Develop Reference

Extends ES5 like in ES6 [duplicate]

I realize that, strictly speaking, this is not subclassing the array type, but will this work in the way one might expect, or am I still going to run into some issues with .length and the like? Are there any drawbacks that I would not have if normal subclassing were an option?
function Vector()
{
var vector = [];
vector.sum = function()
{
sum = 0.0;
for(i = 0; i < this.length; i++)
{
sum += this[i];
}
return sum;
}
return vector;
}
v = Vector();
v.push(1); v.push(2);
console.log(v.sum());

I'd wrap an array inside a proper vector type like this:
window.Vector = function Vector() {
this.data = [];
}
Vector.prototype.push = function push() {
Array.prototype.push.apply(this.data, arguments);
}
Vector.prototype.sum = function sum() {
for(var i = 0, s=0.0, len=this.data.length; i < len; s += this.data[i++]);
return s;
}
var vector1 = new Vector();
vector1.push(1); vector1.push(2);
console.log(vector1.sum());
Alternatively you can build new prototype functions on arrays and then just use normal arrays.
If you are consistent with naming the arrays so they all start with a lowercase v for example or something similar that clearly mark them aw vector and not normal arrays, and you do the same on the vector specific prototype functions, then it should be fairly easy to keep track of.
Array.prototype.vSum = function vSum() {
for(var i = 0, s=0.0, len=this.length; i < len; s += this[i++]);
return s;
}
var vector1 = [];
vector1.push(1); vector1.push(2);
console.log(vector1.vSum());

EDIT -- I originally wrote that you could subclass an Array just like any other object, which was wrong. Learn something new every day. Here is a good discussion
http://perfectionkills.com/how-ecmascript-5-still-does-not-allow-to-subclass-an-array/
In this case, would composition work better? i.e. just create a Vector object, and have it backed by an array. This seems to be the path you are on, you just need to add the push and any other methods to the prototype.

Nowadays you could use subclassing with ES6 classes:
class Vector extends Array {
sum(){
return this.reduce((total, value) => total + value)
}
}
let v2 = new Vector();
v2.push(1);
v2.push(2);
console.log(v2.sum());
console.log(v2.length);
v2.length = 0;
console.log(v2.length);
console.log(v2);

Just another example of the wrapper. Having some fun with .bind.
var _Array = function _Array() {
if ( !( this instanceof _Array ) ) {
return new _Array();
};
};
_Array.prototype.push = function() {
var apContextBound = Array.prototype.push,
pushItAgainst = Function.prototype.apply.bind( apContextBound );
pushItAgainst( this, arguments );
};
_Array.prototype.pushPushItRealGood = function() {
var apContextBound = Array.prototype.push,
pushItAgainst = Function.prototype.apply.bind( apContextBound );
pushItAgainst( this, arguments );
};
_Array.prototype.typeof = (function() { return ( Object.prototype.toString.call( [] ) ); }());

#hvgotcodes answer has an awesome link. I just wanted to summerize the conclusion here.
Wrappers. Prototype chain injection
This seems to be the best method to extend array from the article.
wrappers can be used ... in which object’s prototype chain is augmented, rather than object itself.
function SubArray() {
var arr = [ ];
arr.push.apply(arr, arguments);
arr.__proto__ = SubArray.prototype;
return arr;
}
SubArray.prototype = new Array;
// Add custom functions here to SubArray.prototype.
SubArray.prototype.last = function() {
return this[this.length - 1];
};
var sub = new SubArray(1, 2, 3);
sub instanceof SubArray; // true
sub instanceof Array; // true
Unfortunally for me, this method uses arr.__proto__, unsupported in IE 8-, a browser I have to support.
Wrappers. Direct property injection.
This method is a little slower than the above, but works in IE 8-.
Wrapper approach avoids setting up inheritance or emulating length/indices relation. Instead, a factory-like function can create a plain Array object, and then augment it directly with any custom methods. Since returned object is an Array one, it maintains proper length/indices relation, as well as [[Class]] of “Array”. It also inherits from Array.prototype, naturally.
function makeSubArray() {
var arr = [ ];
arr.push.apply(arr, arguments);
// Add custom functions here to arr.
arr.last = function() {
return this[this.length - 1];
};
return arr;
}
var sub = makeSubArray(1, 2, 3);
sub instanceof Array; // true
sub.length; // 3
sub.last(); // 3

