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Coding my own function constructor for a string object

I am working through the exercises in the book Object-Oriented JavaScript by Stoyan Stefanov. The exercise is asking me to create a function constructor for a String object. None of the built-in String properties or methods can be used. I am trying to recreate returning a character at a certain index of a string. So the following code is the part of the exercise I am having difficulty getting to work:
var s = new MyString('hello');
s[0];
I cannot figure out how to have my function constructor return the character at the index specified. I should be able to display to the screen the character 'h'. I was able to specifically target certain indexes but that would not be usable as there could be any number of characters in the string passed into the function constructor. Here is the code for that, this return value is for the constructor itself:
return {
'0': this.string[0]; // Is this code using built-in String object properties or methods?
}
Okay thanks if you can point me in the right direction.

A simple way to achieve this is to not make it act like a real string, but only deal with the letters that do exist by running over the input string as an array using forEach:
var MyString = function(content) {
var thisObject = this;
var letters = content.split('');
letters.forEach(function(letter, position) {
thisObject[position] = letter;
});
};
JS objects are all dynamic property/value maps, so you can set a binding that is effectively this[0] = 't'; this[1] = 'h'; this[2] = 'e' and have something that works.
Does this make sense to do? Not... really? I don't quite see what this exercise teaches you if it's telling you that your code should allow for yourstring[somenumber], but this would be one way to do it.

javascript: prototype in javascript like reflection in c#, is it possible? [duplicate]

In Ruby I think you can call a method that hasn't been defined and yet capture the name of the method called and do processing of this method at runtime.
Can Javascript do the same kind of thing ?

method_missing does not fit well with JavaScript for the same reason it does not exist in Python: in both languages, methods are just attributes that happen to be functions; and objects often have public attributes that are not callable. Contrast with Ruby, where the public interface of an object is 100% methods.
What is needed in JavaScript is a hook to catch access to missing attributes, whether they are methods or not. Python has it: see the __getattr__ special method.
The __noSuchMethod__ proposal by Mozilla introduced yet another inconsistency in a language riddled with them.
The way forward for JavaScript is the Proxy mechanism (also in ECMAscript Harmony), which is closer to the Python protocol for customizing attribute access than to Ruby's method_missing.

The ruby feature that you are explaining is called "method_missing" http://rubylearning.com/satishtalim/ruby_method_missing.htm.
It's a brand new feature that is present only in some browsers like Firefox (in the spider monkey Javascript engine). In SpiderMonkey it's called "__noSuchMethod__" https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/NoSuchMethod
Please read this article from Yehuda Katz http://yehudakatz.com/2008/08/18/method_missing-in-javascript/ for more details about the upcoming implementation.

Not at the moment, no. There is a proposal for ECMAScript Harmony, called proxies, which implements a similar (actually, much more powerful) feature, but ECMAScript Harmony isn't out yet and probably won't be for a couple of years.

You can use the Proxy class.
var myObj = {
someAttr: 'foo'
};
var p = new Proxy(myObj, {
get: function (target, methodOrAttributeName) {
// target is the first argument passed into new Proxy, aka. target is myObj
// First give the target a chance to handle it
if (Object.keys(target).indexOf(methodOrAttributeName) !== -1) {
return target[methodOrAttributeName];
}
// If the target did not have the method/attribute return whatever we want
// Explicitly handle certain cases
if (methodOrAttributeName === 'specialPants') {
return 'trousers';
}
// return our generic method_missing function
return function () {
// Use the special "arguments" object to access a variable number arguments
return 'For show, myObj.someAttr="' + target.someAttr + '" and "'
+ methodOrAttributeName + '" called with: ['
+ Array.prototype.slice.call(arguments).join(',') + ']';
}
}
});
console.log(p.specialPants);
// outputs: trousers
console.log(p.unknownMethod('hi', 'bye', 'ok'));
// outputs:
// For show, myObj.someAttr="foo" and "unknownMethod" called with: [hi,bye,ok]
About
You would use p in place of myObj.
You should be careful with get because it intercepts all attribute requests of p. So, p.specialPants() would result in an error because specialPants returns a string and not a function.
What's really going on with unknownMethod is equivalent to the following:
var unk = p.unkownMethod;
unk('hi', 'bye', 'ok');
This works because functions are objects in javascript.
Bonus
If you know the number of arguments you expect, you can declare them as normal in the returned function.
eg:
...
get: function (target, name) {
return function(expectedArg1, expectedArg2) {
...

