I'm trying to find a good reason to use Reflect.construct to achieve something noteworthy that I could not achieve before.
I'm not looking for an answer like the one here, because that example don't seem to be very useful. For example, why would I write
function greetingFactory(name) {
return Reflect.construct(Greeting, [name]);
}
when I can just write
function greetingFactory(name) {
return new Greeting(name);
}
?
Do you know any notable use cases for Reflect.construct?
EDIT: Seems like I may have found a use case myself, but I'm not sure if it is solid and if it won't fall apart, but basically it seems like I can make new.target work with ES5-style classes by writing them like this:
function Foo() {
console.log('Foo, new.target:', new.target)
this.name = "foo"
}
Foo.prototype.sayHello = function sayHello() {
return this.name
}
function Bar() {
console.log('Bar, new.target:', new.target)
let _ = Reflect.construct(Foo, [], new.target)
_.name = _.name + " bar"
return _
}
Bar.prototype = Object.create(Foo.prototype)
Bar.prototype.sayHello = function() {
return "Hello " + Foo.prototype.sayHello.call(this) + "!"
}
function Baz() {
console.log('Baz, new.target:', new.target)
let _ = Reflect.construct(Bar, [], new.target)
_.name = _.name + " baz"
return _
}
Baz.prototype = Object.create(Bar.prototype)
Baz.prototype.sayHello = function() {
return Bar.prototype.sayHello.call(this) + " Hello again!"
}
let baz = new Baz
console.log(baz.sayHello())
The cool thing about it is that this is as expected inside the prototype methods!
The only three use cases I know of for Reflect.construct are:
Using it within a Proxy construct trap to get the default behavior (or to get slightly-modified default behavior).
Using it to avoid creating and using an iterator when you need to call a constructor function using an array whose elements need to be passed as discrete arguments. You can just do
t = new Thing(...theArray);
but that involves creating and using an iterator, whereas
t = Reflect.construct(Thing, theArray);
doesn't use an iterator, which is much less work (not that it usually matters; this is for a situation where you know time is crucial). (Instead of an iterator, construct just uses length and directly accesses the 0, 1, etc. properties.)
Neither of those options was available before ES2015. Instead, you had to do this:
t = Thing.apply(Object.create(Thing.prototype), theArray);
which worked with ES5 constructor functions. (It wouldn't work with an ES2015+ constructor function created via class, but you don't need it to — you'd use one of the two options above instead.)
Using it to avoid using class when constructing an instance of a subtype of Error or Array or a web component (some people don't like to use class, and there are good arguments for that in projects that may need to be transpiled). (That said, Reflect.construct can't be perfectly polyfilled, either.)
I've been trying to sort out a useful application of the Reflect.construct as well. I think I may have found something but it would be nice to bounce the idea off other people on a similar path.
What I was thinking is that you could use Reflect.construct to wedge a [[Prototype]] between the instantiated object and it's intended [[Prototype]]. This would allow you to shadow properties and methods that belong to the intended [[Prototype]] without being too intrusive.
function Wedge() {};
Wedge.prototype = Object.create(String.prototype);
Wedge.prototype.toUpperCase = function () {
return "nope";
}
let someString = Reflect.construct(String, ['Foo Bar'], Wedge)
someString.toUpperCase() // "nope"
It's late on the topic, but it is in fact possible to properly subclass native classes in ES5 code. It looks like this:
//ES6-class version
class MyArray extends Array {
peekLast() {
return this[this.length - 1];
}
}
//ES5-class version
function MyArray() {
//No need to use new since that instance gets discarded
var retval = new Array();
retval.__proto__ = MyArray.prototype; //This is the key line
//Do any other initialization to retval here.
return retval;
}
MyArray.prototype = Object.create(Array.prototype, {
constructor: {
configurable: true,
writable: true,
value: MyArray
},
peekLast: {
configurable: true,
writable: true,
value: function peekLast() { return this[this.length - 1]; }
}
});
MyArray.__proto__ = Array;
I get that __proto__ was never in the ES standard before ES6, but it was a defacto web standard that all major browsers and engines supported. That's what makes it possible.
It accepts the constructor whose prototype should be used in Reflect.constructor 3rd parameter.
Try this code below and take a look to the prototype in array1Proto.
It set Array as constructor despite was Object/func1 at the beginning.
function func1(a, b, c) {
this.sum = a + b + c;
}
const args = [1, 2, 3];
const object1 = new func1(...args);
const object2 = Reflect.construct(func1, args);
const array1Proto = Reflect.construct(func1, args, Array);
console.log(object1)
console.log(object2)
console.log(array1Proto)
Every time I create some class, I need to do the same boring procedure:
class Something {
constructor(param1, param2, param3, ...) {
this.param1 = param1;
this.param2 = param2;
this.param3 = param3;
...
}
}
Is there any way to make it more elegant and shorter? I use Babel, so some ES7 experimental features are allowed. Maybe decorators can help?
You can use Object.assign:
class Something {
constructor(param1, param2, param3) {
Object.assign(this, {param1, param2, param3});
}
}
It's an ES2015 (aka ES6) feature that assigns the own enumerable properties of one or more source objects to a target object.
Granted, you have to write the arg names twice, but at least it's a lot shorter, and if you establish this as your idiom, it handles it well when you have arguments you do want on the instance and others you don't, e.g.:
class Something {
constructor(param1, param2, param3) {
Object.assign(this, {param1, param3});
// ...do something with param2, since we're not keeping it as a property...
}
}
Example: (live copy on Babel's REPL):
class Something {
constructor(param1, param2, param3) {
Object.assign(this, {param1, param2, param3});
}
}
let s = new Something('a', 'b', 'c');
console.log(s.param1);
console.log(s.param2);
console.log(s.param3);
Output:
a
b
c
Unfortunately, all you can do are simple things like Object.assign, but if you're trying to remove the redundancy of typing all the params twice (once in constructor signature and once in assignment) there isn't much you can do.
That said, you could do a hack like this. Though I'm not sure the effort and modicum of obfuscation that comes with it is worth it.
var dependencies = ['param1', 'param2', 'param3'];
class Something {
constructor(...params) {
params.forEach((param, index) => this[dependencies[index]] = param);
}
}
var something = new Something('foo', 'bar', 'baz');
// something.param1 === 'foo'
This way you're using a single array of argument names, then using that same array as a reference when creating the properties on your instance of Something. This pattern would work well in an Angular application where you're trying to preserve dependency names through minification by setting the $inject property.
Something.$inject = dependencies;
PS - Welcome to the redundant hell of classical languages that I thought I got away from when I became a JS developer :P
Honestly, you should probably just use a classic object literal unless you really need the formality of an actual class.
