document.getElementById('seats' + index).classList.removeALl code example
Example 1: add multiple classList
element.classList.add(class1,class2,class3...);
Example 2: add to classlist javascript
Add multiple classes to a <div> element:
document.getElementById("myDIV").classList.add("mystyle", "anotherClass", "thirdClass");
Remove a class from a <div> element:
document.getElementById("myDIV").classList.remove("mystyle");
Remove multiple classes from a <div> element:
document.getElementById("myDIV").classList.remove("mystyle", "anotherClass", "thirdClass");
Toggle between two classes for a <div> element:
document.getElementById("myDIV").classList.toggle("newClassName");
Find out if an element has a "mystyle" class:
document.getElementById("myDIV").classList.contains("mystyle");