How to dynamically change CSS class of an HTML tag?

Question

I am using JavaScript. I have a variable var boolVal that either evaluates to true/false. On my page, I have a <div> tag:

<div id='div1' class="redClass"></div> 

Based on the value of var boolVal, I want to change the CSS class of the <div> tag to blueClass.

For example: present class makes <div> color red, then the new class should make the page blue at runtime without need for page refresh.

Can we achieve this in simple JavaScript? Can we use

document.getElementById("MyElement").className = "MyClass"; 

or should we use AddClass?

Answer 1

You can add a CSS class based on id dynamically using classList API as follows:

document.getElementById('idOfElement').classList.add('newClassName'); 

Or the old way:

document.getElementById('idOfElement').className = 'newClassName'; // += to keep existing classes 

---

Alternatively you can use other DOM query methods shown below to find elements. The last three return a collection so you'll have to iterate over it and apply the class to each element in the collection (similar to the example given below each).

• querySelector

• querySelectorAll

  elements.forEach(element => element.classList.add('newClassName')); 

• getElementsByClassName

  Array.from(elements).forEach(element => element.classList.add('newName')); 

• getElementsByTagName

  Array.from(elements).forEach(element => element.classList.add('newName')); 

---

In your case

var element = document.getElementById('div1'); if(boolVal)    element.className= 'blueClass'; // += ' blueClass'; to keep existing classes else    element.className= 'redClass'; 

Answer 2

First of all, AddClass is not a pure Javascript method. It's related to jQuery.

You can use Javascript for adding a class:

setAttribute and className both method are used to set class (class attribute), these method are not used to adding another class with existing one.

document.getElementById('div1').setAttribute( "class", "blueClass" ); 

OR

document.getElementById('div1').className="redClass"; 

Demo Fiddle

So if you want append a css class to an element, you can do it like this -

 document.getElementById('div1').setAttribute( "class", document.getElementById('div1').getAttribute('class') + " blueClass" ); 

OR

 document.getElementById('div1').className +=" redClass"; 

Note: Using this way, the same class can be added multiple times. It's only a trick to do this work using javascript, it's not a perfect solution

OR simply use jQuery to add class -

$("#div1").addClass("blueClass"); 

Working Fiddle