Edit: I suppose you could accept an object literal in your constructor if you want the ease of a literal and the formality of an actual class.
class Something {
constructor(params) {
Object.keys(params).forEach((name) => this[name] = params[name]);
}
}
var something = new Something({
param1: 'foo',
param2: 'bar',
param3: 'baz'
});
But now you've just turned a class into a dynamic class that can be instantiated with any properties, kinda like an object literal :P
Usually I want a class because I want to formalize the object and present a consistent and strictly testable API.
We could create a static method within each class that takes the arguments object and an array of names and returns an object that can be assigned to the new instance using Object.assign.
Check it out using the Babel REPL.
class Something {
static buildArgs (ctx, args, paramNames) {
let obj = {}
Array.from(args).forEach(function (arg, i) {
let name = paramNames[i] || i
obj[name] = args[i]
})
Object.assign(ctx, obj)
}
constructor () {
Something.buildArgs(this, arguments, [
'param1',
'param2'
]);
console.log(this)
}
}
new Something('one', 'two')
Admittedly the addition of a method buildArgs means that this solution is not shorter, however the body of the constructor is and we also have these advantages:
You are only writing the parameter names once.
You are protected against minification.
The code above accommodates extra arguments (i >= paramNames.length) however we could modify it if this behaviour is undesirable such that these are still parsed, but not assigned to the instance:
class Something {
static buildArgs (ctx, args, paramNames) {
let obj = {instance: {}, extra: {}}
Array.from(args).forEach(function (arg, i) {
let name = paramNames[i] || i
if (name) {
obj.instance[name] = args[i]
} else {
obj.extra[i] = args[i]
}
})
Object.assign(ctx, obj)
}
constructor () {
let args = Something.buildArgs(this, arguments, ['param1', 'param2']);
// Do stuff with `args.extra`
}
}
Or ignored altogether:
static buildArgs (args, paramNames) {
let obj = {}
Array.from(args).forEach(function (arg, i) {
let name = paramNames[i]
if (name) obj[name] = args[i]
})
return obj
}
I've come to a point where I need to have some sort of rudimentary multiple inheritance happening in JavaScript. (I'm not here to discuss whether this is a good idea or not, so please kindly keep those comments to yourself.)
I just want to know if anyone's attempted this with any (or not) success, and how they went about it.
To boil it down, what I really need is to be able to have an object capable of inheriting a property from more than one prototype chain (i.e. each prototype could have its own proper chain), but in a given order of precedence (it will search the chains in order for the first definition).
To demonstrate how this is theoretically possible, it could be achieved by attaching the secondary chain onto the end of the primary chain, but this would affect all instances of any of those previous prototypes and that's not what I want.
Thoughts?
Multiple inheritance can be achieved in ECMAScript 6 by using Proxy objects.
Implementation
function getDesc (obj, prop) {
var desc = Object.getOwnPropertyDescriptor(obj, prop);
return desc || (obj=Object.getPrototypeOf(obj) ? getDesc(obj, prop) : void 0);
}
function multiInherit (...protos) {
return Object.create(new Proxy(Object.create(null), {
has: (target, prop) => protos.some(obj => prop in obj),
get (target, prop, receiver) {
var obj = protos.find(obj => prop in obj);
return obj ? Reflect.get(obj, prop, receiver) : void 0;
},
set (target, prop, value, receiver) {
var obj = protos.find(obj => prop in obj);
return Reflect.set(obj || Object.create(null), prop, value, receiver);
},
*enumerate (target) { yield* this.ownKeys(target); },
ownKeys(target) {
var hash = Object.create(null);
for(var obj of protos) for(var p in obj) if(!hash[p]) hash[p] = true;
return Object.getOwnPropertyNames(hash);
},
getOwnPropertyDescriptor(target, prop) {
var obj = protos.find(obj => prop in obj);
var desc = obj ? getDesc(obj, prop) : void 0;
if(desc) desc.configurable = true;
return desc;
},
preventExtensions: (target) => false,
defineProperty: (target, prop, desc) => false,
}));
}
Explanation
A proxy object consists of a target object and some traps, which define custom behavior for fundamental operations.
When creating an object which inherits from another one, we use Object.create(obj). But in this case we want multiple inheritance, so instead of obj I use a proxy that will redirect fundamental operations to the appropriate object.
I use these traps:
The has trap is a trap for the in operator. I use some to check if at least one prototype contains the property.
The get trap is a trap for getting property values. I use find to find the first prototype which contains that property, and I return the value, or call the getter on the appropriate receiver. This is handled by Reflect.get. If no prototype contains the property, I return undefined.
The set trap is a trap for setting property values. I use find to find the first prototype which contains that property, and I call its setter on the appropriate receiver. If there is no setter or no prototype contains the property, the value is defined on the appropriate receiver. This is handled by Reflect.set.
The enumerate trap is a trap for for...in loops. I iterate the enumerable properties from the first prototype, then from the second, and so on. Once a property has been iterated, I store it in a hash table to avoid iterating it again.
Warning: This trap has been removed in ES7 draft and is deprecated in browsers.
The ownKeys trap is a trap for Object.getOwnPropertyNames(). Since ES7, for...in loops keep calling [[GetPrototypeOf]] and getting the own properties of each one. So in order to make it iterate the properties of all prototypes, I use this trap to make all enumerable inherited properties appear like own properties.
The getOwnPropertyDescriptor trap is a trap for Object.getOwnPropertyDescriptor(). Making all enumerable properties appear like own properties in the ownKeys trap is not enough, for...in loops will get the descriptor to check if they are enumerable. So I use find to find the first prototype which contains that property, and I iterate its prototypical chain until I find the property owner, and I return its descriptor. If no prototype contains the property, I return undefined. The descriptor is modified to make it configurable, otherwise we could break some proxy invariants.
The preventExtensions and defineProperty traps are only included to prevent these operations from modifying the proxy target. Otherwise we could end up breaking some proxy invariants.
There are more traps available, which I don't use
The getPrototypeOf trap could be added, but there is no proper way to return the multiple prototypes. This implies instanceof won't work neither. Therefore, I let it get the prototype of the target, which initially is null.
The setPrototypeOf trap could be added and accept an array of objects, which would replace the prototypes. This is left as an exercice for the reader. Here I just let it modify the prototype of the target, which is not much useful because no trap uses the target.
The deleteProperty trap is a trap for deleting own properties. The proxy represents the inheritance, so this wouldn't make much sense. I let it attempt the deletion on the target, which should have no property anyway.
The isExtensible trap is a trap for getting the extensibility. Not much useful, given that an invariant forces it to return the same extensibility as the target. So I just let it redirect the operation to the target, which will be extensible.
