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Adding properties to an object using function and bracket notation

I have an assignment on a basic javascript class that I'm taking and I can't seem to get this to work. I have this unit test that was given to me:
describe('AddSixthProperty', function() {
it('should add a food property with the value of bbq using bracket notation', function() {
expect(objects.addSixthProperty()['food']).to.equal('BBQ');
});
});
I was given an empty function:
// don't touch this line
var mysticalAnimal = objects.mysticalAnimal();
function addSixthElement(){
return
}
So I tried this:
var mysticalAnimal = objects.mysticalAnimal();
objects.addSixthProperty = function(){
mysticalAnimal['food'] = "bbq";
return mysticalAnimal["food"];
};
It doesn't work. Our test page doesn't pass that. Any help is greatly appreciated!
Thanks in advance!

You're returning mysticalAnimal['food'], and then the test tries to access ['food'] again, so it ends up accessing 'bbq'['food'], which is undefined. You need to just return mysticalAnimal, as well as get all your letter cases right. Here's a little proof of concept:
var objects = (function() {
var animal = { mystical: true };
return {
mysticalAnimal: function() { return animal; }
};
})();
var mysticalAnimal = objects.mysticalAnimal();
objects.addSixthProperty = function(){
mysticalAnimal['food'] = "bbq";
return mysticalAnimal;
};
var capturedAnimal = objects.addSixthProperty();
document.getElementById('result').innerText = capturedAnimal['food'];
<p id="result" />

Here is the function:
var mysticalAnimal = objects.mysticalAnimal();
objects.addSixthProperty = function(){
mysticalAnimal['food'] = "BBQ";
return mysticalAnimal;
};
// Test the function
console.log(objects.addSixthProperty()['food'])

How to iterate an Knockout ObservableArray? (length 0)

I'm new to knockout.js (and this is also my first stackoverflow post) and I'm now facing the following problem.
I'm not able to bind the data from web api to a ko.observablearray. Why is the length of this Announcements ko.observablearray always 0? The code works fine with client side data (by adding new announcements)..
Here's the JS-code:
var AnnouncementModel = function () {
var self = this;
self.AnnouncementText = ko.observable();
self.AllDepartmentsBool = ko.observable();
self.Editable = ko.observable(false);
self.Add = function () {
viewModel.Announcements.push(self);
viewModel.AnnouncementToEdit(new AnnouncementModel());
};
self.Delete = function () {
ajaxHelper(announcementsUri + self.ID, 'DELETE').done(
viewModel.Announcements.remove(self));
};
self.Edit = function () {
self.Editable(!self.Editable());
};
}
//The ViewModel
function AnnouncementsViewModel() {
var self = this;
self.InitialData = ko.observableArray();
self.Announcements = ko.observableArray();
self.AnnouncementToEdit = ko.observable(new AnnouncementModel());
self.error = ko.observable();
function getAllAnnouncements() {
ajaxHelper(announcementsUri, 'GET').done(function(data) {
self.InitialData(data);
});
};
getAllAnnouncements();
};
var viewModel = new AnnouncementsViewModel();
ko.applyBindings(viewModel, document.getElementById("announcements-container"));
function createAnnouncement(announcementDto) {
var announcement = new AnnouncementModel();
announcement.AnnouncementText = ko.observable(announcementDto.AnnouncementText);
announcement.AllDepartmentsBool = ko.observable(announcementDto.AllDepartmentsBool);
announcement.Editable = ko.observable(false);
return announcement;
}
var length = viewModel.InitialData.length;
for (i = 0; i < length; i++) {
var newAnnouncement = createAnnouncement(InitialData[i]);
viewModel.Announcements.push(newAnnouncement);
}
The HTML:
<div id="announcements-container" style="display: inline-block; float: right">
<ul id="announcements-list" class="newsticker" data-bind="foreach: Announcements">
<li>
<span data-bind="html: AnnouncementText"></span>
</li>
</ul>
#Html.Partial("_AnnouncementsModal")
</div>
The InitialData gets populated from the api as it should:
GOT IT WORKING! :
Thanks for the quick answers. I got the code working by iterating the data with .forEach(). Another problem was that the initialData didn't get populated in it's current scope so I edited the getAnnouncements function to work like this :
function getAllAnnouncements() {
ajaxHelper(announcementsUri, 'GET').done(function(data) {
data.forEach(function (entry) {
var newAnnouncement = createAnnouncement(entry);
self.Announcements.push(newAnnouncement);
});
});
};

