth(256))() // to get 256 at this call?
Any method of class gets reference to the class instance via this. But when it is called as a function, this refers to window. How can I get the reference to the class instance when it is called as a function?
PS: Same question in Russian.
The super call will invoke the Function constructor, which expects a code string. If you want to access your instance data, you could just hardcode it:
class Smth extends Function {
constructor(x) {
super("return "+JSON.stringify(x)+";");
}
}
but that's not really satisfying. We want to use a closure.
Having the returned function be a closure that can access your instance variables is possible, but not easy. The good thing is that you don't have to call super if you don't want to - you still can return arbitrary objects from your ES6 class constructors. In this case, we'd do
class Smth extends Function {
constructor(x) {
// refer to `smth` instead of `this`
function smth() { return x; };
Object.setPrototypeOf(smth, Smth.prototype);
return smth;
}
}
But we can do even better, and abstract this thing out of Smth:
class ExtensibleFunction extends Function {
constructor(f) {
return Object.setPrototypeOf(f, new.target.prototype);
}
}
class Smth extends ExtensibleFunction {
constructor(x) {
super(function() { return x; }); // closure
// console.log(this); // function() { return x; }
// console.log(this.prototype); // {constructor: …}
}
}
class Anth extends ExtensibleFunction {
constructor(x) {
super(() => { return this.x; }); // arrow function, no prototype object created
this.x = x;
}
}
class Evth extends ExtensibleFunction {
constructor(x) {
super(function f() { return f.x; }); // named function
this.x = x;
}
}
Admittedly, this creates an additional level of indirection in the inheritance chain, but that's not necessarily a bad thing (you can extend it instead of the native Function). If you want to avoid it, use
function ExtensibleFunction(f) {
return Object.setPrototypeOf(f, new.target.prototype);
}
ExtensibleFunction.prototype = Function.prototype;
but notice that Smth will not dynamically inherit static Function properties.
This is an approach to creating callable objects that correctly reference their object members, and maintain correct inheritance,
without messing with prototypes.
Simply:
class ExFunc extends Function {
constructor() {
super('...args', 'return this.__self__.__call__(...args)')
var self = this.bind(this)
this.__self__ = self
return self
}
// Example `__call__` method.
__call__(a, b, c) {
return [a, b, c];
}
}
Extend this class and add a __call__ method, more below...
An explanation in code and comments:
// This is an approach to creating callable objects
// that correctly reference their own object and object members,
// without messing with prototypes.
// A Class that extends Function so we can create
// objects that also behave like functions, i.e. callable objects.
class ExFunc extends Function {
constructor() {
super('...args', 'return this.__self__.__call__(...args)');
// Here we create a function dynamically using `super`, which calls
// the `Function` constructor which we are inheriting from. Our aim is to create
// a `Function` object that, when called, will pass the call along to an internal
// method `__call__`, to appear as though the object is callable. Our problem is
// that the code inside our function can't find the `__call__` method, because it
// has no reference to itself, the `this` object we just created.
// The `this` reference inside a function is called its context. We need to give
// our new `Function` object a `this` context of itself, so that it can access
// the `__call__` method and any other properties/methods attached to it.
// We can do this with `bind`:
var self = this.bind(this);
// We've wrapped our function object `this` in a bound function object, that
// provides a fixed context to the function, in this case itself.
this.__self__ = self;
// Now we have a new wrinkle, our function has a context of our `this` object but
// we are going to return the bound function from our constructor instead of the
// original `this`, so that it is callable. But the bound function is a wrapper
// around our original `this`, so anything we add to it won't be seen by the
// code running inside our function. An easy fix is to add a reference to the
// new `this` stored in `self` to the old `this` as `__self__`. Now our functions
// context can find the bound version of itself by following `this.__self__`.
self.person = 'Hank'
return self;
}
// An example property to demonstrate member access.
get venture() {
return this.person;
}
// Override this method in subclasses of ExFunc to take whatever arguments
// you want and perform whatever logic you like. It will be called whenever
// you use the obj as a function.
