Develop Reference

Adding two same rows in array than modifying only one of them

First, I am adding two same rows in array and later I need to modify only the last one, adding new property to it. The way I do that:
for(var index in arrayOne) {
var arrayOneItem = arrayOne[index];
var new_row = {
address: arrayOne[index].address,
date: arrayOne[index].date,
category: arrayOne[index].category,
};
rows.push(new_row);
if(arrayOne[index].refund_status == 'refunded') {
rows.push(new_row);
rows[rows.length - 1].refund_status = 'refunded';
}
}
But the problem is that the code inside if statement does not only modify last row, but also the one before it, so the refund_status = 'refunded' is added both to the last and one before last row. Why is this happening and is there a way to modify the last row only?

When you are using the same object twice it's best to create a copy (shallow in this case) using Object.assign(). This will avoid referencing the same object from multiple variables or array indexes in your case.
eg.
rows.push(new_row);
becomes
rows.push(Object.assign({}, new_row));

This is because the object you push into the array is passed by reference and not by value, thus when you change the original object you will change both references to it in the array, see example below:
let someArray = [];
let someObj = {foo: "bar"};
someArray.push(someObj);
someArray.push(someObj);
someArray[0].foo = "baz";
console.log(someArray[1]);
To avoid this, you would need to clone the values of the object to create a new one. This question has some ways to do so, using JSON.parse and JSON.stringify is the shortest way to deep-copy an object without an external library, see example below:
let someArray = [];
let someObj = {foo: "bar"};
someArray.push(someObj);
let newObj = JSON.parse(JSON.stringify(someObj));
someArray.push(newObj);
someArray[0].foo = "baz";
console.log(someArray[1]);

Because you are changing property of an object, and object in javaScript is accessed through a link not a separate instance. In another words, you have the same object in memory and you change its property. It means, new_row is object you create and push it several times, and it's the same.
You can avoid it by copying it when pushing second times:
if(arrayOne[index].refund_status == 'refunded') {
rows.push({ ...new_row });
rows[rows.length - 1].refund_status = 'refunded';
}
where { ...new_row } basically creates new copy.
When you do rows[rows.length - 1].refund_status = 'refunded'; only second last will change.
Another solution i'd suggest is a bit more accurate:
const rows = []; // empty
const arrayOne = []; // SOME DATA HERE as I understand
const refundedStatus = ;
arrayOne.forEach(element=> {
rows.push(element);
if (value.refund_status === 'refunded') {
rows[rows.length].refund_status = 'refunded';
rows.push({ ...element});
}
});

How to initialize 4d array in javascript?

Here is my code:
var arr = [[[[[]]]]];
var c = 20;
for (i=0;i<5;i++)
arr[i][0][0][0] = c;
alert(arr[2][0][0][0]);
It doesn't work, but how can I do this?

Most people here are using for loops, which I think are mostly obsolete in the age of anonymous functions in JavaScript. You people should know better :P
Anyway, you can solve this quite nicely in a one-liner. Here are a few scripts that can initialize your array...
If you already have a 4-dimensional array, you can initialize it elegantly like this:
arr.forEach(function(e) { e[0][0][0] = c })
Or, if you're more into map:
arr.map(function(e) { e[0][0][0] = c })
These are assuming you already have c defined, which you do in your code sample (20).
From now on, though, please Google your questions before asking them on stackoverflow. You will receive an answer that has already been accepted :)

It doesn't work because you haven't specified any elements beyond the first one, so the length of array is one and accessing further keys is incorrect.
I think, the most convenient way would be to push a new 3d array with c inside on every iteration (actually I have no idea what you're trying to achieve with this xD):
var arr = [];
var c = 20;
for (i=0;i<5;i++)
arr.push([[[c]]])
alert(arr[2][0][0][0]);
(in your example it's actually 5d, but as you've asked for 4d, writing 4d there)