The apply and construct traps are traps for calling or instantiating. They are only useful when the target is a function or a constructor.
Example
// Creating objects
var o1, o2, o3,
obj = multiInherit(o1={a:1}, o2={b:2}, o3={a:3, b:3});
// Checking property existences
'a' in obj; // true (inherited from o1)
'b' in obj; // true (inherited from o2)
'c' in obj; // false (not found)
// Setting properties
obj.c = 3;
// Reading properties
obj.a; // 1 (inherited from o1)
obj.b; // 2 (inherited from o2)
obj.c; // 3 (own property)
obj.d; // undefined (not found)
// The inheritance is "live"
obj.a; // 1 (inherited from o1)
delete o1.a;
obj.a; // 3 (inherited from o3)
// Property enumeration
for(var p in obj) p; // "c", "b", "a"
Update (2019): The original post is getting pretty outdated. This article (now internet archive link, since domain went away) and its associated GitHub library are a good modern approach.
Original post:
Multiple inheritance [edit, not proper inheritance of type, but of properties; mixins] in Javascript is pretty straightforward if you use constructed prototypes rather than generic-object ones. Here are two parent classes to inherit from:
function FoodPrototype() {
this.eat = function () {
console.log("Eating", this.name);
};
}
function Food(name) {
this.name = name;
}
Food.prototype = new FoodPrototype();
function PlantPrototype() {
this.grow = function () {
console.log("Growing", this.name);
};
}
function Plant(name) {
this.name = name;
}
Plant.prototype = new PlantPrototype();
Note that I have used the same "name" member in each case, which could be a problem if the parents did not agree about how "name" should be handled. But they're compatible (redundant, really) in this case.
Now we just need a class that inherits from both. Inheritance is done by calling the constructor function (without using the new keyword) for the prototypes and the object constructors. First, the prototype has to inherit from the parent prototypes
function FoodPlantPrototype() {
FoodPrototype.call(this);
PlantPrototype.call(this);
// plus a function of its own
this.harvest = function () {
console.log("harvest at", this.maturity);
};
}
And the constructor has to inherit from the parent constructors:
function FoodPlant(name, maturity) {
Food.call(this, name);
Plant.call(this, name);
// plus a property of its own
this.maturity = maturity;
}
FoodPlant.prototype = new FoodPlantPrototype();
Now you can grow, eat, and harvest different instances:
var fp1 = new FoodPlant('Radish', 28);
var fp2 = new FoodPlant('Corn', 90);
fp1.grow();
fp2.grow();
fp1.harvest();
fp1.eat();
fp2.harvest();
fp2.eat();
This one uses Object.create to make a real prototype chain:
function makeChain(chains) {
var c = Object.prototype;
while(chains.length) {
c = Object.create(c);
$.extend(c, chains.pop()); // some function that does mixin
}
return c;
}
For example:
var obj = makeChain([{a:1}, {a: 2, b: 3}, {c: 4}]);
will return:
a: 1
a: 2
b: 3
c: 4
<Object.prototype stuff>
so that obj.a === 1, obj.b === 3, etc.
I like John Resig's implementation of a class structure: http://ejohn.org/blog/simple-javascript-inheritance/
This can be simply extended to something like:
Class.extend = function(prop /*, prop, prop, prop */) {
for( var i=1, l=arguments.length; i<l; i++ ){
prop = $.extend( prop, arguments[i] );
}
// same code
}
which will allow you to pass in multiple objects of which to inherit. You're going to lose instanceOf capability here, but that's a given if you want multiple inheritance.
my rather convoluted example of the above is available at https://github.com/cwolves/Fetch/blob/master/support/plugins/klass/klass.js
Note that there is some dead code in that file, but it allows multiple inheritance if you want to take a look.
If you want chained inheritance (NOT multiple inheritance, but for most people it's the same thing), it can be accomplished with Class like:
var newClass = Class.extend( cls1 ).extend( cls2 ).extend( cls3 )
which will preserve the original prototype chain, but you'll also have a lot of pointless code running.
I offer a function to allow classes to be defined with multiple inheritance. It allows for code like the following:
let human = new Running({ name: 'human', numLegs: 2 });
human.run();
let airplane = new Flying({ name: 'airplane', numWings: 2 });
airplane.fly();
let dragon = new RunningFlying({ name: 'dragon', numLegs: 4, numWings: 6 });
dragon.takeFlight();
to produce output like this:
human runs with 2 legs.
airplane flies away with 2 wings!
dragon runs with 4 legs.
dragon flies away with 6 wings!
Here are what the class definitions look like:
let Named = makeClass('Named', {}, () => ({
init: function({ name }) {
this.name = name;
}
}));
let Running = makeClass('Running', { Named }, protos => ({
init: function({ name, numLegs }) {
protos.Named.init.call(this, { name });
this.numLegs = numLegs;
},
run: function() {
console.log(`${this.name} runs with ${this.numLegs} legs.`);
}
}));
let Flying = makeClass('Flying', { Named }, protos => ({
init: function({ name, numWings }) {
protos.Named.init.call(this, { name });
this.numWings = numWings;
},
fly: function( ){
console.log(`${this.name} flies away with ${this.numWings} wings!`);
}
}));
let RunningFlying = makeClass('RunningFlying', { Running, Flying }, protos => ({
init: function({ name, numLegs, numWings }) {
protos.Running.init.call(this, { name, numLegs });
protos.Flying.init.call(this, { name, numWings });
},
takeFlight: function() {
this.run();
this.fly();
}
}));
We can see that each class definition using the makeClass function accepts an Object of parent-class names mapped to parent-classes. It also accepts a function that returns an Object containing properties for the class being defined. This function has a parameter protos, which contains enough information to access any property defined by any of the parent-classes.