I've created a library for javascript that let you use method_missing in javascript: https://github.com/ramadis/unmiss
It uses ES6 Proxies to work. Here is an example using ES6 Class inheritance. However you can also use decorators to achieve the same results.
import { MethodMissingClass } from 'unmiss'
class Example extends MethodMissingClass {
methodMissing(name, ...args) {
console.log(`Method ${name} was called with arguments: ${args.join(' ')}`);
}
}
const instance = new Example;
instance.what('is', 'this');
> Method what was called with arguments: is this

No, there is no metaprogramming capability in javascript directly analogous to ruby's method_missing hook. The interpreter simply raises an Error which the calling code can catch but cannot be detected by the object being accessed. There are some answers here about defining functions at run time, but that's not the same thing. You can do lots of metaprogramming, changing specific instances of objects, defining functions, doing functional things like memoizing and decorators. But there's no dynamic metaprogramming of missing functions as there is in ruby or python.

I came to this question because I was looking for a way to fall through to another object if the method wasn't present on the first object. It's not quite as flexible as what your asking - for instance if a method is missing from both then it will fail.
I was thinking of doing this for a little library I've got that helps configure extjs objects in a way that also makes them more testable. I had seperate calls to actually get hold of the objects for interaction and thought this might be a nice way of sticking those calls together by effectively returning an augmented type
I can think of two ways of doing this:
Prototypes
You can do this using prototypes - as stuff falls through to the prototype if it isn't on the actual object. It seems like this wouldn't work if the set of functions you want drop through to use the this keyword - obviously your object wont know or care about stuff that the other one knows about.
If its all your own code and you aren't using this and constructors ... which is a good idea for lots of reasons then you can do it like this:
var makeHorse = function () {
var neigh = "neigh";
return {
doTheNoise: function () {
return neigh + " is all im saying"
},
setNeigh: function (newNoise) {
neigh = newNoise;
}
}
};
var createSomething = function (fallThrough) {
var constructor = function () {};
constructor.prototype = fallThrough;
var instance = new constructor();
instance.someMethod = function () {
console.log("aaaaa");
};
instance.callTheOther = function () {
var theNoise = instance.doTheNoise();
console.log(theNoise);
};
return instance;
};
var firstHorse = makeHorse();
var secondHorse = makeHorse();
secondHorse.setNeigh("mooo");
var firstWrapper = createSomething(firstHorse);
var secondWrapper = createSomething(secondHorse);
var nothingWrapper = createSomething();
firstWrapper.someMethod();
firstWrapper.callTheOther();
console.log(firstWrapper.doTheNoise());
secondWrapper.someMethod();
secondWrapper.callTheOther();
console.log(secondWrapper.doTheNoise());
nothingWrapper.someMethod();
//this call fails as we dont have this method on the fall through object (which is undefined)
console.log(nothingWrapper.doTheNoise());
This doesn't work for my use case as the extjs guys have not only mistakenly used 'this' they've also built a whole crazy classical inheritance type system on the principal of using prototypes and 'this'.
This is actually the first time I've used prototypes/constructors and I was slightly baffled that you can't just set the prototype - you also have to use a constructor. There is a magic field in objects (at least in firefox) call __proto which is basically the real prototype. it seems the actual prototype field is only used at construction time... how confusing!
Copying methods
This method is probably more expensive but seems more elegant to me and will also work on code that is using this (eg so you can use it to wrap library objects). It will also work on stuff written using the functional/closure style aswell - I've just illustrated it with this/constructors to show it works with stuff like that.
Here's the mods:
//this is now a constructor
var MakeHorse = function () {
this.neigh = "neigh";
};
MakeHorse.prototype.doTheNoise = function () {
return this.neigh + " is all im saying"
};
MakeHorse.prototype.setNeigh = function (newNoise) {
this.neigh = newNoise;
};
var createSomething = function (fallThrough) {
var instance = {
someMethod : function () {
console.log("aaaaa");
},
callTheOther : function () {
//note this has had to change to directly call the fallThrough object
var theNoise = fallThrough.doTheNoise();
console.log(theNoise);
}
};
//copy stuff over but not if it already exists
for (var propertyName in fallThrough)
if (!instance.hasOwnProperty(propertyName))
instance[propertyName] = fallThrough[propertyName];
return instance;
};
var firstHorse = new MakeHorse();
var secondHorse = new MakeHorse();
secondHorse.setNeigh("mooo");
var firstWrapper = createSomething(firstHorse);
var secondWrapper = createSomething(secondHorse);
var nothingWrapper = createSomething();
firstWrapper.someMethod();
firstWrapper.callTheOther();
console.log(firstWrapper.doTheNoise());
secondWrapper.someMethod();
secondWrapper.callTheOther();
console.log(secondWrapper.doTheNoise());
nothingWrapper.someMethod();
//this call fails as we dont have this method on the fall through object (which is undefined)
console.log(nothingWrapper.doTheNoise());
I was actually anticipating having to use bind in there somewhere but it appears not to be necessary.

