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When in JS you need to identify element by array order?

Ok so I finally have a code example to show this!
if ($('#Snowsports-row')[0].classList.contains("hidden") == false) {
$('#snowsports-only').removeClass("hidden")
}
The code works ONLY as written above, i.e., if the [0] were moved to the second line and removed from the first line, or if it were present/absent in both lines, it would fail.
I understand the output difference...
$('#Snowsports-row')
=> [<div>...]
$('#Snowsports-row')[0]
=> <div>...
...but I'm not understanding under what circumstances you're OK to get an array of element(s) and in which you need to tease out the exact element.
THANKS FOR ALL ANSWERS! Very clearly helped me to figure out that the problem may have been confusing JS/jQuery methods. Final version:
if ($('#Snowsports-row').hasClass("hidden") == false) {
$('#snowsports-only').removeClass("hidden")
}

The .classList method is not widely supported (not in MSIE 9.0 for example) so it's not portable, although where it exists it's fast.
Since every ID in a document is supposed to be unique, and since calling removeClass for a class that isn't present is harmless, just replace your entire call with:
$('#Snowsports-row').removeClass('hidden')
Or better yet, if that class means what I think it does, use .hide() and let jQuery do its job for you, potentially animation the transition in the process.
Alternatively, if you actually wanted to stick with using DOM and classList, you should use the .remove() method that classList already supports:
document.getElementById('#Snowsports-row').classList.remove('hidden')
although there's a minor disadvantage in that this code will crash if that element isn't found (since .getElementById will return null) whereas jQuery silently ignores calls made on empty selectors.
As for the meta-question - you use [n] if you want to access the single DOM element at position n within the jQuery object, as you've done when you use .classList.
You use .eq(n) to obtain a jQuery object representing that DOM element, e.g. if you want to apply jQuery methods to that (single) element.
If there's only a single element, or you want the jQuery method to apply to every matching element, just call the method directly on the selector, as I've done above.

First off, by using jQuery for what it's good at, you can replace this:
if ($('#Snowsports-row')[0].classList.contains("hidden") == false) {
$('#snowsports-only').removeClass("hidden")
}
with this:
$('#Snowsports-row').removeClass("hidden");
Your first block of code does the following:
With $('#Snowsports-row'), make a jQuery object that contains all DOM elements that match the select '#Snowsports-row'.
Then reach into the jQuery object with [0] and get the first DOM object in that jQuery object.
Then, use a property/method on that DOM element to determine if a class exists on that DOM element with your .classList.contains("hidden") reference.
Then, if you find that class, remove it.
A jQuery object contains inside it an array of DOM elements. If you call a method on the jQuery object itself like:
$('.tableRows').html("hello");
Then, you are asking jQuery to operate on ALL the DOM elements inside the jQuery object. You must use jQuery methods, not DOM methods.
If, on the other hand, you want to use a method such as .classList.contains(), that is only a method on an actual DOM element. That isn't a jQuery method. So, you have to reach inside of the jQuery object to get a specific DOM element out of it. That's what the [0] does. It reaches into the jQuery object and gets the first DOM element from its internal data structure. Once you have that DOM element, you can then use any DOM element methods on that DOM object.
FYI, if you ever want to get just the first DOM element from a jQuery object, but want the result to be a jQuery object, not just a DOM element, instead of [0], you can use .eq(0) like ths:
$('#Snowsports-row').eq(0).removeClass("hidden");
Now, in this specific case, this is never necessary because $('#Snowsports-row') cannot ever contain more than one DOM element because internally jQuery will only return the first matching DOM element when you are searching for a ID value (since there's never supposed to be more than one matching element with the same ID).
Just keep in mind that DOM element and a jQuery object are completely different types of objects with different methods on them. What makes it slightly confusing is that a jQuery object contains an internal list of DOM elements. But, if the object you are operating on is a jQuery object, then you can only call jQuery methods on it. If you reach into the jQuery object and pull out a DOM element, then you can only call DOM methods on it.

First of all, ids must be unique, so if you have more than one #Snowsports-only elements you can experience problems.
In your question, you are mixing jQuery code with pure Javascript code.
This:
if ($('#Snowsports-row')[0].classList.contains("hidden") {
...
}
Means that you get the first instance of #Snowsports-row (remember that is better if there is only one element with this id), but you get the DOM object (pure javascript) with the jQuery selector. You can do the same thing in jQuery like this:
$('#Snowsports-row').hasClass("hidden")
See more:
https://api.jquery.com/hasclass/
https://developer.mozilla.org/es/docs/Web/API/Element/classList

