The item() method returns the element at a specified index in an HTMLCollection.
Iterate Through Elements Returned by getElementsByClassName getElementsByClassName method is to use the for-of loop. We call document. getElementsByClassName with 'slide' to return an HTML element collection object with all the divs. Then we use the for-of loop to loop through each item.
Sometimes, we want to convert an HTMLCollection object into a JavaScript array. This way, we can use array methods to manipulate HTML element objects in the HTMLCollection object.
In response to the original question, you are using for/in
incorrectly. In your code, key
is the index. So, to get the value from the pseudo-array, you'd have to do list[key]
and to get the id, you'd do list[key].id
. But, you should not be doing this with for/in
in the first place.
Summary (added in Dec 2018)
Do not ever use for/in
to iterate a nodeList or an HTMLCollection. The reasons to avoid it are described below.
All recent versions of modern browsers (Safari, Firefox, Chrome, Edge) all support for/of
iteration on DOM lists such nodeList
or HTMLCollection
.
Here's an example:
var list = document.getElementsByClassName("events");
for (let item of list) {
console.log(item.id);
}
To include older browsers (including things like IE), this will work everywhere:
var list = document.getElementsByClassName("events");
for (var i = 0; i < list.length; i++) {
console.log(list[i].id); //second console output
}
Explanation For Why You Should Not Use for/in
for/in
is meant for iterating the properties of an object. That means it will return all iterable properties of an object. While it may appear to work for an array (returning array elements or pseudo-array elements), it can also return other properties of the object that are not what you are expecting from the array-like elements. And, guess what, an HTMLCollection
or nodeList
object can both have other properties that will be returned with a for/in
iteration. I just tried this in Chrome and iterating it the way you were iterating it will retrieve the items in the list (indexes 0, 1, 2, etc...), but also will retrieve the length
and item
properties. The for/in
iteration simply won't work for an HTMLCollection.
See http://jsfiddle.net/jfriend00/FzZ2H/ for why you can't iterate an HTMLCollection with for/in
.
In Firefox, your for/in
iteration would return these items (all the iterable properties of the object):
0
1
2
item
namedItem
@@iterator
length
Hopefully, now you can see why you want to use for (var i = 0; i < list.length; i++)
instead so you just get 0
, 1
and 2
in your iteration.
Evolution of Browser Support for NodeList and HTMLCollection iteration
Following below is an evolution of how browsers have evolved through the time period 2015-2018 giving you additional ways to iterate. None of these are now needed in modern browsers since you can use the options described above.
Update for ES6 in 2015
Added to ES6 is Array.from()
that will convert an array-like structure to an actual array. That allows one to enumerate a list directly like this:
"use strict";
Array.from(document.getElementsByClassName("events")).forEach(function(item) {
console.log(item.id);
});
Working demo (in Firefox, Chrome, and Edge as of April 2016): https://jsfiddle.net/jfriend00/8ar4xn2s/
Update for ES6 in 2016
You can now use the ES6 for/of construct with a NodeList
and an HTMLCollection
by just adding this to your code:
NodeList.prototype[Symbol.iterator] = Array.prototype[Symbol.iterator];
HTMLCollection.prototype[Symbol.iterator] = Array.prototype[Symbol.iterator];
Then, you can do:
var list = document.getElementsByClassName("events");
for (var item of list) {
console.log(item.id);
}
This works in the current version of Chrome, Firefox, and Edge. This works because it attaches the Array iterator to both the NodeList and HTMLCollection prototypes so that when for/of iterates them, it uses the Array iterator to iterate them.
Working demo: http://jsfiddle.net/jfriend00/joy06u4e/.