__call__(a, b, c) {
return [this.venture, a, b, c];
}
}
// A subclass of ExFunc with an overridden __call__ method.
class DaFunc extends ExFunc {
constructor() {
super()
this.a = 'a1'
this.b = 'b2'
this.person = 'Dean'
}
ab() {
return this.a + this.b
}
__call__(ans) {
return [this.ab(), this.venture, ans];
}
}
// Create objects from ExFunc and its subclass.
var callable1 = new ExFunc();
var callable2 = new DaFunc();
// Inheritance is correctly maintained.
console.log('\nInheritance maintained:');
console.log(callable2 instanceof Function); // true
console.log(callable2 instanceof ExFunc); // true
console.log(callable2 instanceof DaFunc); // true
// Test ExFunc and its subclass objects by calling them like functions.
console.log('\nCallable objects:');
console.log( callable1(1, 2, 3) ); // [ 'Hank', 1, 2, 3 ]
console.log( callable2(42) ); // [ 'a1b2', Dean', 42 ]
// Test property and method access
console.log(callable2.a, callable2.b, callable2.ab())
View on repl.it
Further explanation of bind:
function.bind() works much like function.call(), and they share a similar method signature:
fn.call(this, arg1, arg2, arg3, ...); more on mdn
fn.bind(this, arg1, arg2, arg3, ...); more on mdn
In both the first argument redefines the this context inside the function. Additional arguments can also be bound to a value.
But where call immediately calls the function with the bound values, bind returns an "exotic" function object that transparently wraps the original, with this and any arguments preset.
So when you define a function then bind some of its arguments:
var foo = function(a, b) {
console.log(this);
return a * b;
}
foo = foo.bind(['hello'], 2);
You call the bound function with only the remaining arguments, its context is preset, in this case to ['hello'].
// We pass in arg `b` only because arg `a` is already set.
foo(2); // returns 4, logs `['hello']`
You can wrap the Smth instance in a Proxy with an apply (and maybe construct) trap:
class Smth extends Function {
constructor (x) {
super();
return new Proxy(this, {
apply: function(target, thisArg, argumentsList) {
return x;
}
});
}
}
new Smth(256)(); // 256
Update:
Unfortunately this doesn't quite work because it's now returning a function object instead of a class, so it seems this actually can't be done without modifying the prototype. Lame.
Basically the problem is there is no way of setting the this value for the Function constructor. The only way to really do this would be to use the .bind method afterwards, however this is not very Class-friendly.
We could do this in a helper base class, however this does does not become available until after the initial super call, so it's a bit tricky.
Working Example:
'use strict';
class ClassFunction extends function() {
const func = Function.apply(null, arguments);
let bound;
return function() {
if (!bound) {
bound = arguments[0];
return;
}
return func.apply(bound, arguments);
}
} {
constructor(...args) {
(super(...args))(this);
}
}
class Smth extends ClassFunction {
constructor(x) {
super('return this.x');
this.x = x;
}
}
console.log((new Smth(90))());
(Example requires modern browser or node --harmony.)
Basically the base function ClassFunction extends will wrap the Function constructor call with a custom function which is similar to .bind, but allows binding later, on the first call. Then in the ClassFunction constructor itself, it calls the returned function from super which is now the bound function, passing this to finish setting up the custom bind function.
(super(...))(this);
This is all quite a bit complicated, but it does avoid mutating the prototype, which is considered bad-form for optimization reasons and can generate warnings in browser consoles.
I took the advice from Bergi's answer and wrapped it into an NPM module.
var CallableInstance = require('callable-instance');
class ExampleClass extends CallableInstance {
constructor() {
// CallableInstance accepts the name of the property to use as the callable
// method.
super('instanceMethod');
}
instanceMethod() {
console.log("instanceMethod called!");
}
}
var test = new ExampleClass();
// Invoke the method normally
test.instanceMethod();
// Call the instance itself, redirects to instanceMethod
test();
// The instance is actually a closure bound to itself and can be used like a
// normal function.
test.apply(null, [ 1, 2, 3 ]);
This is the solution I've worked out that serves all my needs of extending functions and has served me quite well. The benefits of this technique are:
When extending ExtensibleFunction, the code is idiomatic of extending any ES6 class (no, mucking about with pretend constructors or proxies).
The prototype chain is retained through all subclasses, and instanceof / .constructor return the expected values.
.bind() .apply() and .call() all function as expected. This is done by overriding these methods to alter the context of the "inner" function as opposed to the ExtensibleFunction (or it's subclass') instance.
