Ajax Without jQuery

ajax.js

window.ajax = function( params ) {

	var method = 'method' in params ? params['method'] : 'get';
	var queryURL = 'queryURL' in params ? params['queryURL'] : '';

	var data = 'data' in params ? params['data'] : '';
	var dataArray = [];
	var dataString = '';

	var successCallback = 'success' in params ? params['success'] : function(params){console.log('Successfully completed AJAX request.')};
	var errorCallback = 'error' in params ? params['error'] : function(params){console.log('Error during AJAX request.');};

	var ajaxRequest = new XMLHttpRequest();
		ajaxRequest.onreadystatechange = function () {
			if ( ajaxRequest.readyState === 4 ) {
				if ( ajaxRequest.status === 200 ) {
					successCallback(ajaxRequest.responseText, ajaxRequest.status);
				} else {
					errorCallback(ajaxRequest.responseText, ajaxRequest.status);
				}
			}
		};

	switch ( typeof data ) {
		case 'string':
			dataString = data;
			break;
		case 'object':
			for ( key in data ) {
				dataArray += key + '=' + data[key];
			}
			dataString = dataArray.join('&');
			break;
	}

	if ( queryURL.includes('?') ) {

		var tempArray = queryURL.split('?');
		queryURL = tempArray[0];
		dataString = ( dataString == '' ? tempArray[1] : tempArray[1] + '&' + dataString );

	}

	if ( method.toLowerCase() == 'post' ) {

		ajaxRequest.open(method, queryURL, true);

		ajaxRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

		ajaxRequest.send( dataString );

	} else {

		ajaxRequest.open(method, queryURL + ( dataString == '' ? '' : '?' + dataString ), true);

		ajaxRequest.send( null );

	}
	
};

Recent update allows data to be passed as an object as well as string.

ajax({
    method: form.method,
    queryURL: form.action,
    data: {
        key1: 'value',
        key2: 'value',
    },
    success: function(data, status){
        data = JSON.parse(data);
        // Do stuff with data.
    }
    failure: function(data, status) {
        // tell user that things failed.
    }
});

Latest update allows for data in the action of form element. Example, both of the below would function identically:

<form method="get" action="/rest-api/">
    <input type="hidden" name="user-action" value="update" />
    <input type="hidden" name="user" value="a.smith" />
</form>

<form method="get" action="/rest-api/?user-action=update&user=a.smith"></form>