Using Queryselectorall(). Is the Result Returned by the Method Ordered

Using querySelectorAll(). Is the result returned by the method ordered?

The returned node list is ordered. A quick test proved it:

document.querySelectorAll("body, head")[0]; //Returned [object HTMLHeadElement]

Obviously, the <head> tag appears before <body> in a HTML document. The first element of the NodeList is also a <head> element, even if the selector shows body before `head.

From http://www.w3.org/TR/selectors-api/#queryselectorall:

The querySelectorAll() method on the NodeSelector interface must, when
invoked, return a NodeList containing all of the matching Element
nodes within the node’s subtrees, in document order. If there are no
such nodes, the method must return an empty NodeList.

Javascript querySelectorAll('*') return format

'elems' will be a non-live NodeList of element objects. Traversal will be DFS (depth first search)

docs

What type of data does querySelectorAll return?

The Element method querySelectorAll() returns a static (not live) NodeList representing a list of the document's elements that match the specified group of selectors.

For the differences please visit: Difference between HTMLCollection, NodeLists, and arrays of objects

You can use Spread syntax to make that as an array:

var obj1 = {  fname: "Mirajul",  lname: "Momin",  age: 24};console.log(obj1.length);var paraList = [...document.querySelectorAll("p")];console.log(paraList.length);console.log(Array.isArray(paraList));

<p>This is paragraph one</p><p>This is paragraph two</p><p>This is paragraph three</p><p>This is paragraph four</p>

What do querySelectorAll and getElementsBy* methods return?

Your getElementById code works since IDs have to be unique and thus the function always returns exactly one element (or null if none was found).

However, the methods
getElementsByClassName,
getElementsByName,
getElementsByTagName, and
getElementsByTagNameNS
return an iterable collection of elements.

The method names provide the hint: getElement implies singular, whereas getElements implies plural.

The method querySelector also returns a single element, and querySelectorAll returns an iterable collection.

The iterable collection can either be a NodeList or an HTMLCollection.

getElementsByName and querySelectorAll are both specified to return a NodeList; the other getElementsBy* methods are specified to return an HTMLCollection, but please note that some browser versions implement this differently.

Both of these collection types don’t offer the same properties that Elements, Nodes, or similar types offer; that’s why reading style off of document.getElements…(…) fails.
In other words: a NodeList or an HTMLCollection doesn’t have a style; only an Element has a style.

These “array-like” collections are lists that contain zero or more elements, which you need to iterate over, in order to access them.
While you can iterate over them similarly to an array, note that they are different from Arrays.

In modern browsers, you can convert these iterables to a proper Array with Array.from; then you can use forEach and other Array methods, e.g. iteration methods:

Array.from(document.getElementsByClassName("myElement"))
  .forEach((element) => element.style.size = "100px");

In old browsers that don’t support Array.from or the iteration methods, you can still use Array.prototype.slice.call.
Then you can iterate over it like you would with a real array:

var elements = Array.prototype.slice
    .call(document.getElementsByClassName("myElement"));

for(var i = 0; i < elements.length; ++i){
  elements[i].style.size = "100px";
}

You can also iterate over the NodeList or HTMLCollection itself, but be aware that in most circumstances, these collections are live (MDN docs, DOM spec), i.e. they are updated as the DOM changes.
So if you insert or remove elements as you loop, make sure to not accidentally skip over some elements or create an infinite loop.
MDN documentation should always note if a method returns a live collection or a static one.

For example, a NodeList offers some iteration methods such as forEach in modern browsers:

document.querySelectorAll(".myElement")
  .forEach((element) => element.style.size = "100px");

A simple for loop can also be used:

var elements = document.getElementsByClassName("myElement");

for(var i = 0; i < elements.length; ++i){
  elements[i].style.size = "100px";
}

Aside: .childNodes yields a live NodeList and .children yields a live HTMLCollection, so these two getters also need to be handled carefully.