There is a way that looks and feels like prototypical inheritance, but it's different in only one way.
First lets take a look at one of the standard ways of implementing prototypical inheritance in javascript:
var MyClass = function(bar){
this.foo = bar;
};
MyClass.prototype.awesomeMethod = function(){
alert("I'm awesome")
};
// extends MyClass
var MySubClass = function(bar){
MyClass.call(this, bar); // <- call super constructor
}
// which happens here
MySubClass.prototype = Object.create(MyClass.prototype); // prototype object with MyClass as its prototype
// allows us to still walk up the prototype chain as expected
Object.defineProperty(MySubClass.prototype, "constructor", {
enumerable: false, // this is merely a preference, but worth considering, it won't affect the inheritance aspect
value: MySubClass
});
// place extended/overridden methods here
MySubClass.prototype.superAwesomeMethod = function(){
alert("I'm super awesome!");
};
var testInstance = new MySubClass("hello");
alert(testInstance instanceof MyClass); // true
alert(testInstance instanceof MySubClass); // true
The next example just wraps up the above structure to keep everything clean. And there is a slight tweak that seems at first glance to perform a miracle. However, all that is really happening is each instance of the subclass is using not the Array prototype as a template for construction, but rather an instance of an Array - so the prototype of the subclass comes hooked onto the end of a fully loaded object which passes the ducktype of an array - which it then copies. If you still see something strange here and it bothers you, I'm not sure that I can explain it better - so maybe how it works is a good topic for another question. :)
var extend = function(child, parent, optionalArgs){ //...
if(parent.toString() === "function "+parent.name+"() { [native code] }"){
optionalArgs = [parent].concat(Array.prototype.slice.call(arguments, 2));
child.prototype = Object.create(new parent.bind.apply(null, optionalArgs));
}else{
child.prototype = Object.create(parent.prototype);
}
Object.defineProperties(child.prototype, {
constructor: {enumerable: false, value: child},
_super_: {enumerable: false, value: parent} // merely for convenience (for future use), its not used here because our prototype is already constructed!
});
};
var Vector = (function(){
// we can extend Vector prototype here because functions are hoisted
// so it keeps the extend declaration close to the class declaration
// where we would expect to see it
extend(Vector, Array);
function Vector(){
// from here on out we are an instance of Array as well as an instance of Vector
// not needed here
// this._super_.call(this, arguments); // applies parent constructor (in this case Array, but we already did it during prototyping, so use this when extending your own classes)
// construct a Vector as needed from arguments
this.push.apply(this, arguments);
}
// just in case the prototype description warrants a closure
(function(){
var _Vector = this;
_Vector.sum = function sum(){
var i=0, s=0.0, l=this.length;
while(i<l){
s = s + this[i++];
}
return s;
};
}).call(Vector.prototype);
return Vector;
})();
var a = new Vector(1,2,3); // 1,2,3
var b = new Vector(4,5,6,7); // 4,5,6,7
alert(a instanceof Array && a instanceof Vector); // true
alert(a === b); // false
alert(a.length); // 3
alert(b.length); // 4
alert(a.sum()); // 6
alert(b.sum()); // 22
Soon we'll have class and the ability to extend native classes in ES6 but that may be a another year yet. In the mean time I hope this helps someone.

function SubArray(arrayToInitWith){
Array.call(this);
var subArrayInstance = this;
subArrayInstance.length = arrayToInitWith.length;
arrayToInitWith.forEach(function(e, i){
subArrayInstance[i] = e;
});
}
SubArray.prototype = Object.create(Array.prototype);
SubArray.prototype.specialMethod = function(){alert("baz");};
var subclassedArray = new SubArray(["Some", "old", "values"]);

Array with method that sets itself

I'm trying to make an array object that can set itself.
Something like this...
array = new Array(10)
array.someMethod = function () {
this = new Array(20)
}
Except this doesn't work because this can't be set like that.
More or less I want an object that I use array[index] on.