Sure, because you are operating over a list. Now, you're kind of mistaking the jQuery/javascript code. If you would like to use the same line twice you can basically drop jQuery altogether and write something like this:
var el = document.getElementById('Snowsports-row');
if (el.classList.contains('hidden')){
el.classList.remove('hidden');
}

In the first line you're selecting one specific DOM element, whereas in the second line you are selecting ALL elements in the DOM that fit that selector and removing the "hidden" class from all of them. Basically checking whether the element has a class can only be performed over an element (that's why you need to select the index, specifying a given element), but jQuery allows you to remove the class of every element inside a list (hence your second line)

Use jQuery's .eq() function. So:
var el = $('#Snowsports-row').eq(0);
if (el.hasClass("hidden")) {
$(el.removeClass("hidden")
}
There's also no harm in calling removeClass on an element that might not have that class... so:
$('#Snowsports-row').eq(0).removeClass('hidden');

Checking if an element created by javascript exists using only javascript [duplicate]

This question already has answers here:
How can I check if an element exists in the visible DOM?
(27 answers)
Closed 8 years ago.
The question is quite straightforward. Is there a javascript only (no jQuery/other library pls) method of checking if an element created via javascript exists?
For instance, if I did this:
var foo=document.createElement("div");
document.getElementById("bar").appendChild(foo);
Would there be a way to check if foo already exists?

You could simply get the element by id and see if its not null.
var foo=document.createElement("div");
document.getElementById("bar").appendChild(foo);
foo.setAttribute("id","foo");
var ele = document.getElementById("foo");
if(ele !== null){ alert("Hi");}

foo is not an element, it's a variable whose value is an element. There are different ways to interpret your question.
Is the value of the variable foo a DOM element?
If you want to check if the variable foo exists and has as its value an element, you can say
foo && typeof foo === 'object' && foo.nodeType
or something similar. This will check whether the element is any kind of Node. To check for an element, use foo.nodeType === Node.ELEMENT_NODE.
Is a particular HTML element in the DOM?
If you want to check if the element held in variable foo is in the DOM, as opposed to having been created, but not yet inserted, then:
foo.baseURI
since baseURI is set only upon DOM insertion. You might think you could check parentNode, which is also not set until DOM insertion, but children of the non-inserted element, if any, will report a parentNode, as will elements appended to a document fragment.
The other alternative is to use Node#contains:
document.body.contains(foo)
which essentially does the same thing as scanning the entire DOM for the element:
Array.prototype.some.call(document.querySelector('*'), function(elt) {
return elt === foo;
})
In any case, this all seems pointless, because as far as I can imagine, if you hold a reference to a DOM element, and it's not not in the DOM, then it is in the DOM.
Is there already an element in the place I'm going to be inserting this one?
If you know where the element is going to go, in this case as a child of bar, and that's enough to convince you that the "element already exists", you can check easily enough with something like:
document.getElementById("bar").children.length > 0
Is there already an element with this ID?
As other commenters and responders have noted, you can obviously find an element in the DOM if you have arranged to provide it with some uniquely identifiable characteristics, most likely an id.
I hope it's obvious that element ID is completely separate from the name of any JavaScript variables which happen to be holding references to that element. It is true that elements become available on the root (window) object under their ID or name attributes, a feature best left unused.
Was the element in the source HTML, or created by JavaScript?
From the wording of your question, it seems that you might be interested in whether the element was created by JS, as opposed to originating from the source HTML. There is no way I know of to detect that. If you are intent on doing so, override document.createElement as follows:
document.createElement = (function() {
var old = document.createElement;
return function(tagName) {
var elt = old.call(document, tagName);
elt.I_CREATED_THIS = true;
return elt;
};
}());
> document.body.I_CREATED_THIS
undefined
> elt = document.createElement('span');
> elt.I_CREATED_THIS
true

can I use document.prototype.getElementById to get an element in another element, not in the whole document

I recently found on this javascript tip :
element_number = Array.prototype.indexOf.call(element_1, element_2);
It allows developers to use the indexOf method on an object wich is not an Array.
I would like to know if it is possible to use a similar syntax to call the getElementById method but not on the whole document (document.getElementById), just on an element like this :
my_div_2 = document.prototype.getElementById.call(div_1, "id_of_my_div_2");
The idea is that my document contains tabs and elements having the same id can be present several times in the document.
If it is not possible, did somebody write a function doing that :
Search in an element another element by id.

No, but you can accomplish what you want using querySelector instead.
my_div_2 = div_1.querySelector("#id_of_my_div_2");
Not supported in IE6/7.
Docs: MDN element.querySelector()
That said, duplicate IDs are invalid.
This technique is useful if the same script is used on different pages where the ID may or may not be in a particular container.