The final piece required is the makeClass function itself, which does quite a bit of work. Here it is, along with the rest of the code. I've commented makeClass quite heavily:
let makeClass = (name, parents={}, propertiesFn=()=>({})) => {
// The constructor just curries to a Function named "init"
let Class = function(...args) { this.init(...args); };
// This allows instances to be named properly in the terminal
Object.defineProperty(Class, 'name', { value: name });
// Tracking parents of `Class` allows for inheritance queries later
Class.parents = parents;
// Initialize prototype
Class.prototype = Object.create(null);
// Collect all parent-class prototypes. `Object.getOwnPropertyNames`
// will get us the best results. Finally, we'll be able to reference
// a property like "usefulMethod" of Class "ParentClass3" with:
// `parProtos.ParentClass3.usefulMethod`
let parProtos = {};
for (let parName in parents) {
let proto = parents[parName].prototype;
parProtos[parName] = {};
for (let k of Object.getOwnPropertyNames(proto)) {
parProtos[parName][k] = proto[k];
}
}
// Resolve `properties` as the result of calling `propertiesFn`. Pass
// `parProtos`, so a child-class can access parent-class methods, and
// pass `Class` so methods of the child-class have a reference to it
let properties = propertiesFn(parProtos, Class);
properties.constructor = Class; // Ensure "constructor" prop exists
// If two parent-classes define a property under the same name, we
// have a "collision". In cases of collisions, the child-class *must*
// define a method (and within that method it can decide how to call
// the parent-class methods of the same name). For every named
// property of every parent-class, we'll track a `Set` containing all
// the methods that fall under that name. Any `Set` of size greater
// than one indicates a collision.
let propsByName = {}; // Will map property names to `Set`s
for (let parName in parProtos) {
for (let propName in parProtos[parName]) {
// Now track the property `parProtos[parName][propName]` under the
// label of `propName`
if (!propsByName.hasOwnProperty(propName))
propsByName[propName] = new Set();
propsByName[propName].add(parProtos[parName][propName]);
}
}
// For all methods defined by the child-class, create or replace the
// entry in `propsByName` with a Set containing a single item; the
// child-class' property at that property name (this also guarantees
// there is no collision at this property name). Note property names
// prefixed with "$" will be considered class properties (and the "$"
// will be removed).
for (let propName in properties) {
if (propName[0] === '$') {
// The "$" indicates a class property; attach to `Class`:
Class[propName.slice(1)] = properties[propName];
} else {
// No "$" indicates an instance property; attach to `propsByName`:
propsByName[propName] = new Set([ properties[propName] ]);
}
}
// Ensure that "init" is defined by a parent-class or by the child:
if (!propsByName.hasOwnProperty('init'))
throw Error(`Class "${name}" is missing an "init" method`);
// For each property name in `propsByName`, ensure that there is no
// collision at that property name, and if there isn't, attach it to
// the prototype! `Object.defineProperty` can ensure that prototype
// properties won't appear during iteration with `in` keyword:
for (let propName in propsByName) {
let propsAtName = propsByName[propName];
if (propsAtName.size > 1)
throw new Error(`Class "${name}" has conflict at "${propName}"`);
Object.defineProperty(Class.prototype, propName, {
enumerable: false,
writable: true,
value: propsAtName.values().next().value // Get 1st item in Set
});
}
return Class;
};
let Named = makeClass('Named', {}, () => ({
init: function({ name }) {
this.name = name;
}
}));
let Running = makeClass('Running', { Named }, protos => ({
init: function({ name, numLegs }) {
protos.Named.init.call(this, { name });
this.numLegs = numLegs;
},
run: function() {
console.log(`${this.name} runs with ${this.numLegs} legs.`);
}
}));
let Flying = makeClass('Flying', { Named }, protos => ({
init: function({ name, numWings }) {
protos.Named.init.call(this, { name });
this.numWings = numWings;
},
fly: function( ){
console.log(`${this.name} flies away with ${this.numWings} wings!`);
}
}));
let RunningFlying = makeClass('RunningFlying', { Running, Flying }, protos => ({
init: function({ name, numLegs, numWings }) {
protos.Running.init.call(this, { name, numLegs });
protos.Flying.init.call(this, { name, numWings });
},
takeFlight: function() {
this.run();
this.fly();
}
}));
let human = new Running({ name: 'human', numLegs: 2 });
human.run();
let airplane = new Flying({ name: 'airplane', numWings: 2 });
airplane.fly();
let dragon = new RunningFlying({ name: 'dragon', numLegs: 4, numWings: 6 });
dragon.takeFlight();
The makeClass function also supports class properties; these are defined by prefixing property names with the $ symbol (note that the final property name that results will have the $ removed). With this in mind, we could write a specialized Dragon class that models the "type" of the Dragon, where the list of available Dragon types is stored on the Class itself, as opposed to on the instances:
let Dragon = makeClass('Dragon', { RunningFlying }, protos => ({
$types: {
wyvern: 'wyvern',
drake: 'drake',
hydra: 'hydra'
},
init: function({ name, numLegs, numWings, type }) {
protos.RunningFlying.init.call(this, { name, numLegs, numWings });
this.type = type;
},
description: function() {
return `A ${this.type}-type dragon with ${this.numLegs} legs and ${this.numWings} wings`;
}
}));
let dragon1 = new Dragon({ name: 'dragon1', numLegs: 2, numWings: 4, type: Dragon.types.drake });
let dragon2 = new Dragon({ name: 'dragon2', numLegs: 4, numWings: 2, type: Dragon.types.hydra });
The Challenges of Multiple Inheritance
Anyone who followed the code for makeClass closely will note a rather significant undesirable phenomenon occurring silently when the above code runs: instantiating a RunningFlying will result in TWO calls to the Named constructor!
This is because the inheritance graph looks like this:
(^^ More Specialized ^^)
RunningFlying
/ \
/ \
Running Flying
\ /
\ /
Named
(vv More Abstract vv)
When there are multiple paths to the same parent-class in a sub-class' inheritance graph, instantiations of the sub-class will invoke that parent-class' constructor multiple times.
Combatting this is non-trivial. Let's look at some examples with simplified classnames. We'll consider class A, the most abstract parent-class, classes B and C, which both inherit from A, and class BC which inherits from B and C (and hence conceptually "double-inherits" from A):
let A = makeClass('A', {}, () => ({
init: function() {
console.log('Construct A');
}
}));
let B = makeClass('B', { A }, protos => ({
init: function() {
protos.A.init.call(this);
console.log('Construct B');
}
}));
let C = makeClass('C', { A }, protos => ({
init: function() {
protos.A.init.call(this);
console.log('Construct C');
}
}));
let BC = makeClass('BC', { B, C }, protos => ({
init: function() {
// Overall "Construct A" is logged twice:
protos.B.init.call(this); // -> console.log('Construct A'); console.log('Construct B');
protos.C.init.call(this); // -> console.log('Construct A'); console.log('Construct C');
console.log('Construct BC');
}
}));
If we want to prevent BC from double-invoking A.prototype.init we may need to abandon the style of directly calling inherited constructors. We will need some level of indirection to check whether duplicate calls are occurring, and short-circuit before they happen.
