ajaxform.js

const ajaxform = (form, next) => {

	let url  = form.action
	let xhr  = new XMLHttpRequest()
	let data = new FormData(form)

	xhr.open('POST', url)
	xhr.onload = () => next(form, xhr)
	xhr.send(data)

}

Example:

let form = document.querySelector('form')

const next = function(form, xhr) {

    if (xhr.status===200 && xhr.responseText===true) { … }

}

ajaxform(form, next)