<div id="before"></div>
<div id="after"></div>

body {
    font-family: monospace;
    margin: 20px;
    font-size: 20px;
}

var arr = ["here", "are", undefined, "some", "values", null, "hi"];

var newArr = arr.map(function (item) {
    return item ? item : "n/a";
});

// join the arrays with commas and print to page
print("before", "before: " + arr.join(', '));
print("after", "after: " + newArr.join(', '));

function print(id, html) {
    document.getElementById(id).innerHTML = html;
}