JavaScript HTML DOM Collections

The HTMLCollection Object

The getElementsByTagName() method returns an HTMLCollection object.

An HTMLCollection object is an array-like list (collection) of HTML elements.

The following code selects all <p> elements in a document:

Example

var x = document.getElementsByTagName(“p”);

The elements in the collection can be accessed by an index number.

To access the second <p> element you can write:

y = x[1];

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Note: The index starts at 0.


HTML HTMLCollection Length

The length property defines the number of elements in an HTMLCollection:

Example

var myCollection = document.getElementsByTagName(“p”);
document.getElementById(“demo”).innerHTML = myCollection.length;

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Example explained:

  1. Create a collection of all <p> elements
  2. Display the length of the collection

The length property is useful when you want to loop through the elements in a collection:

Example

Change the background color of all <p> elements:

var myCollection = document.getElementsByTagName(“p”);
var i;
for (i = 0; i < myCollection.length; i++) {
myCollection[i].style.backgroundColor = “red”;
}

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An HTMLCollection is NOT an array!

An HTMLCollection may look like an array, but it is not.

You can loop through the list and refer to the elements with a number (just like an array).

However, you cannot use array methods like valueOf(), pop(), push(), or join() on an HTMLCollection.