It is unclear what you want, but I imagine a 4 dimension array is an array that has a set of arrays nested 3 deep, each of which has an array nested 2 deep, each of which has a single array that contains values.
In a one dimension array, you access the value at index 2 by:
arr[2];
In a two dimension array, you'd access the value at (2,3) by:
arr[2][3]
and so on until you get to the value at (2,3,1,2) in a four dimension array by:
arr[2][3][1][2]
and if that was the only value in the array, it would look like:
[,,[,,,[,[,,'value at 2312']]]];
If there was also a value at (1,1,0,2) the array would now look like:
[,[,[[,,'value at 1102']]],[,,,[,[,,'value at 2312']]]];
There can only be values in the last nested array, the value at indexes in every other array must be another array (for the lower dimensions), so to insert at value at, say (2,1,3,1) and assign it a value of 6, you need to loop over the array and inspect each index. If it's not already an array, insert an array and keep going, e.g.:
// Insert value in arrary at coord
// coord is a comma separated list of coordinates.
function insertValue( array, coord, value) {
var coords = coord.split(',');
var arr = array;
for (var c, i=0, iLen=coords.length-1; i < iLen; i++) {
c = coords[i];
if (!Array.isArray(arr[c])) arr[c] = [];
arr = arr[c];
}
arr[coords[i]] = value;
return array;
}
document.write('result: ' + JSON.stringify(insertValue([],'1,2,1,3','at 1213')));
I don't understand what you are trying to do in the OP: are you trying to create a value of 20 at coordinates (0,0,0,0), (1,0,0,0), (2,0,0,0), etc.? If that is the case, you also need a fill function that will iterate for the required number of times and pass suitable arguments to insertValue.
If that's what you want, then given the above you should be able to write such a function. On the first iteration it would pass:
insertValue(array, '0,0,0,0', 20)
and on the second:
insertValue(array, '1,0,0,0', 20)
and so on. You may wish to modify the function so that instead of the coords being a CSV string, you pass an array like [0,0,0,0] (which is what split turns the CSV string into), but that's up to you.
Note that you must pass all 4 dimensions, otherwise you will replace one of the dimension arrays with a value and effectively delete all other points in that dimension sector.
PS
ES5 introduced forEach, which helps encapsulate loops but doesn't necessarily mean less code, or faster execution, than an equivalent for loop:
// Insert value in arr at coord
// coord is a comma separated list of coordinates.
function insertValue( array, coord, value) {
var arr = array;
var coords = coord.split(',');
var last = coords.pop();
coords.forEach(function(c) {
if (!Array.isArray(arr[c])) arr[c] = [];
arr = arr[c];
})
arr[last] = value;
return array;
}

Create array with 5 nested arrays:
var arr = [[[[[]]]], [[[[]]]], [[[[]]]], [[[[]]]], [[[[]]]], [[[[]]]]];
var c = 20;
for (i=0;i<5;i++)
arr[i][0][0][0] = c;
alert(arr[2][0][0][0]);
EDIT: if you dig into functional programming and recursion, you can initialize your multidimensional array with just a few lines of code. Let's say you want 4-dimensional array with length 10 of each dimension:
function createNDimensionalArray(n, length) {
return n === 1
? new Array(length)
: Array.apply(null, Array(length)).map(createNDimensionalArray.bind(null, n - 1, length));
}
var arr = createNDimensionalArray(4, 10);
console.log(arr); // creates 4-dimensional array 10x10x10x10
Notice that initialization like this could be very slow if you create very big arrays (e.g. createNDimensionalArray(5, 10000).
If you prefer to set length of each dimension, you can modify previous the solution like this:
function createNDimensionalArray(dims) {
return dims.length === 1
? new Array(dims[0])
: Array.apply(null, Array(dims[0])).map(createNDimensionalArray.bind(null, dims.slice(1)));
}
var arr = createNDimensionalArray([2, 3, 4, 5]);
console.log(arr); // creates 4-dimensional array 2x3x4x5

