Difference between HTMLCollection, NodeLists, and

I've always been confused between HTMLCollections, objects, and arrays when it comes to DOM. For instance...

What is the difference between document.getElementsByTagName("td") and $("td")?
$("#myTable") and $("td") are objects (jQuery objects). Why is console.log also showing the array of DOM elements beside them, and are they not objects and not an array?
What is the elusive "NodeLists" all about, and how do I select one?

Please also provide any interpretation of the below script.

Thank you

[123,"abc",321,"cba"]=[123,"abc",321,"cba"]
{123:123,abc:"abc",321:321,cba:"cba"}=Object { 123=123, abc="abc", 321=321, more...}
Node= Node { ELEMENT_NODE=1, ATTRIBUTE_NODE=2, TEXT_NODE=3, more...}
document.links= HTMLCollection[a #, a #]
document.getElementById("myTable")= <table id="myTable">
document.getElementsByClassName("myRow")= HTMLCollection[tr.myRow, tr.myRow]
document.getElementsByTagName("td")= HTMLCollection[td, td, td, td]
$("#myTable")= Object[table#myTable]
$("td")= Object[td, td, td, td]


<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
<html xmlns="http://www.w3.org/1999/xhtml"> 
    <head> 
        <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1" /> 
        <title>Collections?</title>  
        <script src="http://code.jquery.com/jquery-latest.js" type="text/javascript"></script> 
        <script type="text/javascript">
            $(function(){
                console.log('[123,"abc",321,"cba"]=',[123,"abc",321,"cba"]);
                console.log('{123:123,abc:"abc",321:321,cba:"cba"}=',{123:123,abc:"abc",321:321,cba:"cba"});
                console.log('Node=',Node);
                console.log('document.links=',document.links);
                console.log('document.getElementById("myTable")=',document.getElementById("myTable"));
                console.log('document.getElementsByClassName("myRow")=',document.getElementsByClassName("myRow"))
                console.log('document.getElementsByTagName("td")=',document.getElementsByTagName("td"));
                console.log('$("#myTable")=',$("#myTable"));
                console.log('$("td")=',$("td"));
            });
        </script>
    </head>

    <body>
        <a href="#">Link1</a>
        <a href="#">Link2</a>
        <table id="myTable">
            <tr class="myRow"><td>td11</td><td>td12</td></tr>
            <tr class="myRow"><td>td21</td><td>td22</td></tr>
        </table>
    </body> 
</html>

标签： javascript jquery dom

5条回答

刘海飞了

2楼-- · 2019-01-02 14:54

Additional note

What is the difference between a HTMLCollection and a NodeList?

A HTMLCollection contains only element nodes (tags) and a NodeList contains all nodes.

There are four node types:

element node
attribute node
text node
comment node

nodeTypes

Whitespace inside elements is considered as text, and text is considered as nodes.

Consider the following:

<ul id="myList">
  <!-- List items -->
  <li>List item 1</li> 
  <li>List item 2</li>
  <li>List item 3</li>
  <li>List item 4</li>
  <li>List item 5</li>
</ul>

Whitespace: <ul id="myList"> <li>List item</li></ul>

No whitespace: <ul id="myList"><li>List item</li></ul>

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梦寄多情

3楼-- · 2019-01-02 14:57

$("td") is extended jQuery object and it has jQuery methods, it returns jquery object that contains array of html objects. document.getElementsByTagName("td") is raw js method and returns NodeList. See this article

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不流泪的眼

4楼-- · 2019-01-02 14:59

The NodeList objects are collections of Node's, returned for example by x.childNodes property or document.querySelectorAll() method. In some cases, the NodeList is live, which means that changes in the DOM automatically update the collection! For example, Node.childNodes is live:

var c = parent.childNodes; //assume c.length is 2
parent.appendChild(document.createElement('div'));
//now c.length is 3, despite the `c` variable is assigned before appendChild()!!
//so, do not cache the list's length in a loop.

But in some other cases, the NodeList is static, where any changes in the DOM does not affect the content of the collection. querySelectorAll() returns a static NodeList.

The HTMLCollection is a live and ordered collection of elements (it is automatically updated when the underlying document is changed). It can be result of properties like as children or methods like as document.getElementsByTagName(), and could only have HTMLElement's as their items.

HTMLCollection also exposes its members directly as properties by both name and index:

var f = document.forms; // this is an HTMLCollection
f[0] === f.item(0) === f.myForm //assume first form id is 'myForm'

The HTMLElement is just one type of Nodes:

The Node can be several types. Most important ones are as following:

element (1): An Element node such as <p> or <div>.
attribute (2): An Attribute of an Element. The element attributes are no longer implementing the Node interface in DOM4 specification!
text (3): The actual Text of Element or Attribute.
comment (8): A comment node.
document (9): A document node.