No. The getElementById method must be applied on objects of type Document, otherwise it would throw a WRONG_THIS_VALUE exception. You could try it with
myDiv1 = document.getElementById("div1");
myDiv2 = document.getElementById.call(myDiv1, "div2"); // no ".prototype"!

This makes no sense as only one element with a given id is possible in a document.
So the only right syntax would be
my_div_2 = document.getElementById("id_of_my_div_2");
Don't reuse an id in a document. You probably can achieve the desired result using classes instead of id.

Javascript - extract object ID from a Javascript object

So I am not sure if my title is clear enough. I essentially have a div saved as a Javascript object which looks like this: [div#field_30.checkbox_group]
The field_30 is the ID which I am trying to extract here. doing something like object.id is not working. Does anyone know how to get the ID?
Note: I saved the object like this: var object = $(".workspace .selected"); which grabs the currently selected div inside the object called workspace. Sorry is this is a rookie mistake, I just can't seem to find anything anywhere. Thanks for the help...

var object = $(".workspace .selected"); will return a jQuery wrapped element that has jQuery properties and methods rather than element properties and methods. This means that any of
object[0].id
object.prop("id")
object.attr("id")
should work, but the 1st option should be the best performance-wise. It gets the id property of the the 1st element contained by the jQuery object, which is your div.

Your object is in fact a jQuery object, not a dom object.
To use the dom object use,
object[0].id
Or using, jquery, (Since it is already there)
object.prop('id');

You can use either $jquery_object.attr('id') or $jquery_object.eq(0).id
See this for exemple: http://jsfiddle.net/cquuT/

In this case it looks like object is the result of a jQuery select. To get to the actual DOM object you need to use [0]. Then you can access the id property
object[0].id

I don't see a complete answer here, so I'll provide my own.
If you're using jQuery selector $(), then you'll get jQuery-wrapped collection, not a single element.
(I assume now that you're using jQuery 1.5.2, the same as StackOverflow uses now.)
Universal solution to get ids of all elements returned by selector is:
.map(function(){ return this.id; })
Running $(".post-text").map(function(){ return this.id; }) on current page will return something like: ["", "", "", "", ""]
To get id of the first element returned by selector use:
.attr('id')
Running $("div").attr('id') on current page will return "notify-container".
Since jQuery 1.6 you can also use .prop('id') here.
If you know, that query will return only one element or you just want the first element matching given selector, then use .attr which is obviously a simpler solution.

Unique identifier for HTML elements

Besides the ID, if you say you want a unique identifier for an HTML element (let’s say a div).
I browsed the DOM for something (like a number or string) that was unique for each element; but the DOM was big and I failed to find that on the Internet.
Is there a property (in the DOM obviously) that is unique only to that element? (Other than the ID and also you don't specify it, but it comes when the DOM is constructed.)

Depending on the objective, here are two suggestions.
Unless you actually need to express the id as some kind of string, you can save the normal DOM reference.
If you do need to express it as a string for some reason, then you'll need to assign a unique id yourself.
var getId = (function () {
var incrementingId = 0;
return function(element) {
if (!element.id) {
element.id = "id_" + incrementingId++;
// Possibly add a check if this ID really is unique
}
return element.id;
};
}());

The only other identifier I can think of is the XPath of the element in the document.
For instance, the title link inside the heading of this very page has an XPath of
/html/body/div[3]/div[2]/div/h1/a
But like Pekka already said, it depends on what you want to do. And I don’t think you can get the XPath easily from the DOM in JavaScript, despite XPath being available nowadays in JavaScript engines.

Internet Explorer has a property, "uniqueID", for every element. The problem is that the other browsers don't support it.

You can use a library or roll your own to create a unique identifier. jQuery has .data():
Store arbitrary data associated with the matched elements or return
the value at the named data store for the first element in the set of
matched elements.

I just encountered the same situation, and while I was looking into some DOM elements in the Chrome developer tools inspector, I noticed that they all seem to have a property like jQuery11230892710877873282 assigned with a unique number.
Obviously the number after 'jQuery' is different every time you load the page. My guess is that jQuery is generating this internally every time it tries to access or manipulate any DOM element.
I played a little bit with it, and it looks like elements that are never accessed/manipulated by jQuery may not have this property, but the moment you do something like $(elem), the property will be there. So, since we're using both jQuery and Lodash, I devised the following function to return a unique ID regardless of whether the element actually has a DOM id attribute.
_.reduce(
$(elem),
function(result, value, key) {
if(_.startsWith(key, 'jQuery'))
return value;
},
0)

There is the name attribute that can be addressed by document.getElementByName.
I don't think other unique identifiers exist - even though you could simulate one by setting a property (like title) to a unique value, and then query for that. But that is kludgy.