widget pops up as "NaN"

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function () {
    if(xhr.readyState === 4) {
        var employees = JSON.parse(xhr.responseText)
        var statusHTML = '<ul class="bulleted">';
        for (var i=0; i<employees.length; i +=1) {
            if (employees[i].inoffice === true) {
                statusHTML =+ '<li class="in">';
            } else {
                statusHTML =+ '<li class="out">';
            }
            statusHTML =+ employees[i].name;
            statusHTML =+ '</li>';
        }
        statusHTML += '</ul>';
        document.getElementById('employeeList').innerHTML = statusHTML;
    }
};
xhr.open('GET', 'data/employees.json');
xhr.send();