Not to my knowledge, but you can simulate it by initializing the function to null at first and then replacing the implementation later.
var foo = null;
var bar = function() { alert(foo()); } // Appear to use foo before definition
// ...
foo = function() { return "ABC"; } /* Define the function */
bar(); /* Alert box pops up with "ABC" */
This trick is similar to a C# trick for implementing recursive lambdas, as described here.
The only downside is that if you do use foo before it's defined, you'll get an error for trying to call null as though it were a function, rather than a more descriptive error message. But you would expect to get some error message for using a function before it's defined.

Confused about JavaScript prototypal inheritance with constructors

I've read pages and pages about JavaScript prototypal inheritance, but I haven't found anything that addresses using constructors that involve validation. I've managed to get this constructor to work but I know it's not ideal, i.e. it's not taking advantage of prototypal inheritance:
function Card(value) {
if (!isNumber(value)) {
value = Math.floor(Math.random() * 14) + 2;
}
this.value = value;
}
var card1 = new Card();
var card2 = new Card();
var card3 = new Card();
This results in three Card objects with random values. However, the way I understand it is that each time I create a new Card object this way, it is copying the constructor code. I should instead use prototypal inheritance, but this doesn't work:
function Card(value) {
this.value = value;
}
Object.defineProperty( Card, "value", {
set: function (value) {
if (!isNumber(value)) {
value = Math.floor(Math.random() * 14) + 2;
}
this.value = value;
}
});
This doesn't work either:
Card.prototype.setValue = function (value) {
if (!isNumber(value)) {
value = Math.floor(Math.random() * 14) + 2;
}
this.value = value;
};
For one thing, I can no longer call new Card(). Instead, I have to call var card1 = new Card(); card1.setValue(); This seems very inefficient and ugly to me. But the real problem is it sets the value property of each Card object to the same value. Help!
Edit
Per Bergi's suggestion, I've modified the code as follows:
function Card(value) {
this.setValue(value);
}
Card.prototype.setValue = function (value) {
if (!isNumber(value)) {
value = Math.floor(Math.random() * 14) + 2;
}
this.value = value;
};
var card1 = new Card();
var card2 = new Card();
var card3 = new Card();
This results in three Card objects with random values, which is great, and I can call the setValue method later on. It doesn't seem to transfer when I try to extend the class though:
function SpecialCard(suit, value) {
Card.call(this, value);
this.suit = suit;
}
var specialCard1 = new SpecialCard("Club");
var specialCard2 = new SpecialCard("Diamond");
var specialCard3 = new SpecialCard("Spade");
I get the error this.setValue is not a function now.
Edit 2
This seems to work:
function SpecialCard(suit, value) {
Card.call(this, value);
this.suit = suit;
}
SpecialCard.prototype = Object.create(Card.prototype);
SpecialCard.prototype.constructor = SpecialCard;
Is this a good way to do it?
Final Edit!
Thanks to Bergi and Norguard, I finally landed on this implementation:
function Card(value) {
this.setValue = function (val) {
if (!isNumber(val)) {
val = Math.floor(Math.random() * 14) + 2;
}
this.value = val;
};
this.setValue(value);
}
function SpecialCard(suit, value) {
Card.call(this, value);
this.suit = suit;
}
Bergi helped me identify why I wasn't able to inherit the prototype chain, and Norguard explained why it's better not to muck with the prototype chain at all. I like this approach because the code is cleaner and easier to understand.