We could consider changing the parameters supplied to the properties function: alongside protos, an Object containing raw data describing inherited properties, we could also include a utility function for calling an instance method in such a way that parent methods are also called, but duplicate calls are detected and prevented. Let's take a look at where we establish the parameters for the propertiesFn Function:
let makeClass = (name, parents, propertiesFn) => {
/* ... a bunch of makeClass logic ... */
// Allows referencing inherited functions; e.g. `parProtos.ParentClass3.usefulMethod`
let parProtos = {};
/* ... collect all parent methods in `parProtos` ... */
// Utility functions for calling inherited methods:
let util = {};
util.invokeNoDuplicates = (instance, fnName, args, dups=new Set()) => {
// Invoke every parent method of name `fnName` first...
for (let parName of parProtos) {
if (parProtos[parName].hasOwnProperty(fnName)) {
// Our parent named `parName` defines the function named `fnName`
let fn = parProtos[parName][fnName];
// Check if this function has already been encountered.
// This solves our duplicate-invocation problem!!
if (dups.has(fn)) continue;
dups.add(fn);
// This is the first time this Function has been encountered.
// Call it on `instance`, with the desired args. Make sure we
// include `dups`, so that if the parent method invokes further
// inherited methods we don't lose track of what functions have
// have already been called.
fn.call(instance, ...args, dups);
}
}
};
// Now we can call `propertiesFn` with an additional `util` param:
// Resolve `properties` as the result of calling `propertiesFn`:
let properties = propertiesFn(parProtos, util, Class);
/* ... a bunch more makeClass logic ... */
};
The whole purpose of the above change to makeClass is so that we have an additional argument supplied to our propertiesFn when we invoke makeClass. We should also be aware that every function defined in any class may now receive a parameter after all its others, named dup, which is a Set that holds all functions that have already been called as a result of calling the inherited method:
let A = makeClass('A', {}, () => ({
init: function() {
console.log('Construct A');
}
}));
let B = makeClass('B', { A }, (protos, util) => ({
init: function(dups) {
util.invokeNoDuplicates(this, 'init', [ /* no args */ ], dups);
console.log('Construct B');
}
}));
let C = makeClass('C', { A }, (protos, util) => ({
init: function(dups) {
util.invokeNoDuplicates(this, 'init', [ /* no args */ ], dups);
console.log('Construct C');
}
}));
let BC = makeClass('BC', { B, C }, (protos, util) => ({
init: function(dups) {
util.invokeNoDuplicates(this, 'init', [ /* no args */ ], dups);
console.log('Construct BC');
}
}));
This new style actually succeeds in ensuring "Construct A" is only logged once when an instance of BC is initialized. But there are three downsides, the third of which is very critical:
This code has become less readable and maintainable. A lot of complexity hides behind the util.invokeNoDuplicates function, and thinking about how this style avoids multi-invocation is non-intuitive and headache inducing. We also have that pesky dups parameter, which really needs to be defined on every single function in the class. Ouch.
This code is slower - quite a bit more indirection and computation is required to achieve desirable results with multiple inheritance. Unfortunately this is likely to be the case with any solution to our multiple-invocation problem.
Most significantly, the structure of functions which rely on inheritance has become very rigid. If a sub-class NiftyClass overrides a function niftyFunction, and uses util.invokeNoDuplicates(this, 'niftyFunction', ...) to run it without duplicate-invocation, NiftyClass.prototype.niftyFunction will call the function named niftyFunction of every parent class that defines it, ignore any return values from those classes, and finally perform the specialized logic of NiftyClass.prototype.niftyFunction. This is the only possible structure. If NiftyClass inherits CoolClass and GoodClass, and both these parent-classes provide niftyFunction definitions of their own, NiftyClass.prototype.niftyFunction will never (without risking multiple-invocation) be able to:
A. Run the specialized logic of NiftyClass first, then the specialized logic of parent-classes
B. Run the specialized logic of NiftyClass at any point other than after all specialized parent logic has completed
C. Behave conditionally depending on the return values of its parent's specialized logic
D. Avoid running a particular parent's specialized niftyFunction altogether
Of course, we could solve each lettered problem above by defining specialized functions under util:
A. define util.invokeNoDuplicatesSubClassLogicFirst(instance, fnName, ...)
B. define util.invokeNoDuplicatesSubClassAfterParent(parentName, instance, fnName, ...) (Where parentName is the name of the parent whose specialized logic will be immediately followed by the child-classes' specialized logic)
C. define util.invokeNoDuplicatesCanShortCircuitOnParent(parentName, testFn, instance, fnName, ...) (In this case testFn would receive the result of the specialized logic for the parent named parentName, and would return a true/false value indicating whether the short-circuit should happen)
D. define util.invokeNoDuplicatesBlackListedParents(blackList, instance, fnName, ...) (In this case blackList would be an Array of parent names whose specialized logic should be skipped altogether)
These solutions are all available, but this is total mayhem! For every unique structure that an inherited function call can take, we would need a specialized method defined under util. What an absolute disaster.
With this in mind we can start to see the challenges of implementing good multiple inheritance. The full implementation of makeClass I provided in this answer does not even consider the multiple-invocation problem, or many other problems which arise regarding multiple inheritance.
This answer is getting very long. I hope the makeClass implementation I included is still useful, even if it isn't perfect. I also hope anyone interested in this topic has gained more context to keep in mind as they do further reading!
Don't get confused with JavaScript framework implementations of multiple inheritance.
All you need to do is use Object.create() to create a new object each time with the specified prototype object and properties, then be sure to change the Object.prototype.constructor each step of the way if you plan on instantiating B in the future.
To inherit instance properties thisA and thisB we use Function.prototype.call() at the end of each object function. This is optional if you only care about inheriting the prototype.
Run the following code somewhere and observe objC:
function A() {
this.thisA = 4; // objC will contain this property
}
A.prototype.a = 2; // objC will contain this property
B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;
function B() {
this.thisB = 55; // objC will contain this property
A.call(this);
}
B.prototype.b = 3; // objC will contain this property
C.prototype = Object.create(B.prototype);
C.prototype.constructor = C;
function C() {
this.thisC = 123; // objC will contain this property
B.call(this);
}
C.prototype.c = 2; // objC will contain this property
var objC = new C();
B inherits the prototype from A
C inherits the prototype from B
objC is an instance of C
This is a good explanation of the steps above:
OOP In JavaScript: What You NEED to Know
I’m in no way an expert on javascript OOP, but if I understand you correctly you want something like (pseudo-code):
Earth.shape = 'round';
Animal.shape = 'random';
Cat inherit from (Earth, Animal);
Cat.shape = 'random' or 'round' depending on inheritance order;
In that case, I’d try something like:
var Earth = function(){};
Earth.prototype.shape = 'round';
var Animal = function(){};
Animal.prototype.shape = 'random';
Animal.prototype.head = true;
var Cat = function(){};
MultiInherit(Cat, Earth, Animal);
console.log(new Cat().shape); // yields "round", since I reversed the inheritance order
console.log(new Cat().head); // true
function MultiInherit() {
var c = [].shift.call(arguments),
len = arguments.length
while(len--) {
$.extend(c.prototype, new arguments[len]());
}
}
It's possible to implement multiple inheritance in JavaScript, although very few libraries does it.
