NodeList

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NodeList objects are collections of nodes such as those returned by Node.childNodes and the document.querySelectorAll method.

Properties

length
The number of nodes in the NodeList.

Methods

item ( idx )
Returns an item in the list by its index, or null if the index is out-of-bounds; can be used as an alternative to simply accessing nodeList[idx] (which instead returns  undefined when idx is out-of-bounds).

Description

A sometimes-live collection

In some cases, the NodeList is a live collection, which means that changes in the DOM are reflected in the collection. For example, Node.childNodes is live:

var parent = document.getElementById('parent');
var child_nodes = parent.childNodes;
console.log(child_nodes.length); // let's assume "2"
parent.appendChild(document.createElement('div'));
console.log(child_nodes.length); // should output "3"

In other cases, the NodeList is a static collection, meaning any subsequent change in the DOM does not affect the content of the collection. document.querySelectorAll returns a static NodeList.

It's good to keep this distinction in mind when you choose how to iterate over the items in the NodeList, and how you cache the length of the list in particular.

Why is NodeList not an Array?

NodeList are used very much like arrays and it's tempting to invoke Array.prototype methods on them, however NodeList objects don't have any of the familiar Array methods.

JavaScript has an inheritance mechanism based on prototypes for both built–in objects (like Arrays) and host objects (like NodeLists). Array instances inherit array methods (such as forEach or map) because their prototype chain looks like the following:

myArray --> Array.prototype --> Object.prototype --> null (The prototype chain of an object can be obtained by calling Object.getPrototypeOf several times.)

forEach, map and the likes are own properties of the Array.prototype object.

Unlike arrays, NodeList prototype chain looks like the following:

myNodeList --> NodeList.prototype --> Object.prototype --> null

NodeList.prototype contains the item method, but none of the Array.prototype methods, so they cannot be used on NodeLists.

Workarounds

One idea would be to add Array.prototype methods to NodeList.prototype. However, be aware that Extending the DOM is dangerous, especially in old version of Internet Explorer (6, 7, 8).

var arrayMethods = Object.getOwnPropertyNames( Array.prototype );

arrayMethods.forEach( attachArrayMethodsToNodeList );

function attachArrayMethodsToNodeList(methodName)
{
  if(methodName !== "length") {
    NodeList.prototype[methodName] = Array.prototype[methodName];
  }
};
 
var divs = document.getElementsByTagName( 'div' );
var firstDiv = divs[ 0 ];

firstDiv.childNodes.forEach(function( divChild ){
  divChild.parentNode.style.color = '#0F0';
});

Another approach without extending the DOM:

var forEach = Array.prototype.forEach;

var divs = document.getElementsByTagName( 'div' );
var firstDiv = divs[ 0 ];

forEach.call(firstDiv.childNodes, function( divChild ){
  divChild.parentNode.style.color = '#0F0';
});

Note that in the above, passing a host object (like a NodeList) as this to a native method (such as forEach) is not guaranteed to work in all browsers and is known to fail in some.

Example

It's possible to loop over the items in a NodeList using:

for (var i = 0; i < myNodeList.length; ++i) {
  var item = myNodeList[i];  // Calling myNodeList.item(i) isn't necessary in JavaScript
}

Don't be tempted to use for...in or for each...in to enumerate the items in the list, since that will also enumerate the length and item properties of the NodeList 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...of loops will loop over NodeList objects correctly, in browsers that support for...of (like Firefox 13 and later):

var list = document.querySelectorAll( 'input[type=checkbox]' );
for (var item of list) {
  item.checked = true;
}

Converting a NodeList to an Array

Sometimes it's more convenient to work with the content of a NodeList using familiar Array methods. Here is a technique for converting a NodeList object to an Array:

var div_list = document.querySelectorAll('div'); // returns NodeList
var div_array = Array.prototype.slice.call(div_list); // converts NodeList to Array

Specification

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Contributors to this page: fscholz
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