.bind() returns a new instance of the functions constructor (be it ExtensibleFunction or a subclass). It uses Object.assign() to ensure the properties stored on the bound function are consistent with those of the originating function.
Closures are honored, and arrow functions continue to maintain the proper context.
The "inner" function is stored via a Symbol, which can be obfuscated by modules or an IIFE (or any other common technique of privatizing references).
And without further ado, the code:
// The Symbol that becomes the key to the "inner" function
const EFN_KEY = Symbol('ExtensibleFunctionKey');
// Here it is, the `ExtensibleFunction`!!!
class ExtensibleFunction extends Function {
// Just pass in your function.
constructor (fn) {
// This essentially calls Function() making this function look like:
// `function (EFN_KEY, ...args) { return this[EFN_KEY](...args); }`
// `EFN_KEY` is passed in because this function will escape the closure
super('EFN_KEY, ...args','return this[EFN_KEY](...args)');
// Create a new function from `this` that binds to `this` as the context
// and `EFN_KEY` as the first argument.
let ret = Function.prototype.bind.apply(this, [this, EFN_KEY]);
// For both the original and bound funcitons, we need to set the `[EFN_KEY]`
// property to the "inner" function. This is done with a getter to avoid
// potential overwrites/enumeration
Object.defineProperty(this, EFN_KEY, {get: ()=>fn});
Object.defineProperty(ret, EFN_KEY, {get: ()=>fn});
// Return the bound function
return ret;
}
// We'll make `bind()` work just like it does normally
bind (...args) {
// We don't want to bind `this` because `this` doesn't have the execution context
// It's the "inner" function that has the execution context.
let fn = this[EFN_KEY].bind(...args);
// Now we want to return a new instance of `this.constructor` with the newly bound
// "inner" function. We also use `Object.assign` so the instance properties of `this`
// are copied to the bound function.
return Object.assign(new this.constructor(fn), this);
}
// Pretty much the same as `bind()`
apply (...args) {
// Self explanatory
return this[EFN_KEY].apply(...args);
}
// Definitely the same as `apply()`
call (...args) {
return this[EFN_KEY].call(...args);
}
}
/**
* Below is just a bunch of code that tests many scenarios.
* If you run this snippet and check your console (provided all ES6 features
* and console.table are available in your browser [Chrome, Firefox?, Edge?])
* you should get a fancy printout of the test results.
*/
// Just a couple constants so I don't have to type my strings out twice (or thrice).
const CONSTRUCTED_PROPERTY_VALUE = `Hi, I'm a property set during construction`;
const ADDITIONAL_PROPERTY_VALUE = `Hi, I'm a property added after construction`;
// Lets extend our `ExtensibleFunction` into an `ExtendedFunction`
class ExtendedFunction extends ExtensibleFunction {
constructor (fn, ...args) {
// Just use `super()` like any other class
// You don't need to pass ...args here, but if you used them
// in the super class, you might want to.
super(fn, ...args);
// Just use `this` like any other class. No more messing with fake return values!
let [constructedPropertyValue, ...rest] = args;
this.constructedProperty = constructedPropertyValue;
}
}
// An instance of the extended function that can test both context and arguments
// It would work with arrow functions as well, but that would make testing `this` impossible.
// We pass in CONSTRUCTED_PROPERTY_VALUE just to prove that arguments can be passed
// into the constructor and used as normal
let fn = new ExtendedFunction(function (x) {
// Add `this.y` to `x`
// If either value isn't a number, coax it to one, else it's `0`
return (this.y>>0) + (x>>0)
}, CONSTRUCTED_PROPERTY_VALUE);
// Add an additional property outside of the constructor
// to see if it works as expected
fn.additionalProperty = ADDITIONAL_PROPERTY_VALUE;
// Queue up my tests in a handy array of functions
// All of these should return true if it works
let tests = [
()=> fn instanceof Function, // true
()=> fn instanceof ExtensibleFunction, // true
()=> fn instanceof ExtendedFunction, // true
()=> fn.bind() instanceof Function, // true
()=> fn.bind() instanceof ExtensibleFunction, // true
()=> fn.bind() instanceof ExtendedFunction, // true
()=> fn.constructedProperty == CONSTRUCTED_PROPERTY_VALUE, // true
()=> fn.additionalProperty == ADDITIONAL_PROPERTY_VALUE, // true
()=> fn.constructor == ExtendedFunction, // true
()=> fn.constructedProperty == fn.bind().constructedProperty, // true
()=> fn.additionalProperty == fn.bind().additionalProperty, // true
()=> fn() == 0, // true
()=> fn(10) == 10, // true
()=> fn.apply({y:10}, [10]) == 20, // true
()=> fn.call({y:10}, 20) == 30, // true
()=> fn.bind({y:30})(10) == 40, // true
];
// Turn the tests / results into a printable object
let table = tests.map((test)=>(
{test: test+'', result: test()}
));
// Print the test and result in a fancy table in the console.
