The difference between using document.getElementsByTagName("img") and document.getElementById('testimg')

My HTML code is as follows:

<a href="index.php"><img id="testimg"   src="images/logo.png"/></a>

This is my JavaScript code:

function getW(){
    var theImg = document.getElementById('testimg');
    return theImg;
}

theImg = getW();

if (theImg.width > 119){
    document.write(theImg.width);
}

When I run this script, it outputs the width of the image.

However, when I use the following script:

function getW(){
    var theImg = document.getElementsByTagName("img"); 
    return theImg;
}

theImg = getW();

if (theImg.width > 119){
    document.write(theImg.width);
}

It does not output anything. What could be the reason for this difference in behavior between the two scripts?

Thank you!

Answer №1

When using the getElementsByTagName() method, it retrieves a collection of elements rather than just one. In order to access a specific element within this collection, you would need to specify its index, such as [0].

Conversely, the getElementById() method is designed to target an element with a unique identifier (id) on the page. This means that it will always return a reference to a single element.

Answer №2

The function gEBTN will give you a list of nodes. Use theImg[0] to access the first element.

To address your second question, simply create a for loop based on the length property of the nodeList.

Answer №3

The method getElementsByTagName() will retrieve a collection of nodes (elements) that correspond to the tag name specified. This means that while the first code snippet retrieves a single element, the second one engages with an array.

If you intend to extract a specific image using getElementsByTagName, you must conduct either an attribute search (such as locating a relevant name or id tag) or be aware of its position on the page.

In the provided instance, theImg[0] will specifically return the desired image.

Answer №4

Update the code snippet in your project:

let image = document.getElementsByTagName("img");

to this alternative:

let image = document.getElementsByTagName("img").item(0);

or you can use this version:

let image = document.getElementsByTagName("img")[0];

Both options are identical, note that .item(0) is the same as using [0].

If the image you want is not the first one, make sure to adjust the index number accordingly.

Answer №5

When using the method

document.getElementById('testimg')
, a real element object with an id of testimg will be returned. Similarly, calling
document.getElementsByTagName("img")
will provide an array containing all element objects with the tag img.

It is important to note that if there are multiple elements with the same id of testimg, the method

document.getElementById('testimg')
will only return the first one it encounters.

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