the way I understand it is that each time I create a new Card object this way, it is copying the constructor code
No, it is executing it. No problems, and your constructor works perfect - this is how it should look like.
Problems will only arise when you create values. Each invocation of a function creates its own set of values, e.g. private variables (you don't have any). They usually get garbage collected, unless you create another special value, a privileged method, which is an exposed function that holds a reference to the scope it lives in. And yes, every object has its own "copy" of such functions, which is why you should push everything that does not access private variables to the prototype.
Object.defineProperty( Card, "value", ...
Wait, no. Here you define a property on the constructor, the function Card. This is not what you want. You could call this code on instances, yes, but note that when evaluating this.value = value; it would recursively call itself.
Card.prototype.setValue = function(){ ... }
This looks good. You could need this method on Card objects when you are going to use the validation code later on, for example when changing the value of a Card instance (I don't think so, but I don't know?).
but then I can no longer call new Card()
Oh, surely you can. The method is inherited by all Card instances, and that includes the one on which the constructor is applied (this). You can easily call it from there, so declare your constructor like this:
function Card(val) {
this.setValue(val);
}
Card.prototype...
It doesn't seem to transfer when I try to extend the class though.
Yes, it does not. Calling the constructor function does not set up the prototype chain. With the new keyword the object with its inheritance is instantiated, then the constructor is applied. With your code, SpecialCards inherit from the SpecialCard.prototype object (which itself inherits from the default Object prototype). Now, we could either just set it to the same object as normal cards, or let it inherit from that one.
SpecialCard.prototype = Card.prototype;
So now every instance inherits from the same object. That means, SpecialCards will have no special methods (from the prototype) that normal Cards don't have... Also, the instanceof operator won't work correctly any more.
So, there is a better solution. Let the SpecialCards prototype object inherit from Card.prototype! This can be done by using Object.create (not supported by all browsers, you might need a workaround), which is designed to do exactly this job:
SpecialCard.prototype = Object.create(Card.prototype, {
constructor: {value:SpecialCard}
});
SpecialCard.prototype.specialMethod = ... // now possible

In terms of the constructor, each card IS getting its own, unique copy of any methods defined inside of the constructor:
this.doStuffToMyPrivateVars = function () { };
or
var doStuffAsAPrivateFunction = function () {};
The reason they get their own unique copies is because only unique copies of functions, instantiated at the same time as the object itself, are going to have access to the enclosed values.
By putting them in the prototype chain, you:
Limit them to one copy (unless manually-overridden per-instance, after creation)
Remove the method's ability to access ANY private variables
Make it really easy to frustrate friends and family by changing prototype methods/properties on EVERY instance, mid-program.
The reality of the matter is that unless you're planning on making a game that runs on old Blackberries or an ancient iPod Touch, you don't have to worry too much about the extra overhead of the enclosed functions.
Also, in day-to-day JS programming, the extra security from properly-encapsulated objects, plus the extra benefit of the module/revealing-module patterns and sandboxing with closures VASTLY OUTWEIGHS the cost of having redundant copies of methods attached to functions.
Also, if you're really, truly that concerned, you might do to look at Entity/System patterns, where entities are pretty much just data-objects (with their own unique get/set methods, if privacy is needed)... ...and each of those entities of a particular kind is registered to a system which is custom made for that entity/component-type.
IE: You'd have a Card-Entity to define each card in a deck.
Each card has a CardValueComponent, a CardWorldPositionComponent, a CardRenderableComponent, a CardClickableComponent, et cetera.
CardWorldPositionComponent = { x : 238, y : 600 };
Each of those components is then registered to a system:
CardWorldPositionSystem.register(this.c_worldPos);
Each system holds ALL of the methods which would normally be run on the values stored in the component.
The systems (and not the components) will chat back and forth, as needed to send data back and forth, between components shared by the same entity (ie: the Ace of Spade's position/value/image might be queried from different systems so that everybody's kept up to date).
Then instead of updating each object -- traditionally it would be something like:
Game.Update = function (timestamp) { forEach(cards, function (card) { card.update(timestamp); }); };
Game.Draw = function (timestamp, renderer) { forEach(cards, function (card) { card.draw(renderer); }); };
Now it's more like:
CardValuesUpdate();
CardImagesUpdate();
CardPositionsUpdate();
RenderCardsToScreen();
Where inside of the traditional Update, each item takes care of its own Input-handling/Movement/Model-Updating/Spritesheet-Animation/AI/et cetera, you're updating each subsystem one after another, and each subsystem is going through each entity which has a registered component in that subsystem, one after another.
So there's a smaller memory-footprint on the number of unique functions.
But it's a very different universe in terms of thinking about how to do it.

JavaScript official keywords shortcuts

I've seen something similar to this code in the Google API JavaScript, I mean the r=Array part. Here is an example of what they have done:
var r = Array;
var t = new r('sdsd' , 'sdsd');
alert(t[0]);
Few questions about this:
Is it legal to write like this and won't cause any problems?
I can do something similar with other keywords like ´For´ loop or with the ´this´ keyword?
Can I have article about this JavaScript official keyword shortcuts etc..?
Thank you in advance.

That works because Array is an object. You can do that with any object. For example, the Date object:
var d = Date;
console.log((new d()).getTime()); //Prints time
You cannot do that for keywords such as for or while because they are language constructs that will be recognised by the interpreter.
You can do it with this:
document.getElementById("b").onclick = function() {
var x = this; //this holds a reference to the DOM element that was clicked
x.value = "Clicked!";
}
In fact, that can be very useful sometimes (to keep a reference to this so you can access it from an anonymous inner function for example). This also works because, to put it simply, this will be a reference to an object.

