metrics.js

// get the length of the longest classname in document

[].slice.call(document.querySelectorAll('body *')).filter(el => el.nodeType === 1 && typeof el.className === 'string').reduce( (m, el) => {
	const l = el.className.split(' ').filter(classname => classname.length > m).sort((a,b) => a.length - b.length).shift();
	return l ? l.length : m;
},0)

// get the longest classname

[].slice.call(document.querySelectorAll('body *')).filter(el => el.nodeType === 1 && typeof el.className === 'string').reduce( (m, el) => {
	const l = el.className.split(' ').filter(classname => classname.length > m.length).sort((a,b) => a.length - b.length).shift();
	return l || m;
},'')

// get the element with the longest class Attribute

[].slice.call(document.querySelectorAll('body *')).filter(el => el.nodeType === 1 && typeof el.className === 'string').reduce( (m, el) => {
	return el.className.length > m.className.length ? el : m;
},document.body)

// get the element with the most classes

[].slice.call(document.querySelectorAll('body *')).filter(el => el.nodeType === 1 && typeof el.className === 'string').reduce( (m, el) => {
	return el.className.split(' ').length > m.className.split(' ').length ? el : m;
},document.body)