I could point Ring.js, the only example I know.
I was working on this a lot today and trying to achieve this myself in ES6. The way I did it was using Browserify, Babel and then I tested it with Wallaby and it seemed to work. My goal is to extend the current Array, include ES6, ES7 and add some additional custom features I need in the prototype for dealing with audio data.
Wallaby passes 4 of my tests. The example.js file can be pasted in the console and you can see that the 'includes' property is in the prototype of the class. I still want to test this more tomorrow.
Here's my method: (I will most likely refactor and repackage as a module after some sleep!)
var includes = require('./polyfills/includes');
var keys = Object.getOwnPropertyNames(includes.prototype);
keys.shift();
class ArrayIncludesPollyfills extends Array {}
function inherit (...keys) {
keys.map(function(key){
ArrayIncludesPollyfills.prototype[key]= includes.prototype[key];
});
}
inherit(keys);
module.exports = ArrayIncludesPollyfills
Github Repo:
https://github.com/danieldram/array-includes-polyfill
I think it is ridiculously simple. The issue here is that the child class will only refer to instanceof for the first class you call
https://jsfiddle.net/1033xzyt/19/
function Foo() {
this.bar = 'bar';
return this;
}
Foo.prototype.test = function(){return 1;}
function Bar() {
this.bro = 'bro';
return this;
}
Bar.prototype.test2 = function(){return 2;}
function Cool() {
Foo.call(this);
Bar.call(this);
return this;
}
var combine = Object.create(Foo.prototype);
$.extend(combine, Object.create(Bar.prototype));
Cool.prototype = Object.create(combine);
Cool.prototype.constructor = Cool;
var cool = new Cool();
console.log(cool.test()); // 1
console.log(cool.test2()); //2
console.log(cool.bro) //bro
console.log(cool.bar) //bar
console.log(cool instanceof Foo); //true
console.log(cool instanceof Bar); //false
Check the code below which IS showing support for multiple inheritance. Done by using PROTOTYPAL INHERITANCE
function A(name) {
this.name = name;
}
A.prototype.setName = function (name) {
this.name = name;
}
function B(age) {
this.age = age;
}
B.prototype.setAge = function (age) {
this.age = age;
}
function AB(name, age) {
A.prototype.setName.call(this, name);
B.prototype.setAge.call(this, age);
}
AB.prototype = Object.assign({}, Object.create(A.prototype), Object.create(B.prototype));
AB.prototype.toString = function () {
return `Name: ${this.name} has age: ${this.age}`
}
const a = new A("shivang");
const b = new B(32);
console.log(a.name);
console.log(b.age);
const ab = new AB("indu", 27);
console.log(ab.toString());
Take a look of the package IeUnit.
The concept assimilation implemented in IeUnit seems to offers what you are looking for in a quite dynamical way.
Here is an example of prototype chaining using constructor functions:
function Lifeform () { // 1st Constructor function
this.isLifeform = true;
}
function Animal () { // 2nd Constructor function
this.isAnimal = true;
}
Animal.prototype = new Lifeform(); // Animal is a lifeform
function Mammal () { // 3rd Constructor function
this.isMammal = true;
}
Mammal.prototype = new Animal(); // Mammal is an animal
function Cat (species) { // 4th Constructor function
this.isCat = true;
this.species = species
}
Cat.prototype = new Mammal(); // Cat is a mammal
This concept uses Yehuda Katz's definition of a "class" for JavaScript:
...a JavaScript "class" is just a Function object that serves as a constructor plus an attached prototype object. (Source: Guru Katz)
Unlike the Object.create approach, when the classes are built in this way and we want to create instances of a "class", we don't need to know what each "class" is inheriting from. We just use new.
// Make an instance object of the Cat "Class"
var tiger = new Cat("tiger");
console.log(tiger.isCat, tiger.isMammal, tiger.isAnimal, tiger.isLifeform);
// Outputs: true true true true
The order of precendence should make sense. First it looks in the instance object, then it's prototype, then the next prototype, etc.
// Let's say we have another instance, a special alien cat
var alienCat = new Cat("alien");
// We can define a property for the instance object and that will take
// precendence over the value in the Mammal class (down the chain)
alienCat.isMammal = false;
// OR maybe all cats are mutated to be non-mammals
Cat.prototype.isMammal = false;
console.log(alienCat);
We can also modify the prototypes which will effect all objects built on the class.
// All cats are mutated to be non-mammals
Cat.prototype.isMammal = false;
console.log(tiger, alienCat);
I originally wrote some of this up with this answer.
A latecomer in the scene is SimpleDeclare. However, when dealing with multiple inheritance, you will still end up with copies of the original constructors. That's a necessity in Javascript...
Merc.
I would use ds.oop. Its similar to prototype.js and others. makes multiple inheritance very easy and its minimalist. (only 2 or 3 kb) Also supports some other neat features like interfaces and dependency injection
/*** multiple inheritance example ***********************************/
var Runner = ds.class({
run: function() { console.log('I am running...'); }
});
var Walker = ds.class({
walk: function() { console.log('I am walking...'); }
});
var Person = ds.class({
inherits: [Runner, Walker],
eat: function() { console.log('I am eating...'); }
});
var person = new Person();
person.run();
person.walk();
person.eat();
How about this, it implements multiple inheritance in JavaScript:
class Car {
constructor(brand) {
this.carname = brand;
}
show() {
return 'I have a ' + this.carname;
}
}
class Asset {
constructor(price) {
this.price = price;
}
show() {
return 'its estimated price is ' + this.price;
}
}
class Model_i1 { // extends Car and Asset (just a comment for ourselves)
//
constructor(brand, price, usefulness) {
specialize_with(this, new Car(brand));
specialize_with(this, new Asset(price));
this.usefulness = usefulness;
}
show() {
return Car.prototype.show.call(this) + ", " + Asset.prototype.show.call(this) + ", Model_i1";
}
}
mycar = new Model_i1("Ford Mustang", "$100K", 16);
document.getElementById("demo").innerHTML = mycar.show();
And here's the code for specialize_with() utility function:
function specialize_with(o, S) { for (var prop in S) { o[prop] = S[prop]; } }
This is real code that runs. You can copy-paste it in html file, and try it yourself. It does work.
That's the effort to implement MI in JavaScript. Not much of code, more of a know-how.