Yes
for - no. this - yes.
You can store references to any JavaScript object in a variable. String, Array, Object, etc. are JavaScript objects that are built-in to the language. for, if, while, etc. are are JavaScript statements, and cannot be stored or referenced any other way.
You can do it the other way around as well (and really mess yourself up in the process):
Array = 0;
var myArray = new Array("a", "b", "c"); // throws error
This is easily undone like this:
Array = [].constructor;
Edit: Being able to assign the value of this to a variable is essential when nesting functions that will execute in a different scope:
function Widget() {
var that = this;
this.IsThis = function() {
return isThis();
};
function isThis() {
return that == this;
}
}
new Widget().IsThis(); // false!
Maybe not the best example, but illustrates losing scope.
You cannot reassign the value of this:
function doSomething() {
this = 0; // throws error
}

javascript question: functionName.VariableName?

I had the following question in a test today. But i had not see something like functionName.VariableName before. Not sure how that works.
Would be great if you can tell me the solution:
function Item(itemName)
{
var next_item_id = 1;
Item.item_name = itemName;
Item.item_id = next_item_id++;
}
var Item1 = Item('Desktop');
var Item2 = Item('Laptop');
var Item3 = Item('Monitor');
Anything wrong with the code above? if yes fix it. (The problem i would see is next_item_id is always 1, need to make it global?)
Modify the function so that the variable “next_item_id”, cannot be modified during run time.
My own question, how does the variable like Item.item_name work? I want to google it, but not sure what I should search for.
Thanks.

Your thinking is close in that next_item_id will always be 1, but it's generally not recommended to pollute the global namespace. Instead, wrap it in an anonymous function:
(function() {
var next_item_id = 1;
function Item(itemName)
{
//Use "this" to apply the property to the instance only
this.item_name = itemName;
this.item_id = next_item_id++;
}
var Item1 = new Item('Desktop');
var Item2 = new Item('Laptop');
var Item3 = new Item('Monitor');
})()
Also, as in Java, the general best practice in Javascript is to use camelCase rather than under_scores. Constructors are generally in UpperCamelCase. Examples:
Array //UpperCamelCase
Object.prototype.toString //toString is camelCase
This question is ambiguous, and, depending on the interpretation, there can be a number of possible answers. next_item_id is a "var" declared inside a function and naturally has an internal [[DontDelete]] and [[DontEnum]] flag. If it's a "var" and we're not using "this" as shown in my modified code, the variable is inherently not accessible outside of the function or its nested functions and therefore cannot be modified. You can use the non-standard const or you can create an object and use Object.defineProperty to create a setter that returns false assuming an ES5-compatible environment, etc.
Functions are objects in Javascript. All objects can have "properties."

Javascript functions are also objects and can have properties.
They behave like static fields in Java.

You're going about it all wrong. I'm assuming that you want to set internal variables.
If you want to create a 'constructor', do something like this:
function Item(itemName)
{
var next_item_id = 1;
this.item_name = itemName;
this.item_id = next_item_id++;
}
var Item1 = new Item('Desktop');
var Item2 = new Item('Laptop');
var Item3 = new Item('Monitor');
Both item_name and item_id will be publicly available. To make use next_item_id like you want, try this:
Item.next_item_id = 1;
Then in your constructor, do Item.next_item_id++;
Your final code should look something like this:
function Item(itemName)
{
this.item_name = itemName;
this.item_id = Item.next_item_id++;
}
Item.next_item_id = 1;
var Item1 = new Item('Desktop');
var Item2 = new Item('Laptop');
var Item3 = new Item('Monitor');
What this is doing is attaching a property to the object Item. You might be thinking, "Item is a function! How is this possible?!". Nearly everything in JavaScript is an object with mutable properties. The only things that aren't are the keywords null and undefined (AFAIK).
Item is still a function, but it also has a property next_item_id.
Also, using new will create a new instance. This is a similar concept as in Java and other programming languages. Just calling Item is like calling a function, and you'll get back whatever is returned from it (through an explicit return statement, otherwise undefined).

Answer for question 2 : Modify the function so that the variable “next_item_id”, cannot be modified during run time.
If I have interpreted your question correctly, you want to have Item.item_id as a constant that cannot be modified any where outside the constructor. I don't think we can have constants in JS.
Item1.item_id = someValue
Above line, used some where can change the item_id value of Item1.