This line is the likely culprit:
var length = viewModel.InitialData.length;
Remember that InitialData is a function. Functions have a length (it's their "arity", the number of formal arguments they have), but the observable function for an observable array's length isn't the array's length..
You probably wanted the length of the array inside it:
var length = viewModel.InitialData().length;
// -------------------------------^^
Your various calls to push on observable arrays work even though length doesn't because Knockout provides push (and several other things) on the observable array function, as James points out.
Similarly, this line:
var newAnnouncement = createAnnouncement(InitialData[i]);
probably wants to be using the array as well (and is missing viewModel. in front of InitialData).
So that whole section probably wants to be refactored a bit:
viewModel.InitialData().forEach(function(entry) {
var newAnnouncement = createAnnouncement(entry);
viewModel.Announcements.push(newAnnouncement);
});
or without forEach (but really, it's nearly 2016, and it's shimmable on obsolete browsers);
var data = viewModel.InitialData();
for (var i = 0; i < data.length; ++i) {
var newAnnouncement = createAnnouncement(data[i]);
viewModel.Announcements.push(newAnnouncement);
}
Side note: Your code (at least as it is in the question) was also falling prey to The Horror of Implicit Globals by not declaring the i that you use in that for loop. I've added a var above, but this is another reason for using forEach to loop through arrays.

You can also use EcmaScript 6 style enumeration as follows:
viewModel.InitialData().forEach(item => {
let newAnnouncement = createAnnouncement(item);
viewModel.Announcements.push(newAnnouncement);
});

Losing scope on recursive call

I'm creating a module that accepts a data set and an integer n and recursively fills that dataset with n products at a time, after the first call, the function loses its scope and errors out. Why, And what's the best practice for fixing this?
Code:
function ProductFactory(){
var bigArr = [0,1,2,3,4,5,6,7,8,9];
var smallArr = [1,2,3,4];
return {
getProductList: getProductList,
getAllProducts: getAllProducts
};
function getProductList(start, size){ return start < 5 ? bigArr : smallArr }
function getAllProducts(batchSizeRequested, dataSet) {
var startPage = dataSet.length / batchSizeRequested;
var productBatch = this.getProductList(startPage, batchSizeRequested);
dataSet = dataSet.concat(productBatch);
if (productBatch.length === batchSizeRequested)
getAllProducts(batchSizeRequested, dataSet);
}
}
var productGetter = new ProductFactory();
productGetter.getAllProducts(10, []);

1) First of all you shouldn't call getProductList using this, in this case you can just call it as it is, because getProductList is not a function that was assigned directly to this object. It is just a closure that uses local variables in it's code. If you want to call function using this, you should assign it using this, for example this.getProductList = function() {}
2) I don't think there are other scoping problems except redundant this, but I found another issue, though.
You are not actually return anything from your function, plus recursive call does not have an exit point.
Fixed code looks like this.
function ProductFactory(){
var bigArr = [0,1,2,3,4,5,6,7,8,9];
var smallArr = [1,2,3,4];
return {
getProductList: getProductList,
getAllProducts: getAllProducts
};
function getProductList(start, size){ return start < 5 ? bigArr : smallArr }
function getAllProducts(batchSizeRequested, dataSet) {
var startPage = dataSet.length / batchSizeRequested;
var productBatch = getProductList(startPage, batchSizeRequested);
dataSet = dataSet.concat(productBatch);
if (productBatch.length === batchSizeRequested) {
return getAllProducts(batchSizeRequested, dataSet);
} else {
return dataSet;
}
}
}
var productGetter = ProductFactory();
var products = productGetter.getAllProducts(10, []);
console.log(products)

The typical approach to a function call like this is to assign an external value to this (typically called self):
function ProductFactory(){
...
var self = this;
function getAllProducts(batchSizeRequested, dataSet) {
...
getAllProducts.apply(self, [batchSizeRequested, dataSet]);
}
}
In this case, however, please try to remember that you have defined a closure function getAllProducts that is only privately accessible internal to the constructor. Instead you should probably do:
function ProductFactory(){
...
var self = this;
this.getAllProducts = function(batchSizeRequested, dataSet) {
...
self.getAllProducts(batchSizeRequested, dataSet);
}
}

Returning a reference in JavaScript

Here is the code:
http://jsfiddle.net/GKBfL/
I am trying to get collection.prototype.add to return a reference such that the final alert will display testing, testing, 123, testing. Is there a way to accomplish what I'm trying to do here?
HTML:
<span id="spantest">testing, testing, 123, testing</span>​
JavaScript:
var collection = function () {
this.items = {};
}
collection.prototype.add = function(sElmtId) {
this.items[sElmtId] = {};
return this.items[sElmtId];
}
collection.prototype.bind = function() {
for (var sElmtId in this.items) {
this.items[sElmtId] = document.getElementById(sElmtId);
}
}
var col = new collection();
var obj = {};
obj = col.add('spantest');
col.bind();
alert(obj.innerHTML);​

You problem is this line:
this.items[sElmtId] = document.getElementById(sElmtId);
This overwrites the object currently assigned to this.items[sElmtId] with the DOM node. Instead, you should assign the node to a property of that object:
this.items[sElmtId].node = document.getElementById(sElmtId);
That way, obj.node will always refer to the current node:
alert(obj.node.innerHTML);
DEMO
Side note: The problem with your fiddle is also that you execute the code when the DOM is not built yet (no wrap (head)), so it cannot find #spantest. You have to run the code once the DOM is ready, either no wrap (body), onDomRead or onLoad.