Named objects and collection of them

not sure how to ask tbh :)
I'm used of PHP's associative arrays so much that I struggle to understand how to create an "named array" of objects.
Example:
I have two arrays, two ints and one boolean. This represents one of my entities. I have multiple entities on which I'm doing some work.
In PHP I would write:
$entitites[$entitity_id]['items'][] = $item;
$entitites[$entitity_id]['items_status'][] = $item_status;
$entitites[$entitity_id]['items_count']++;
and so on..
How do I do this with objects in JS?
var entities = {items:[], items_status: [], items_count: 0};
entities[entity_id].items.push(item)
How does one name his object for later access (via name or in my case, entity_id?)
This code doesnt work for me to this extend that my webpage goes blank without any errors produced :S
I also tried this:
var entities = {};
var entity = {items:[], items_status: [], items_count: 0};
but then I dont know how to always add values to already existing object in entities object and how to call that exact object via name eg. entity_id.
Halp :(

Keep entities as an object. Then you can just go ahead and add each entity_id as a key and an object which has all the details of that entity as the value.
var entities = {};
entities["1234"] = {
"items" : [],
"items_status" : [],
"items_count" : 0
};

There are 2 types involved here: Objects & Arrays.
Arrays are simple and you're probably familiar with them from any other language:
var myArray = []; // this is an empty array
myArray[0] = 1;
myArray[1] = 2;
myArray[2] = 3;
// you could also use "var myArray = [1, 2, 3];" instead
alert(myArray[1]); // alerts the value 2
Note: arrays are actually objects, and can have non-index properties as well
You can also use various array functions such as .push(), .pop(), .shift() and so on to mutate the array instead.
Objects share the square brackets notation, but the purpose is different:
var myObject = {}; // this is an empty object
myObject[0] = 1;
myObject[1] = 2;
myObject[2] = 3;
alert(myObject[1]); // alerts the value 2
// but also...
myObject['prop'] = 4;
alert(myObject['prop']); // alerts the value 4
// and
myObject.prop2 = 5;
alert(myObject.prop2); // alerts the value 5
// and lastly
alert(myObject.prop); // alerts the value 4
So while arrays are accessed by index, objects are accessed by property names.
As for your entities, it looks like an array of objects. Lets see how we can do that:
function Entity() {
this.items = [];
this.items_status = [];
this.items_count = 0;
}
var entitites = [];
entities.push(new Entity());
entities[0].items = [1, 2, 3];
entities[0].items_status = ['good', 'good', 'poor'];
entities[0].items_count = 3;
Or you can wrap insertion in a more elegant function:
Entity.prototype.insert(item, status) {
this.items.push(item);
this.items_status.push(status);
this.items_count++;
}
entities[0].insert(4, 'excellent!');

If you want to keep control of the indexes in your JS array you can do so by not using .push() :
var entities = [];
entities[5] = {items:[], items_status:[], items_count:0};
Just replace 5 by your integer entity_id variable, and there you go.

You can use a regular javascript object to create the associative array you're looking for.
Actually it's PHP's implementation that's abit off but all they do is call it different (associative array) to most other language that simply refer to it as an object or hash.
You can use numeric keys in JS and still access them with the [] square brackets.
It works like this:
var my_obj = {};
my_obj[5] = 'any value';
console.log(my_obj); // {5: 'any value'}
JS will not add any redundant undefined to missing indexes either so when looping over the collection you won't loop over undefined.
Also, I can access the object by using the key as a string or as number so you won't have to check if the key is the right type. Taken from the above example:
console.log(my_obj['5']); // 'any value'
console.log(my_obj[5]); // 'any value'
JS Objects are the equivelant of PHP assoc arrays except JS objects are much more flexible than PHP's associative arrays.
The only downside to this is that you can't have duplicate keys.
No two keys may exist that share the same name, in an array if you .push(an_item) it will create a new index making even a duplicate data entry unique but when overwriting a key with a new value only the last value will persist, mind that :)