// F12 much?
console.table(table);
Edit
Since I was in the mood, I figured I'd publish a package for this on npm.
Firstly I came to solution with arguments.callee, but it was awful.
I expected it to break in global strict mode, but seems like it works even there.
class Smth extends Function {
constructor (x) {
super('return arguments.callee.x');
this.x = x;
}
}
(new Smth(90))()
It was a bad way because of using arguments.callee, passing the code as a string and forcing its execution in non-strict mode. But than idea to override apply appeared.
var global = (1,eval)("this");
class Smth extends Function {
constructor(x) {
super('return arguments.callee.apply(this, arguments)');
this.x = x;
}
apply(me, [y]) {
me = me !== global && me || this;
return me.x + y;
}
}
And the test, showing I'm able to run this as function in different ways:
var f = new Smth(100);
[
f instanceof Smth,
f(1),
f.call(f, 2),
f.apply(f, [3]),
f.call(null, 4),
f.apply(null, [5]),
Function.prototype.apply.call(f, f, [6]),
Function.prototype.apply.call(f, null, [7]),
f.bind(f)(8),
f.bind(null)(9),
(new Smth(200)).call(new Smth(300), 1),
(new Smth(200)).apply(new Smth(300), [2]),
isNaN(f.apply(window, [1])) === isNaN(f.call(window, 1)),
isNaN(f.apply(window, [1])) === isNaN(Function.prototype.apply.call(f, window, [1])),
] == "true,101,102,103,104,105,106,107,108,109,301,302,true,true"
Version with
super('return arguments.callee.apply(arguments.callee, arguments)');
in fact contains bind functionality:
(new Smth(200)).call(new Smth(300), 1) === 201
Version with
super('return arguments.callee.apply(this===(1,eval)("this") ? null : this, arguments)');
...
me = me || this;
makes call and apply on window inconsistent:
isNaN(f.apply(window, [1])) === isNaN(f.call(window, 1)),
isNaN(f.apply(window, [1])) === isNaN(Function.prototype.apply.call(f, window, [1])),
so the check should be moved into apply:
super('return arguments.callee.apply(this, arguments)');
...
me = me !== global && me || this;
Generalizing Oriol's answer:
class Smth extends Function {
constructor(x) {
super();
this.x = x;
return new Proxy(this, {
apply: (target, that, args) => target.__call__(...args)
});
}
__call__(v) {
return this.x * v;
}
}
There is a simple solution which takes advantage of JavaScript's functional capabilities: Pass the "logic" as a function-argument to the constructor of your class, assign the methods of that class to that function, then return that function from the constructor as the result:
class Funk
{
constructor (f)
{ let proto = Funk.prototype;
let methodNames = Object.getOwnPropertyNames (proto);
methodNames.map (k => f[k] = this[k]);
return f;
}
methodX () {return 3}
}
let myFunk = new Funk (x => x + 1);
let two = myFunk(1); // == 2
let three = myFunk.methodX(); // == 3
The above was tested on Node.js 8.
A shortcoming of the example above is it does not support methods inherited from the superclass-chain. To support that, simply replace "Object . getOwnPropertyNames(...)" with something that returns also the names of inherited methods. How to do that I believe is explained in some other question-answer on Stack Overflow :-). BTW. It would be nice if ES7 added a method to produce inherited methods' names as well ;-).
If you need to support inherited methods one possibility is adding a static method to the above class which returns all inherited and local method names. Then call that from the constructor. If you then extend that class Funk, you get that static method inherited along as well.
Having read this article and all the answers here, I finally found a satisfying answer in an older thread.