Creating a reference like you need is impossible in JavaScript. The closest thing you can get is either a nested or closed object, or just copying it over, like so:
var collection = function() {
this.items = {};
};
collection.prototype.add = function(sElmtId) {
return this.items[sElmtId] = {};
};
collection.prototype.bind = function() {
for(var sElmtId in this.items) {
var element = document.getElementById(sElmtId);
for(var x in element) {
this.items[sElmtId][x] = element[x];
}
}
};
var col = new collection();
var obj = {};
obj = col.add('spantest');
col.bind();
alert(obj.innerHTML);
But it won't be truly "bound". You'll have to use nested objects if you need that kind of functionality, and it will probably defeat the point of your syntactic sugar.
http://jsfiddle.net/GKBfL/7/

Javascript array is undefined... and I'm not sure why

I'm trying to translate a PHP class into JavaScript. The only thing I'm having trouble with is getting an item out of an array variable. I've created a simple jsfiddle here. I cannot figure out why it won't work.
(EDIT: I updated this code to better reflect what I'm doing. Sorry for the previous mistake.)
function tattooEightBall() {
this.subjects = ['a bear', 'a tiger', 'a sailor'];
this.prediction = make_prediction();
var that = this;
function array_random_pick(somearray) {
//return array[array_rand(array)];
var length = somearray.length;
var random = somearray[Math.floor(Math.random()*somearray.length)];
return random;
}
function make_prediction() {
var prediction = array_random_pick(this.subjects);
return prediction;
}
}
var test = tattooEightBall();
document.write(test.prediction);
​

Works fine here, you are simple not calling
classname();
After you define the function.
Update
When you make a call to *make_prediction* , this will not be in scope. You are right on the money creating a that variable, use it on *make_prediction* :
var that = this;
this.prediction = make_prediction();
function make_prediction() {
var prediction = ''; //initialize it
prediction = prediction + array_random_pick(that.subjects);
return prediction;
}
You can see a working version here: http://jsfiddle.net/zKcpC/
This is actually pretty complex and I believe someone with more experience in Javascript may be able to clarify the situation.
Edit2: Douglas Crockfords explains it with these words:
By convention, we make a private that variable. This is used to make
the object available to the private methods. This is a workaround for
an error in the ECMAScript Language Specification which causes this to
be set incorrectly for inner functions.
To see the complete article head to: http://javascript.crockford.com/private.html

You never call classname. Seems to be working fine.

Works for me:
(function classname() {
this.list = [];
this.list[0] = "tiger";
this.list[1] = "lion";
this.list[2] = "bear";
function pickone(somearray) {
var length = somearray.length;
var random = somearray[Math.floor(Math.random()*length)];
return random;
}
var random_item = pickone(this.list);
document.write(random_item);
}());
Were you actually calling the classname function? Note I wrapped your code block in:
([your_code]());

I'm not sure what you're trying to accomplish exactly with the class structure you were using so I made some guesses, but this code works by creating a classname object that has instance data and a pickone method:
function classname() {
this.list = [];
this.list[0] = "tiger";
this.list[1] = "lion";
this.list[2] = "bear";
this.pickone = function() {
var length = this.list.length;
var random = this.list[Math.floor(Math.random()*length)];
return random;
}
}
var cls = new classname();
var random = cls.pickone();
You can play with it interactively here: http://jsfiddle.net/jfriend00/ReL2h/.

It's working fine for me: http://jsfiddle.net/YznSE/6/ You just didn't call classname(). If you don't call it, nothing will happen ;)

Make it into a self-executing function like this:
(function classname() {
this.list = [];
this.list[0] = "tiger";
this.list[1] = "lion";
this.list[2] = "bear";
function pickone(somearray) {
var length = somearray.length; //<---WHY ISN'T THIS DEFINED??
var random = somearray[Math.floor(Math.random() * length)];
return random;
}
var random_item = pickone(this.list);
document.write(random_item);
})();

var test = tattooEightBall();
document.write(test.prediction);
Should be:
var test = new tattooEightBall(); //forgot new keyword to create object
document.write(test.prediction()); // forgot parens to fire method
and:
this.prediction = make_prediction();
Should be:
this.prediction = make_prediction;