javascript get element by id and class with code examples

JavaScript is an important programming language used by developers for creating dynamic web pages. One of the most common tasks that developers need to perform while working with JavaScript is to retrieve elements from HTML documents by their ID or class.

In this article, we will explore the concept of getting elements by ID or class and discuss code examples to demonstrate how to do it effectively.

Getting Elements by ID

IDs are unique identifiers assigned to elements in HTML documents. They provide a fast and efficient way to access an element by its unique identifier without having to traverse the entire document tree.

The getElementById() method is used in JavaScript to retrieve elements by their ID. Here is an example of its usage:

<!DOCTYPE html>
<html>
<head>
	<title>Get Element By ID Example</title>
	<script>
		function changeText() {
			document.getElementById("demo").innerHTML = "Hello JavaScript!";
		}
	</script>
</head>
<body>

	<h1 id="demo">Welcome to JavaScript Programming!</h1>

	<button type="button" onclick="changeText()">Click me to change text</button>

</body>
</html>

In the above example, we have a heading element with an ID of "demo". The changeText() function is called when the button is clicked, and it retrieves the heading element using its ID and updates its innerHTML property to display the message "Hello JavaScript!".

We can also use the getElementById() method to retrieve form elements like input fields and select boxes. Here is an example:

<!DOCTYPE html>
<html>
<head>
	<title>Get Form Element By ID Example</title>
	<script>
		function getInputValue() {
			var inputField = document.getElementById("myInput").value;
			alert("You entered: " + inputField);
		}
	</script>
</head>
<body>

	<label for="myInput">Enter your name:</label>

	<input type="text" id="myInput"/>

	<button type="button" onclick="getInputValue()">Click me to get input value</button>

</body>
</html>

In the above example, we have an input field with an ID of "myInput". When the button is clicked, the getInputValue() function retrieves the value of the input field using its ID and displays it in an alert box.

Getting Elements by Class

Class names are used to group elements with similar styling or functionality together. By assigning the same class name to multiple elements, developers can apply styles, add event listeners, or perform other tasks on all of them at once.

The getElementsByClassName() method is used in JavaScript to retrieve elements by their class name. Here is an example of its usage:

<!DOCTYPE html>
<html>
<head>
	<title>Get Element By Class Example</title>
	<script>
		function changeAllText() {
			var elements = document.getElementsByClassName("demo");
			for (var i = 0; i < elements.length; i++) {
				elements[i].innerHTML = "Hello JavaScript!";
			}
		}
	</script>
	<style>
		.demo {
			font-weight: bold;
		}
	</style>
</head>
<body>

	<p class="demo">This is a paragraph.</p>
	<p class="demo">This is another paragraph.</p>

	<button type="button" onclick="changeAllText()">Click me to change text in all paragraphs</button>

</body>
</html>

In the above example, we have two paragraphs with the same class name of "demo". The changeAllText() function retrieves all the paragraphs with the "demo" class using the getElementsByClassName() method, loops through them, and updates their innerHTML property to display the message "Hello JavaScript!".

Note that the getElementsByClassName() method returns a collection of elements, so we need to loop through them and apply our desired changes to each element individually.

Conclusion

Retrieving elements from HTML documents using their ID or class is a common task performed by developers working with JavaScript. The getElementById() and getElementsByClassName() methods provide efficient ways to retrieve elements by their unique identifiers and class names, respectively. By understanding these concepts and using the right methods, developers can create dynamic web pages with ease and efficiency.

let's dive deeper into the concepts of getting elements by ID and class in JavaScript.

Getting Elements by ID

The getElementById() method is used to retrieve an element with a specific ID from an HTML document. It returns the element as an object, which can then be manipulated using JavaScript.

Syntax:

var element = document.getElementById(id);

Here, the id parameter is the ID of the element you want to retrieve. The getElementById() method can be called from within a script tag or a separate JavaScript file.

Code Example:

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript DOM manipulation</h2>
<p id="demo">This is a paragraph</p>

<script>
var para = document.getElementById("demo"); // retrieve the element with ID "demo"
para.style.color = "red"; // change the color of the paragraph to red
para.innerHTML = "Hello World!"; // change the text of the paragraph
</script>

</body>
</html>

In the above example, we have a simple HTML document with a paragraph element that has an ID of "demo". The JavaScript code uses the getElementById() method to retrieve this element and then modifies its style and content using the properties of the returned object.

Getting Elements by Class

The getElementsByClassName() method is used to retrieve a collection of elements with the specified class name from an HTML document. It returns the collection as an array-like object (HTMLCollection or NodeList), which can then be looped through to access and manipulate individual elements.

Syntax:

var elements = document.getElementsByClassName(classname);

Here, the classname parameter is the name of the class that you want to retrieve elements with. You can pass multiple class names separated by spaces, and the method will return all elements with at least one of these classes.

Code Example:

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript DOM manipulation</h2>
<p class="demo">This is a paragraph with class "demo".</p>
<p>This is another paragraph without the "demo" class.</p>
<p class="demo">This is a second paragraph with class "demo".</p>

<script>
var paras = document.getElementsByClassName("demo"); // retrieve all elements with class "demo"
for(var i=0; i<paras.length; i++) {
    paras[i].style.color = "red"; // change the color of each element to red
}
</script>

</body>
</html>

In the above example, we have three paragraph elements, two of which have the "demo" class. The JavaScript code uses the getElementsByClassName() method to retrieve all elements with this class and then loops through them to change their color to red.

Conclusion

The ability to get elements by ID or class is a powerful feature of JavaScript's Document Object Model (DOM) that allows developers to manipulate the contents of HTML documents dynamically. With the right methods and techniques, getting elements by ID or class can greatly simplify the process of working with web pages and create engaging user experiences.

Popular questions

  1. What is the syntax for retrieving an element by its ID in JavaScript?

The syntax for retrieving an element by its ID in JavaScript is:

var element = document.getElementById(id);

Here, the "id" parameter is the ID of the element you want to retrieve.

  1. How do you retrieve elements with a specific class name in JavaScript?

To retrieve elements with a specific class name in JavaScript, you can use the getElementsByClassName() method. The syntax is:

var elements = document.getElementsByClassName(classname);

Here, the "classname" parameter is the name of the class that you want to retrieve elements with.

  1. What is the difference between getElementById() and getElementsByClassName()?

getElementById() is used to retrieve an element by its unique ID, while getElementsByClassName() is used to retrieve a collection of elements by their class name.

getElementById() returns a single object, while getElementsByClassName() returns a collection of objects.

  1. Can you change the style of an element retrieved using getElementById() or getElementsByClassName()?

Yes, you can change the style of an element retrieved using getElementById() or getElementsByClassName() by using its style property. For example, you can change its color, background color, font size, etc.

  1. Are there any limitations to using getElementById() or getElementsByClassName()?

getElementById() can only retrieve elements with unique IDs, while getElementsByClassName() may return a collection of elements with the same class name.

Both methods only retrieve elements within the current document, so if you are looking for elements across multiple documents or frames, you'll need to use other techniques.

Tag

DOM Manipulation

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