NodeList - Web APIs 编辑

NodeList objects are collections of nodes, usually returned by properties such as Node.childNodes and methods such as document.querySelectorAll().

Although NodeList is not an Array, it is possible to iterate over it with forEach(). It can also be converted to a real Array using Array.from().

However, some older browsers have not implemented NodeList.forEach() nor Array.from(). This can be circumvented by using Array.prototype.forEach() — see this document's Example.

Live vs. Static NodeLists

Although they are both considered NodeLists, there are 2 varieties of NodeList: live and static.

Live NodeLists

In some cases, the NodeList is live, which means that changes in the DOM automatically update the collection.

For example, Node.childNodes is live:

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

Static NodeLists

In other cases, the NodeList is static, where any changes in the DOM does not affect the content of the collection. The ubiquitous document.querySelectorAll() method 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 whether you should cache the list's length.

Properties

NodeList.length

The number of nodes in the NodeList.

Methods

NodeList.item()

Returns an item in the list by its index, or null if the index is out-of-bounds.

An alternative to accessing nodeList[i] (which instead returns  undefined when i is out-of-bounds). This is mostly useful for non-JavaScript DOM implementations.

NodeList.entries()

Returns an iterator, allowing code to go through all key/value pairs contained in the collection. (In this case, the keys are numbers starting from 0 and the values are nodes.)

NodeList.forEach()

Executes a provided function once per NodeList element, passing the element as an argument to the function.

NodeList.keys()

Returns an iterator, allowing code to go through all the keys of the key/value pairs contained in the collection. (In this case, the keys are numbers starting from 0.)

NodeList.values()

Returns an iterator allowing code to go through all values (nodes) of the key/value pairs contained in the collection.

Example

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

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

Don't use for...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...of loops will loop over NodeList objects correctly:

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

Recent browsers also support iterator methods (forEach()) as well as entries(), values(), and keys().

There is also an Internet Explorer-compatible way to use Array.prototype.forEach for iteration:

const list = document.querySelectorAll('input[type=checkbox]');
Array.prototype.forEach.call(list, function (checkbox) {
  checkbox.checked = true;
});

Specifications

Browser compatibility

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