A plain JavaScript AJAX Contact Form that is designed to work with http://formspree.io/

formspree-ajax-contact-form.js

/**
 * AJAX Form
 * http://stackoverflow.com/a/13038218/1414881
 */

var requestDemoForm = document.getElementById('aaa_form');
var subscribeNewsLetterForm = document.getElementById('bbb_form');

// Append the form status (you can just alert instead of these)
var formStatus = document.createElement('div');
formStatus.setAttribute('class', 'form-status alert');
form.appendChild(formStatus);

requestDemoForm.onsubmit = submitForm;
subscribeNewsLetterForm.onsubmit = submitForm;

function submitForm (e) {
	// Stop the regular form submission
	e.preventDefault();

	// Collect the form data while iterating over the inputs
	var data = {};
	for (var i = 0, ii = form.length; i < ii; ++i) {
		var input = form[i];
		if (input.name) {
			data[input.name] = input.value;
		}
	}

	// Construct an HTTP request
	var xhr = new XMLHttpRequest();
	xhr.open(form.method, form.action, true);
	xhr.setRequestHeader('Accept', 'application/json; charset=utf-8');
	xhr.setRequestHeader('Content-Type', 'application/json; charset=UTF-8');

	// Send the collected data as JSON
	xhr.send(JSON.stringify(data));

	// Callback function
	xhr.onloadend = function (response) {
		if (response.target.status === 0) {

			// Failed XmlHttpRequest should be considered an undefined error.
			formStatus.className += ' alert-danger';
			formStatus.innerHTML = form.dataset.formError;

		} else if (response.target.status === 400) {

			// Bad Request
			formStatus.className += ' alert-danger';
			formStatus.innerHTML = JSON.parse(responseText).error;

		} else if (response.target.status === 200) {

			// Success
			formStatus.className += ' alert-success';
			formStatus.innerHTML = form.dataset.formSuccess;

		}
	};
};