So, a big difference is that HTMLCollection contains only HTMLElements but NodeList also contains the comments, white-space texts (carriage return chars, spaces..), etc. Check it as in following snippet:

function printList(x, title) {
  console.log("\r\nprinting "+title+" (length="+x.length+"):");
  for(var i=0; i<x.length; i++) {
    console.log("  "+i+":"+x[i]);
  }
}

var elems = document.body.children; //HTMLCollection
var nodes = document.body.childNodes; //NodeList

printList(elems, "children [HTMLCollection]");
printList(nodes, "childNodes [NodeList]");

<div>para 1</div><!-- MyComment -->
<div>para 2</div>

Both HTMLCollection and NodeList contain the length property you can use to loop over their items. Don't use for...in or for each...in to enumerate the items in NodeLists, since they will also enumerate its length and item properties and cause errors if your script assumes it only has to deal with element objects. Also, for..in is not guaranteed to visit the properties in any particular order.

for (var i = 0; i < myNodeList.length; i++) {
  var item = myNodeList[i];
}

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有味是清欢

5楼-- · 2019-01-02 15:00

0. What is the difference between an `HTMLCollection` and a `NodeList`?

Here are some definitions for you.

DOM Level 1 Spec - Miscellaneous Object Definitions:

Interface HTMLCollection

An HTMLCollection is a list of nodes. An individual node may be accessed by either ordinal index or the node's name or id attributes. Note: Collections in the HTML DOM are assumed to be live meaning that they are automatically updated when the underlying document is changed.

DOM Level 3 Spec - NodeList

Interface NodeList

The NodeList interface provides the abstraction of an ordered collection of nodes, without defining or constraining how this collection is implemented. NodeList objects in the DOM are live.

The items in the NodeList are accessible via an integral index, starting from 0.

So they can both contain live data which means that the DOM will update when their values do. They also contain a different set of functions.

You will note if you inspect the console if you run your scripts that the table DOM element contains both a childNodes NodeList[2] and a children HTMLCollection[1]. Why are they different? Because HTMLCollection can only contain element nodes, NodeList also contains a text node.

enter image description here

1. What is the difference between `document.getElementsByTagName("td")` and `$("td")`?

document.getElementsByTagName("td") returns an array of DOM elements (a NodeList), $("td") is called a jQuery object which has the the elements from document.getElementsByTagName("td") on its properties 0, 1, 2, etc. The main difference is that the jQuery object is a little slower to retrieve but gives access to all the handy jQuery functions.

2. `$("#myTable")` and `$("td")` are objects (`jQuery` objects). Why is `console.log` also showing the array of DOM elements beside them, and are they not objects and not an array?

They are objects with their properties 0, 1, 2, etc. set to the DOM elements. Here's a simple example: of how it works:

jsFiddle

    var a = {
        1: "first",
        2: "second"
    }
    alert(a[1]);

3. What is the elusive "NodeLists" all about, and how do I select one?

You have been retrieving them in your code, getElementsByClassName and getElementsByTagName both return NodeLists

NodeList

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十年一品温如言

6楼-- · 2019-01-02 15:05

First I will explain the difference between NodeList and HTMLCollection.

Both interfaces are collections of DOM nodes. They differ in the methods they provide and in the type of nodes they can contain. While a NodeList can contain any node type, an HTMLCollection is supposed to only contain Element nodes.
An HTMLCollection provides the same methods as a NodeList and additionally a method called namedItem.

Collections are always used when access has to be provided to multiple nodes, e.g. most selector methods (such as getElementsByTagName) return multiple nodes or getting a reference to all children (element.childNodes).

For more information, have a look at DOM4 specification - Collections.

What is the difference between document.getElementsByTagName("td") and $("td")?

getElementsByTagName is method of the DOM interface. It accepts a tag name as input and returns a HTMLCollection (see DOM4 specification).

$("td") is presumably jQuery. It accepts any valid CSS/jQuery selector and returns a jQuery object.

The biggest differences between standard DOM collections and jQuery selections is that DOM collections are typically live (not all methods return a live collection though), i.e. any changes to the DOM are reflected in the collections if they are affected. They are like a view on the DOM tree, whereas jQuery selections are snapshots of the DOM tree in the moment the function was called.

Why is console.log also showing the array of DOM elements beside them, and are they not objects and not an array?

jQuery objects are array-like objects, i.e. they have numeric properties and a length property (keep in mind that arrays are just objects themselves). Browsers tend to display arrays and array-like objects in a special way, like [ ... , ... , ... ].

What is the elusive "NodeLists" all about, and how do I select one?

See the first part of my answer. You cannot select NodeLists, they are the result of a selection.

As far as I know there is not even a way to create NodeLists programatically (i.e. creating an empty one and adding nodes later on), they are only returned by some DOM methods/properties.

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