Seems to me this could be accomplished by using an object and storing the array in one of the object's properties. I'm learning about the class syntax, so I did it that way, but I think the same approach would work with regular objects.
class MyArray {
constructor(num) {
console.log(`building MyArray with ${num} objects`);
this.innerArray = new Array(num);
}
update(num) {
console.log(`updating MyArray to have ${num} objects`);
this.innerArray = new Array(num);
}
get() { return this.innerArray }
}
let foo = new MyArray(10);
foo.get()[0] = "bar";
console.log(foo.get());
foo.update(20);
foo.get()[0] = "baz";
console.log(foo.get());

the way you asked your question is not quite right you should explain better.
not sure this is what you want.
you can't change the this in javascript.
but function in prototype will be called with the object as their context. so Array.prototype.length gives the length in arr.length by having arr as it's context. Array.prototype.splice
so
Array.prototype.testA = function() {
// this.splice(0,this.length);
var a = [3,5,7];
// new Array(23).fill().map( (a,_)=>_)
this.splice(0,this.length,...a);
}
Array.prototype.testAA = function() {
var a = [3,5,7];
this.splice(this.length,0,...a);
}
var a = [1];
a.testAA();
console.log(a);
a.testA();
console.log(a);

How does one manipulate a variable in a prototype?

I'm new to Javascript and was wondering how a public variable in a prototype can be modified.
function Thing (val)
{
this.x = val;
this.addToX = function (valIn)
{
this.x += valIn;
};
}
function ChildThing ()
{
this.y = 55;
}
ChildThing.prototype = new Thing(10);
var frank = new ChildThing();
console.log("out1: " + frank.x);
frank.addToX(10);
console.log("out2: " + frank.x);
This code takes the value in the prototype x which is 10 and adds 10 to it in the addToX function. The new x value is stored in the top level object rather than replacing the current x value in the prototype.
Is there a way to overwrite the existing x in the prototype or am I using Javascript wrong?

That depends. What would be the point of altering x on the prototype? Generally you don't want to chang shared properties. But I imagine that there could be a use case (generating new id?).
As for the question: you can simply do:
this.addToX = function(valIn) {
ChildThing.prototype.x += valIn;
};
Again I do not advice doing it.
EDIT You can make it without referencing the child by defining the prototype before setting it as a prototype, i.e.
var my_proto = new Thing(10);
ChildThing.prototype = my_proto;
and then
this.addToX = function(valIn) {
my_proto.x += valIn;
};
Or you can even play with the singleton pattern.

What you seem to be wanting is very similar to what static members are in classical languages. It's very misleading to call a method on an object instance and have that method modify the state of other objects outside of it's scope. Therefore, I believe you shounldn't be relying on prototypes at all for this behavior. Here's what you could do to mimic static members.
function SomeClass() {}
SomeClass.staticMember = 'initial value';
SomeClass.changeStaticMember = function (val) { this.staticMember = val; };
SomeClass.changeStaticMember('another value');
I believe the code above is less cryptic and better at communicating the behavior. However if you still want to share mutable values across instances through the prototype you could simply avoid writing the property directly as a primitive value, but rather wrap it within a mutable shared object like below. Note that the whole inheritance hierarchy will share the same x value.
//Mutable class to encapsulate the value of X
function XValue(val) {
this.value = val;
}
XValue.prototype = {
constructor: XValue,
valueOf: function () { return this.value; },
set: function (val) { this.value = val; },
add: function (val) { this.value += val; }
};
function Thing(x) {
this.x = x;
}
Thing.prototype = {
constructor: Thing,
_x: new XValue(), //shared mutable object representing the value of X
get x() { return this._x.valueOf(); },
set x(val) { this._x.set(val); },
addToX: function (val) { this._x.add(val); }
};
function ChildThing() {
Thing.call(this, 10); //call parent constructor
}
ChildThing.prototype = Object.create(Thing.prototype);
//helper for snippet
function log(text) {
var span = document.createElement('span');
span.innerHTML = text;
document.body.appendChild(span);
document.body.appendChild(document.createElement('br'));
}
var ct = new ChildThing();
ct.addToX(10);
log('ct.x → ' + ct.x);
log('Thing.prototype.x → ' + Thing.prototype.x);