Please feel free to look at my complete article on this, https://github.com/latitov/OOP_MI_Ct_oPlus_in_JS
I just used to assign what classes I need in properties of others, and add a proxy to auto-point to them i like:
class A {
constructor()
{
this.test = "a test";
}
method()
{
console.log("in the method");
}
}
class B {
constructor()
{
this.extends = [new A()];
return new Proxy(this, {
get: function(obj, prop) {
if(prop in obj)
return obj[prop];
let response = obj.extends.find(function (extended) {
if(prop in extended)
return extended[prop];
});
return response ? response[prop] : Reflect.get(...arguments);
},
})
}
}
let b = new B();
b.test ;// "a test";
b.method(); // in the method
I am exporting the following ES6 class from one module:
export class Thingy {
hello() {
console.log("A");
}
world() {
console.log("B");
}
}
And importing it from another module:
import {Thingy} from "thingy";
if (isClass(Thingy)) {
// Do something...
}
How can I check whether a variable is a class? Not a class instance, but a class declaration?
In other words, how would I implement the isClass function in the example above?
If you want to ensure that the value is not only a function, but really a constructor function for a class, you can convert the function to a string and inspect its representation. The spec dictates the string representation of a class constructor.
function isClass(v) {
return typeof v === 'function' && /^\s*class\s+/.test(v.toString());
}
Another solution would be to try to call the value as a normal function. Class constructors are not callable as normal functions, but error messages probably vary between browsers:
function isClass(v) {
if (typeof v !== 'function') {
return false;
}
try {
v();
return false;
} catch(error) {
if (/^Class constructor/.test(error.message)) {
return true;
}
return false;
}
}
The disadvantage is that invoking the function can have all kinds of unknown side effects...
I'll make it clear up front here, any arbitrary function can be a constructor. If you are distinguishing between "class" and "function", you are making poor API design choices. If you assume something must be a class for instance, no-one using Babel or Typescript will be be detected as a class because their code will have been converted to a function instead. It means you are mandating that anyone using your codebase must be running in an ES6 environment in general, so your code will be unusable on older environments.
Your options here are limited to implementation-defined behavior. In ES6, once code is parsed and the syntax is processed, there isn't much class-specific behavior left. All you have is a constructor function. Your best choice is to do
if (typeof Thingy === 'function'){
// It's a function, so it definitely can't be an instance.
} else {
// It could be anything other than a constructor
}
and if someone needs to do a non-constructor function, expose a separate API for that.
Obviously that is not the answer you are looking for, but it's important to make that clear.
As the other answer here mentions, you do have an option because .toString() on functions is required to return a class declaration, e.g.
class Foo {}
Foo.toString() === "class Foo {}" // true
The key thing, however, is that that only applies if it can. It is 100% spec compliant for an implementation to have
class Foo{}
Foo.toString() === "throw SyntaxError();"
No browsers currently do that, but there are several embedded systems that focus on JS programming for instance, and to preserve memory for your program itself, they discard the source code once it has been parsed, meaning they will have no source code to return from .toString() and that is allowed.
Similarly, by using .toString() you are making assumptions about both future-proofing, and general API design. Say you do
const isClass = fn => /^\s*class/.test(fn.toString());
because this relies on string representations, it could easily break.
Take decorators for example:
#decorator class Foo {}
Foo.toString() == ???
Does the .toString() of this include the decorator? What if the decorator itself returns a function instead of a class?
Checking the prototype and its writability should allow to determine the type of function without stringifying, calling or instantiating the input.
/**
* determine if a variable is a class definition or function (and what kind)
* #revised
*/
function isFunction(x) {
return typeof x === 'function'
? x.prototype
? Object.getOwnPropertyDescriptor(x, 'prototype').writable
? 'function'
: 'class'
: x.constructor.name === 'AsyncFunction'
? 'async'
: 'arrow'
: '';
}
console.log({
string: isFunction('foo'), // => ''
null: isFunction(null), // => ''
class: isFunction(class C {}), // => 'class'
function: isFunction(function f() {}), // => 'function'
arrow: isFunction(() => {}), // => 'arrow'
async: isFunction(async function () {}) // => 'async'
});
There are subtle differences between a function and a class, and we can take this advantage to distinguish between them, the following is my implementation:
// is "class" or "function"?
function isClass(obj) {
// if not a function, return false.
if (typeof obj !== 'function') return false;
// ⭐ is a function, has a `prototype`, and can't be deleted!
// ⭐ although a function's prototype is writable (can be reassigned),
// it's not configurable (can't update property flags), so it
// will remain writable.
//
// ⭐ a class's prototype is non-writable.
//
// Table: property flags of function/class prototype
// ---------------------------------
// prototype write enum config
// ---------------------------------
// function v . .
// class . . .
// ---------------------------------
const descriptor = Object.getOwnPropertyDescriptor(obj, 'prototype');
// ❗functions like `Promise.resolve` do have NO `prototype`.
// (I have no idea why this is happening, sorry.)
if (!descriptor) return false;
return !descriptor.writable;
}
Here are some test cases:
class A { }
function F(name) { this.name = name; }
isClass(F), // ❌ false
isClass(3), // ❌ false
isClass(Promise.resolve), // ❌ false
isClass(A), // ✅ true
isClass(Object), // ✅ true
What about:
function isClass(v) {
return typeof v === 'function' && v.prototype.constructor === v;
}
This solution fixes two false positives with Felix's answer:
It works with anonymous classes that don't have space before the class body:
isClass(class{}) // true
It works with native classes:
isClass(Promise) // true
isClass(Proxy) // true
function isClass(value) {
return typeof value === 'function' && (
/^\s*class[^\w]+/.test(value.toString()) ||
// 1. native classes don't have `class` in their name
// 2. However, they are globals and start with a capital letter.
(globalThis[value.name] === value && /^[A-Z]/.test(value.name))
);
}
const A = class{};
class B {}
function f() {}
console.log(isClass(A)); // true
console.log(isClass(B)); // true
console.log(isClass(Promise)); // true
console.log(isClass(Promise.resolve)); // false
console.log(isClass(f)); // false
Shortcomings
Sadly, it still won't work with node built-in (and probably many other platform-specific) classes, e.g.:
const EventEmitter = require('events');
console.log(isClass(EventEmitter)); // `false`, but should be `true` :(
Well going through some of the answers and thinks to #Joe Hildebrand for highlighting edge case thus following solution is updated to reflect most tried edge cases. Open to more identification where edge cases may rest.
Key insights: although we are getting into classes but just like pointers and references debate in JS does not confirm to all the qualities of other languages - JS as such does not have classes as we have in other language constructs.
some debate it is sugar coat syntax of function and some argue other wise. I believe classes are still a function underneath but not so much so as sugar coated but more so as something that could be put on steroids. Classes will do what functions can not do or didn't bother upgrading them to do.
