test.js

var foo = ["one", "two", "three", "four"];

foo.forEach(function(value, index) {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', '/ajax');
    
    // Save the element's index to which this request is
    // related to.
    xhr.index = index;
    
    xhr.send();
 
    // Since the first element doesn't have a predecessor
    // just process it withouth waiting any response.
    processElement(foo[0]);
    
    xhr.addEventListener('load', function() {
            var next_element_index = this.index + 1;
            
            if (foo[next_element_index] !== undefined) {
               processElement(foo[next_element_index], this.response);
            }
    });
});