Simulating Array Functionality

Good day! I have this code:
function MyArray() {}
MyArray.prototype.length = 0;
(function() {
var methods = ['push', 'pop', 'shift', 'unshift',
'slice', 'splice', 'join'];
for (var i = 0; i < methods.length; i++) (function(name) {
MyArray.prototype[ name ] = function() {
return Array.prototype[ name ].apply(this, arguments);
};
})(methods[i]);
})();
I need explanation. I understood that "methods" is array of real methods, which just "exported" to our new class. But, what is this: MyArray.prototype.length = 0; ? Author create new prototype property and assign it zero. And later use this new property!
var mine = new MyArray();
mine.push(1, 2, 3);
assert(mine.length == 3 ...
.....
How it is work? "length" have not instantiation in code above!

Its getting initialized at zero so that if you never call any of its functions, it will return zero (like a real array) and not undefined. Also it needs to start at zero so that the methods update it correctly. in your example, length its 3 because the push method did so.

You can't really subclass Array http://perfectionkills.com/how-ecmascript-5-still-does-not-allow-to-subclass-an-array/
So if you create an instance of MyArray you can't do: MyArr[0]=...
You can wrap an array inside MyArray and take advantage of the Array functions:
var MyArray=function() {
this.arr=[];
[].push.apply(this.arr,arguments);
//following doesn't work in older browsers
Object.defineProperty(this,"length",{
get:function(){return this.arr.length;},
enumerable:true,
configurable:true
});
}
MyArray.prototype.valueOf=function(){return this.arr;};
(function() {
var methods = ['push', 'pop', 'shift', 'unshift',
'slice', 'splice', 'join'],i=methods.length
while(--i!==-1){
;(function(name) {
MyArray.prototype[ name ] = function() {
console.log(arguments);
return Array.prototype[ name ].apply(this.arr, arguments);
};
}(methods[i]));
}
}());
var mArr1=new MyArray(1,2,3);
console.log(mArr1.slice(0,1));
//you cannot do this: myArr1[0]=22;

How can I count the instances of an object?

If i have a Javascript object defined as:
function MyObj(){};
MyObj.prototype.showAlert = function(){
alert("This is an alert");
return;
};
Now a user can call it as:
var a = new MyObj();
a.showAlert();
So far so good, and one can also in the same code run another instance of this:
var b = new MyObj();
b.showAlert();
Now I want to know, how can I hold the number of instances MyObj?
is there some built-in function?
One way i have in my mind is to increment a global variable when MyObj is initialized and that will be the only way to keep track of this counter, but is there anything better than this idea?
EDIT:
Have a look at this as suggestion here:
I mean how can I make it get back to 2 instead of 3

