javascript - 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

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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'));
  

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In your case

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