So dealing with classes as function for the time being open up another Pandora box. Everything in JS is object and everything JS does not understand but is willing to go along with the developer is an object e.g.
Booleans can be objects (if defined with the new keyword)
Numbers can be objects (if defined with the new keyword)
Strings can be objects (if defined with the new keyword)
Dates are always objects
Maths are always objects
Regular expressions are always objects
Arrays are always objects
Functions are always objects
Objects are always objects
Then what the heck are Classes?
Important Classes are a template for creating objects they are not object per say them self at this point.
They become object when you create an instance of the class somewhere, that instance is considered an object.
So with out freaking out we need to sift out
which type of object we are dealing with
Then we need to sift out its properties.
functions are always objects they will always have prototype and arguments property.
arrow function are actually sugar coat of old school function and have no concept of this or more the simple return context so no prototype or arguments even if you attempt at defining them.
classes are kind of blue print of possible function dont have arguments property but have prototypes. these prototypes become after the fact object upon instance.
So i have attempted to capture and log each iteration we check and result of course.
Hope this helps
'use strict';
var isclass,AA,AAA,BB,BBB,BBBB,DD,DDD,E,F;
isclass=function(a) {
if(/null|undefined/.test(a)) return false;
let types = typeof a;
let props = Object.getOwnPropertyNames(a);
console.log(`type: ${types} props: ${props}`);
return ((!props.includes('arguments') && props.includes('prototype')));}
class A{};
class B{constructor(brand) {
this.carname = brand;}};
function C(){};
function D(a){
this.a = a;};
AA = A;
AAA = new A;
BB = B;
BBB = new B;
BBBB = new B('cheking');
DD = D;
DDD = new D('cheking');
E= (a) => a;
F=class {};
console.log('and A is class: '+isclass(A)+'\n'+'-------');
console.log('and AA as ref to A is class: '+isclass(AA)+'\n'+'-------');
console.log('and AAA instance of is class: '+isclass(AAA)+'\n'+'-------');
console.log('and B with implicit constructor is class: '+isclass(B)+'\n'+'-------');
console.log('and BB as ref to B is class: '+isclass(BB)+'\n'+'-------');
console.log('and BBB as instance of B is class: '+isclass(BBB)+'\n'+'-------');
console.log('and BBBB as instance of B is class: '+isclass(BBBB)+'\n'+'-------');
console.log('and C as function is class: '+isclass(C)+'\n'+'-------');
console.log('and D as function method is class: '+isclass(D)+'\n'+'-------');
console.log('and DD as ref to D is class: '+isclass(DD)+'\n'+'-------');
console.log('and DDD as instance of D is class: '+isclass(DDD)+'\n'+'-------');
console.log('and E as arrow function is class: '+isclass(E)+'\n'+'-------');
console.log('and F as variable class is class: '+isclass(F)+'\n'+'-------');
console.log('and isclass as variable function is class: '+isclass(isclass)+'\n'+'-------');
console.log('and 4 as number is class: '+isclass(4)+'\n'+'-------');
console.log('and 4 as string is class: '+isclass('4')+'\n'+'-------');
console.log('and DOMI\'s string is class: '+isclass('class Im a class. Do you believe me?')+'\n'+'-------');
shorter cleaner function covering strict mode, es6 modules, null, undefined and what ever not property manipulation on object.
What I have found so far is since from above discussion classes are blue print not as such object on their own before the fact of instance. Thus running toString function would almost always produce class {} output not [object object] after the instance and so on. Once we know what is consistent then simply run regex test to see if result starts with word class.
"use strict"
let isclass = a =>{
return (!!a && /^class.*{}/.test(a.toString()))
}
class A {}
class HOO {}
let B=A;
let C=new A;
Object.defineProperty(HOO, 'arguments', {
value: 42,
writable: false
});
console.log(isclass(A));
console.log(isclass(B));
console.log(isclass(C));
console.log(isclass(HOO));
console.log(isclass());
console.log(isclass(null));
console.log(HOO.toString());
//proxiy discussion
console.log(Proxy.toString());
//HOO was class and returned true but if we proxify it has been converted to an object
HOO = new Proxy(HOO, {});
console.log(isclass(HOO));
console.log(HOO.toString());
console.log(isclass('class Im a class. Do you believe me?'));
Lead from DOMI's disucssion
class A {
static hello (){console.log('hello')}
hello () {console.log('hello there')}
}
A.hello();
B = new A;
B.hello();
console.log('it gets even more funnier it is properties and prototype mashing');
class C {
constructor() {
this.hello = C.hello;
}
static hello (){console.log('hello')}
}
C.say = ()=>{console.log('I said something')}
C.prototype.shout = ()=>{console.log('I am shouting')}
C.hello();
D = new C;
D.hello();
D.say();//would throw error as it is not prototype and is not passed with instance
C.say();//would not throw error since its property not prototype
C.shout();//would throw error as it is prototype and is passed with instance but is completly aloof from property of static
D.shout();//would not throw error
console.log('its a whole new ball game ctaching these but gassumption is class will always have protoype to be termed as class');
I'm shocked lodash didnt have the answer. Check this out - Just like Domi I just came up with a solution to fix the glitches. I know its a lot of code but its the most working yet understandable thing I could produce by now. Maybe someone can optimize it by a regex-approach:
function isClass(asset) {
const string_match = "function";
const is_fn = !!(typeof asset === string_match);
if(!is_fn){
return false;
}else{
const has_constructor = is_fn && !!(asset.prototype && asset.prototype.constructor && asset.prototype.constructor === asset);
const code = !asset.toString ? "" : asset.toString();
if(has_constructor && !code.startsWith(string_match)){
return true;
}
if(has_constructor && code.startsWith(string_match+"(")){
return false;
}
const [keyword, name] = code.split(" ");
if(name && name[0] && name[0].toLowerCase && name[0].toLowerCase() != name[0]){
return true;
}else{
return false;
}
}
}
Just test it with:
console.log({
_s:isClass(String),
_a:isClass(Array),
_o:isClass(Object),
_c:isClass(class{}),
fn:isClass(function(){}),
fnn:isClass(function namedFunction(){}),
fnc:isClass(()=>{}),
n:isClass(null),
o:isClass({}),
a:isClass([]),
s:isClass(""),
n:isClass(2),
u:isClass(undefined),
b:isClass(false),
pr:isClass(Promise),
px:isClass(Proxy)
});
Just make sure all classes have a frist uppercased letter.
Maybe this can help
let is_class = (obj) => {
try {
new obj();
return true;
} catch(e) {
return false;
};
};