There is nothing built-in; however, you could have your constructor function keep a count of how many times it has been called. Unfortunately, the JavaScript language provides no way to tell when an object has gone out of scope or has been garbage collected, so your counter will only go up, never down.
For example:
function MyObj() {
MyObj.numInstances = (MyObj.numInstances || 0) + 1;
}
new MyObj();
new MyObj();
MyObj.numInstances; // => 2
Of course, if you want to prevent tampering of the count then you should hide the counter via a closure and provide an accessor function to read it.
[Edit]
Per your updated question - there is no way to keep track of when instances are no longer used or "deleted" (for example by assigning null to a variable) because JavaScript provides no finalizer methods for objects.
The best you could do is create a "dispose" method which objects will call when they are no longer active (e.g. by a reference counting scheme) but this requires cooperation of the programmer - the language provides no assistance:
function MyObj() {
MyObj.numInstances = (MyObj.numInstances || 0) + 1;
}
MyObj.prototype.dispose = function() {
return MyObj.numInstances -= 1;
};
MyObj.numInstances; // => 0
var a = new MyObj();
MyObj.numInstances; // => 1
var b = new MyObj();
MyObj.numInstances; // => 2
a.dispose(); // 1 OK: lower the count.
a = null;
MyObj.numInstances; // => 1
b = null; // ERR: didn't call "dispose"!
MyObj.numInstances; // => 1

Create a static property on the MyObj constructor called say count and increment it within the constructor itself.
function MyObj() {
MyObj.count++;
}
MyObj.count = 0;
var a = new MyObj;
var b = new MyObj;
alert(MyObj.count);
This is the way you would normally do it in say Java (using a static property).

var User = (function() {
var id = 0;
return function User(name) {
this.name = name;
this.id = ++id;
}
})();
User.prototype.getName = function() {
return this.name;
}
var a = new User('Ignacio');
var b = new User('foo bar');
a
User {name: "Ignacio", id: 1}
b
User {name: "foo bar", id: 2}

Using ES6 Classes MDN syntax - we can define a static method:
The static keyword defines a static method for a class. Static methods are called without instantiating their class and cannot be called through a class instance. Static methods are often used to create utility functions for an application.
class Item {
static currentId = 0;
_id = ++Item.currentId; // Set Instance's this._id to incremented class's ID
// PS: The above line is same as:
// constructor () { this._id = ++Item.currentId; }
get id() {
return this._id; // Getter for the instance's this._id
}
}
const A = new Item(); // Create instance (Item.currentId is now 1)
const B = new Item(); // Create instance (Item.currentId is now 2)
const C = new Item(); // Create instance (Item.currentId is now 3)
console.log(A.id, B.id, C.id); // 1 2 3
console.log(`Currently at: ${ Item.currentId }`); // Currently at: 3
PS: if you don't want to log-expose the internal currentId property, make it private:
static #currentId = 0;
_id = ++Item.#currentId;
Here's an example with constructor and without the getter:
class Item {
static id = 0;
constructor () {
this.id = ++Item.id;
}
getID() {
console.log(this.id);
}
}
const A = new Item(); // Create instance (Item.id is now 1)
const B = new Item(); // Create instance (Item.id is now 2)
const C = new Item(); // Create instance (Item.id is now 3)
A.getID(); B.getID(); C.getID(); // 1; 2; 3
console.log(`Currently at: ${ Item.id }`); // Currently at: 3

what about such method?
var Greeter = (function ()
{
var numInstances;
function Greeter(message)
{
numInstances = (numInstances || 0) + 1;
this.greeting = message;
}
Greeter.prototype.greet = function ()
{
return "Hello, " + this.greeting;
};
Greeter.prototype.getCounter = function ()
{
return numInstances;
};
return Greeter;
})();
var greeter = new Greeter("world");
greeter.greet();
greeter.getCounter();
var newgreeter = new Greeter("new world");
newgreeter.greet();
newgreeter.getCounter();
greeter.getCounter();

Keeping a global count variable and incrementing every time is an option. Another option is to call counter method after each instance creation by hand (the worst thing I could imagine). But there is another better solution.
Every time we create an instance, the constructor function is being called. The problem is the constructor function is being created for each instance, but we can have a count property inside __proto__ which can be the same for each instance.
function MyObj(){
MyObj.prototype.addCount();
};
MyObj.prototype.count = 0;
MyObj.prototype.addCount = function() {
this.count++;
};
var a = new MyObj();
var b = new MyObj();
This is our a and b variables after all:

Eventually, JS is going to have built-in proxy capability, which will have low-level access to all kinds of things which happen in the background, which will never be exposed to front-end developers (except through the proxy -- think magic-methods in languages like PHP).
At that time, writing a destructor method on your object, which decrements the counter might be entirely trivial, as long as support for destruction/garbage-collection as a trigger is 100% guaranteed across platforms.
The only way to currently, reliably do it might be something like creating an enclosed registry of all created instances, and then manually destructing them (otherwise, they will NEVER be garbage-collected).
var Obj = (function () {
var stack = [],
removeFromStack = function (obj) {
stack.forEach(function (o, i, arr) {
if (obj === o) { arr.splice(i, 1); }
makeObj.count -= 1;
});
};
function makeObj (name) {
this.sayName = function () { console.log("My name is " + this.name); }
this.name = name;
this.explode = function () { removeFromStack(this); };
stack.push(this);
makeObj.count += 1;
}
makeObj.checkInstances = function () { return stack.length; };
makeObj.count = 0;
return makeObj;
}());
// usage:
var a = new Obj("Dave"),
b = new Obj("Bob"),
c = new Obj("Doug");
Obj.count; // 3
// "Dave? Dave's not here, man..."
a.explode();
Obj.count; // 2
a = null; // not 100% necessary, if you're never going to call 'a', ever again
// but you MUST call explode if you ever want it to leave the page's memory
// the horrors of memory-management, all over again
Will this pattern do what you want it to do?
As long as:
you don't turn a into something else
you don't overwrite its explode method
you don't mess with Obj in any way
you don't expect any prototype method to have access to any of the internal variables
...then yes, this method will work just fine for having the counter work properly.
You could even write a general method called recycle, which calls the explode method of any object you pass it (as long as its constructor, or factory, supported such a thing).
function recycle (obj) {
var key;
obj.explode();
for (key in obj) { if (obj.hasOwnProperty(key)) { delete obj[key]; } }
if (obj.__proto__) { obj.__proto__ = null; }
}
Note - this won't actually get rid of the object.
You'll just have removed it from the closure, and removed all methods/properties it once had.
So now it's an empty husk, which you could reuse, expressly set to null after recycling its parts, or let it be collected and forget about it, knowing that you removed necessary references.
Was this useful?
Probably not.
The only time I really see this as being of use would be in a game where your character might only be allowed to fire 3 bullets at a time, and he can't shoot a 4th until the 1st one on screen hits someone or goes off the edge (this is how, say, Contra worked, in the day).
You could also just shift a "disappeared" bullet off the stack, and reuse that bullet for any player/enemy by resetting its trajectory, resetting appropriate flags, and pushing it back onto the stack.
But again, until proxies allow us to define "magic" constructor/destructor methods, which are honoured at a low-level, this is only useful if you're going to micromanage the creation and destruction of all of your own objects (really not a good idea).

My solution is creating an object store instance count and a function to increase them in prototype.
function Person() {
this.countInst();
}
Person.prototype = {
constructor: Person,
static: {
count: 0
},
countInst: function() {
this.static.count += 1;
}
};
var i;
for (i = 0; i < 10; i++) {
var p = new Person();
document.write('Instance count: ');
document.write(p.static.count);
document.write('<br />');
}
Here is my plunker: https://plnkr.co/edit/hPtIR2MQnV08L9o1oyY9?p=preview

class Patient{
constructor(name,age,id){
Object.assign(this,{name, age, id});
}
static patientList = []; // declare a static variable
static addPatient(obj){
this.patientList.push(...obj); // push to array
return this.patientList.length; // find the array length to get the number of objects
}
}
let p1 = new Patient('shreyas',20, 1);
let p2 = new Patient('jack',25, 2);
let p3 = new Patient('smith',22, 3);
let patientCount = Patient.addPatient([p1,p2,p3]); // call static method to update the count value with the newly created object
console.log(Patient.patientList);
console.log(patientCount);