Second Update for ES6 in Dec 2016
As of Dec 2016, Symbol.iterator
support has been built-in to Chrome v54 and Firefox v50 so the code below works by itself. It is not yet built-in for Edge.
var list = document.getElementsByClassName("events");
for (let item of list) {
console.log(item.id);
}
Working demo (in Chrome and Firefox): http://jsfiddle.net/jfriend00/3ddpz8sp/
Third Update for ES6 in Dec 2017
As of Dec. 2017, this capability works in Edge 41.16299.15.0 for a nodeList
as in document.querySelectorAll()
, but not an HTMLCollection
as in document.getElementsByClassName()
so you have to manually assign the iterator to use it in Edge for an HTMLCollection
. It is a total mystery why they'd fix one collection type, but not the other. But, you can at least use the result of document.querySelectorAll()
with ES6 for/of
syntax in current versions of Edge now.
I've also updated the above jsFiddle so it tests both HTMLCollection
and nodeList
separately and captures the output in the jsFiddle itself.
Fourth Update for ES6 in Mar 2018
Per mesqueeeb, Symbol.iterator
support has been built-in to Safari too, so you can use for (let item of list)
for either document.getElementsByClassName()
or document.querySelectorAll()
.
Fifth Update for ES6 in Apr 2018
Apparently, support for iterating an HTMLCollection
with for/of
will be coming to Edge 18 in Fall 2018.
Sixth Update for ES6 in Nov 2018
I can confirm that with Microsoft Edge v18 (that is included in the Fall 2018 Windows Update), you can now iterate both an HTMLCollection and a NodeList with for/of in Edge.
So, now all modern browsers contain native support for for/of
iteration of both the HTMLCollection and NodeList objects.
You can't use for
/in
on NodeList
s or HTMLCollection
s. However, you can use some Array.prototype
methods, as long as you .call()
them and pass in the NodeList
or HTMLCollection
as this
.
So consider the following as an alternative to jfriend00's for
loop:
var list= document.getElementsByClassName("events");
[].forEach.call(list, function(el) {
console.log(el.id);
});
There's a good article on MDN that covers this technique. Note their warning about browser compatibility though:
[...] passing a host object (like a
NodeList
) asthis
to a native method (such asforEach
) is not guaranteed to work in all browsers and is known to fail in some.
So while this approach is convenient, a for
loop may be the most browser-compatible solution.
Update (Aug 30, 2014): Eventually you'll be able to use ES6 for
/of
!
var list = document.getElementsByClassName("events");
for (const el of list)
console.log(el.id);
It's already supported in recent versions of Chrome and Firefox.
In ES6, you could do something like [...collection]
, or Array.from(collection)
,
let someCollection = document.querySelectorAll(someSelector)
[...someCollection].forEach(someFn)
//or
Array.from(collection).forEach(someFn)
Eg:-
navDoms = document.getElementsByClassName('nav-container');
Array.from(navDoms).forEach(function(navDom){
//implement function operations
});
you can add this two lines:
HTMLCollection.prototype.forEach = Array.prototype.forEach;
NodeList.prototype.forEach = Array.prototype.forEach;
HTMLCollection is return by getElementsByClassName and getElementsByTagName
NodeList is return by querySelectorAll
Like this you can do a forEach:
var selections = document.getElementsByClassName('myClass');
/* alternative :
var selections = document.querySelectorAll('.myClass');
*/
selections.forEach(function(element, i){
//do your stuffs
});
Alternative to Array.from
is to use Array.prototype.forEach.call
forEach:
Array.prototype.forEach.call(htmlCollection, i => { console.log(i) });
map: Array.prototype.map.call(htmlCollection, i => { console.log(i) });
ect...
There's no reason to use es6 features to escape for
looping if you're on IE9 or above.
In ES5, there are two good options. First, you can "borrow" Array
's forEach
as evan mentions.
But even better...
Object.keys()
, which does have forEach
and filters to "own properties" automatically.That is, Object.keys
is essentially equivalent to doing a for... in
with a HasOwnProperty
, but is much smoother.
var eventNodes = document.getElementsByClassName("events");
Object.keys(eventNodes).forEach(function (key) {
console.log(eventNodes[key].id);
});
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