Get the length of an array within a JSON object

I want to find out how many items are in my JSON object. I have seen many answers but they all deal with a common value to loop through. I am wanting the results to tell me that there are 2 items in this object. Any help would be great!
[{"manager_first_name":"jim","manager_last_name":"gaffigan"}]

You could use Object.keys in newer browsers. It would return an array of all the keys in the object, and that array would have a length property that will tell you how many items there are in the object :
var arr = [{"manager_first_name":"jim","manager_last_name":"gaffigan"}];
var length = Object.keys(arr[0]).length;
FIDDLE
In non-supporting browsers, you have to iterate
var arr = [{"manager_first_name":"jim","manager_last_name":"gaffigan"}];
var i = 0;
for (var key in arr[0]) i++;
FIDDLE

You can do this:
var arr = [{"manager_first_name":"jim","manager_last_name":"gaffigan"}],
length = 0,
obj = arr[0]; // Get first obj from array
for(var k in obj){
if( obj.hasOwnProperty(k) ) {
length++;
}
}
console.log(length); // Shows 2
You should use hasOwnProperty because you can also extend an Object with functions, there could otherwise also be count als keys.
Links:
hasOwnProperty

Try
var jsonArr = [{"manager_first_name":"jim","manager_last_name":"gaffigan"}];
var itemCount = JSON.stringify(jsonArr).split('":"').length - 1;
This is, of course, a rather coarse(and unreliable) way of doing it, but if you just want the item count, this should work like a charm.

Can't reference array by index

I have an array defined as:
var subjectCache = [];
I then have some code to build it up, which is working ok.
However, if I try to reference the array by an index, e.g.:
var x = subjectCache[0];
or
var x = subjectCache[1];
I get undefined.
Also subjectCache.length is always 0 (zero).
if I try to reference it by its key, e.g.:
var x = subjectCache['12345'];
it works.
Is this normal? Shouldn't I be able to reference it by its index whatever?
I'm using Internet Explorer, if it makes a difference (and it probably does :( )
[Edit]
this is the code I'm using to build the array, although I really don't think it is to blame.
It's a callback from a webservice call. This is working fine and the array is being populated.
var subjectCache = [];
var subjectCacheCount = 0;
function refreshSubjectsCallback(data) {
// update subjects
// loop through retrieved subjects and add to cache
for( i=0; i < data.length; i++ )
{
var subject = data[i];
var subjectid = subject.SubjectId;
subjectCache[subjectid] = subject;
subjectCacheCount += 1;
}
}
[/Edit]

You're probably assigning keys manually instead of using subjectCache.push() to add new elements to the array:
var array = [];
array['foo'] = 'bar';
console.log(array.length); // 0
The length attribute isn't going to reflect those changes the way you'd expect:
> var a = [];
undefined
> a[100] = 2; // The previous `100` entries evaluate to `undefined`
2
> a.length;
101
Instead, use an object:
var object = {};
object['foo'] = 'bar';
for (var key in object) {
var value = object[key];
console.log(value);
}

From your symptoms, it sounds like you are trying to treat the array as an associative array.
In Javascript, arrays work like this:
var a = [];
a[1] = 10;
alert(a.length);
Objects work like this:
var o = {};
o.myProp = true;
o["myOtherProp"] = false;

Arrays only work with numeric keys not strings. Strings assign properties to the object, and aren't counted as part of length nor it's numeric indices.
When building the array, make sure you are assigning to a numeric position within the array.

No, it will not work, because you haven't created arrays but objects.
you will have to access it by its key.
var x = subjectCache['12345'];
If this works and subjectCache.length doesn't, I think you are making an object not an array. You are confused.

Somewhere along the road you lost the array, and the variable subjectCache points to a different kind of object.
If it was an array, it can't have the length zero and contain an item that is reachable using subjectCache['12345']. When you access an item in an array it doesn't make any difference if you use a numeric index or a string representing a number.