Here's an example:
class Hey {
constructor() {
function hey() {
return "hey";
}
this.value = "yo";
Object.assign(hey, this);
Object.setPrototypeOf(hey, Object.getPrototypeOf(this));
return hey;
}
yo() {
return this.value;
}
}
const hey = new Hey();
console.log(hey()); // it's callable 👍
console.log(hey.yo()); // methods are correctly bound to `this` 👍
console.log(hey instanceof Hey); // it type-checks 👍
Inheritance works too:
class HeyHey extends Hey {
constructor() {
super();
}
yoyo() {
return this.value + "!";
}
}
const lol = new HeyHey();
console.log(lol()); // it's callable 👍
console.log(lol.yo()); // inherited methods are correctly bound to `this` 👍
console.log(lol.yoyo()); // new methods are correctly bound to `this` as well 👍
console.log(lol instanceof Hey); // it type-checks for the super class 👍
console.log(lol instanceof HeyHey); // it type-checks for the derived class 👍
You can run the example here to see for yourself.
This approach:
correctly binds methods to this
is type-safe (can be type-checked using instanceof)
correctly supports inheritance (correctly binds and type-checks derived classes)
doesn't rely on any features newer than class - getPrototypeOf and setPrototypeOf were widely available some years before class
doesn't rely on the Function constructor (avoids parsing source code at run-time)
doesn't rely on Proxy (which isn't great for performance)
All in all, this approach is definitely simpler and easier to implement, and it should perform better as well.
(In theory - please feel free to benchmark this and post your results.)
Came up with a solution that works without using Object.setPrototypeOf since MDN has big red warning signs around that. Can run the example JSFiddle here. One limitation I can't figure out is how to get access to the this context at call time of the produced function within the arbitrary execute.
class ExtendedFunction extends Function {
// Arbitrary private properties
#foo
#baz() { return this.#foo + 'qux' }
// The thing that happens when you call your extended function
// context is optional if you want access to the `this`
// provides to your extended function at call time
#execute() {
// Arbitrary code that can call anything in closure here
return this.#baz()
}
constructor(a) {
// Set `this` to simple wrapper function
// that takes another function and returns its value
// Use super we get an instance of Function and ExtendedFucntion
super('execute', 'return execute()')
this.#foo = a
// Bind our arbitrary function to the current `this`
// allowing it to access private properties even when passed around
const boundExecute = this.#execute.bind(this)
// Bind the simple wrapper and the boundExecute together and return that
// When called our extended function will do whatever is in #execute
var self = this.bind(null, boundExecute)
return self
}
}
const a = new ExtendedFunction(256)
console.log(a instanceof Function) // true
console.log(a instanceof ExtendedFunction) // true
console.log(a()) // 256qux
A little late but let me leave this here.
Recently I had to discover a way to subclass Function in order to turn normal functions into Threadable / Promisified functions without messing with the Function.prototype. I think this particular necessity forms a very reasonable basis to this question on how and why one can use class abstraction to extend Function.
So the idea is, we create a Threadable class of which the member functions are threadable. By this I mean, any normal function can easily be made Threadable and when spawned runs on a seperate thread and gives us a promise to be resolved or rejected depending on the outcome of the worker operation. However you should still be able to invoke it syncronously if need be.
class Threadable extends Function {
// just use super() to wrap the given f function with a transparent function layer
constructor(f){
super("...as",`return ${f.toString()}.apply(this,as)`);
}
// spawn is the only method of the Threadable class.
// Returns a promise and runs callee function on a separate thread.
spawn(...as){
var code = `self.onmessage = m => self.postMessage((${this.toString()}).apply(self,m.data));`,
blob = new Blob([code], {type: "text/javascript"}),
wrkr = new Worker(window.URL.createObjectURL(blob));
return new Promise( (v,x) => ( wrkr.onmessage = m => (v(m.data), wrkr.terminate())
, wrkr.onerror = e => (x(e.message), wrkr.terminate())
, wrkr.postMessage(as)
)
);
}
}
function add(...ns) {
return ns.reduce((a,b) => a+b);
}
var addT = new Threadable(add);
addT.spawn(1,2,3,4)
.then(m => console.log(`Promisified thread returned ${m}`));
console.log(`Synchronous invocation of addT returned ${addT(1,2,3,4,5)}`);