There are some libraries like jQuery which make DOM querying a bit shorter and create a layer of abstraction over “one element” and “a collection of elements”:

$(".myElement").css("size", "100px");

JavaScript why cannot select the elements

According to mdn:

The Document method querySelectorAll() returns a static (not live)
NodeList representing a list of the document's elements that match the
specified group of selectors.

It is like the document.getElementsByClassName which return you a nodelist and you have to specify the index or you could use document.querySelector

const content = document.querySelectorAll('.test');

content[0].classList.add('hide-content');

.Content {
  width: 180px;
  height: 90px;
  background-color: green;
}

.hide-content {
  display: none
}

<div class="Content"></div>
<div class="Content test"></div>
<div class="Content test"></div>

Is order of resulting array of document.querySelectorAll(input[type=checkbox) guaranteed?

1

Yes, the order is guaranteed.

The specification states

The querySelectorAll() methods on the Document, DocumentFragment, and
Element interfaces must return a NodeList containing all of the
matching Element nodes within the subtrees of the context node, in
document order. If there are no matching nodes, the method must return
an empty NodeList.

meaning the elements will be returned in the order they appear in the document

2

If checkboxes are added, the best way to get those elements would be to keep track of them when inserting.

Other than that, wrapping them in another element with an ID or class would work as well, or just giving the new checkboxes a class

var new_boxes = document.querySelectorAll('.new_boxes');

3

Generally you'd just submit the form, and the checked boxes will be sent automatically.

If you're sending the data with ajax, you can get the checked boxes with an attribute selector

var boxes = document.querySelector('input[type="checkbox"][checked]');

How do I parse the results of a querySelectorAll selector engine & allow method chaining?

I want to use it like this...:
$("div").innerHTML='It works!';
...not like this...:
$("div")[0].innerHTML='It works only on the specified index!';

It sounds like you want to have assigning to innerHTML on your set of results assign to the innerHTML of all of the results.

To do that, you'll have to use a function, either directly or indirectly.

Directly:

var $ = function(selector, node) { // Selector engine
    var selector = selector.trim(),
        node = node || document.body,
        rv;
    if (selector != null) {
        rv = Array.prototype.slice.call(node.querySelectorAll(selector), 0); }
        rv.setInnerHTML = setInnerHTML;
    }
    return rv;
}
function setInnerHTML(html) {
    var index;

    for (index = 0; index < this.length; ++index) {
        this[index].innerHTML = html;
    }
}

// Usage
$("div").setInnerHTML("The new HTML");

There, we define a function, and we assign it to the array you're returning as a property. You can then call that function on the array. (You might want to use Object.defineProperty if it's available to set the setInnerHTML property, so you can make it non-enumerable.)

Indirectly (requires an ES5-enabled JavaScript engine):

var $ = function(selector, node) { // Selector engine
    var selector = selector.trim(),
        node = node || document.body,
        rv;
    if (selector != null) {
        rv = Array.prototype.slice.call(node.querySelectorAll(selector), 0); }
        Object.defineProperty(rv, "innerHTML", {
            set: setInnerHTML
        });
    }
    return rv;
}
function setInnerHTML(html) {
    var index;

    for (index = 0; index < this.length; ++index) {
        this[index].innerHTML = html;
    }
}

// Usage
$("div").innerHTML = "The new HTML";

There, we use Object.defineProperty to define a setter for the property.

In the comments below you say

I have a few prototypes that work when individually attached to the $ function. Example: $('div').makeClass('this'); They do not work when they are chained together. Example: $('div').makeClass('this').takeClass('that');

To make chaining work, you do return this; from each of the functions (so the end of makeClass would do return this;). That's because when you're chaining, such as obj.foo().bar(), you're calling bar on the return value of foo. So to make chaining work, you make sure foo returns this (the object on which foo was called).

Using Queryselectorall(). Is the Result Returned by the Method Ordered