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

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

var arr = [1, 2, 3, 4, 5];

// reduce to a single value by summing them
var result = arr.reduce(function (accumulator, currentValue) {
    return accumulator + currentValue;
}, 0);

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

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