middleware-realworld-example.js

var middlewarify = function(pre, done) {
    var middleware = [];

    var func = function(...args) {
        if (done) middleware.push(done);

        return middleware.reduce( (chain, fn) => chain.then(fn)
                , Promise.resolve(pre.apply(null, args)))
    };

    func.use = function(fn){  
        middleware.push(fn);
    };

    return func;
};

var myLibrary = (function() {
    var authToken = "ABC123";

    var getEmployees = function() {
        // assume api.fetchEmployees makes an AJAX request for a JSON array of employees
        return api.fetchEmployees(authToken, searchFilter)
    };

    // takes a flat list of employees and returns and array of departments with employees underneath
    // [ { name: "Finance", employees: [...] }, { name: "IT", employees: [...] }]
    var groupByDepartment = function(employees) {

        var deptsObj = employees.reduce((depts, employee) => {
            if(!depts[employee.department]) depts[employee.department] = [];
            depts[employee.department].push(employee);
        }, {});

        return Object.keys(deptsObj).map(deptName => {
            return {
                name: deptName,
                employees: deptsObj[deptName]
            }
        });
    };

    return {
        getDepartments: middlewarify(getEmployees, groupByDepartment)
    };
    
})();


// Set empty 'department' to "Unknown" before the employees get grouped by department
var handleNullDepartment = function(employees) {
    employees.forEach(e => e.department = e.department || "Unknown");
    return employees;
}

// Consumers would then use your public, middlewarified method like so:
myLibrary.getDepartments.use(handleNullDepartment);
myLibrary.getDepartments().then(departments => {
    console.log("Done")
    console.log(departments);
});