Rendering comma separated list of links

At Khan Academy we use a helper called intersperse for this:

/* intersperse: Return an array with the separator interspersed between
 * each element of the input array.
 *
 * > _([1,2,3]).intersperse(0)
 * [1,0,2,0,3]
 */
function intersperse(arr, sep) {
    if (arr.length === 0) {
        return [];
    }

    return arr.slice(1).reduce(function(xs, x, i) {
        return xs.concat([sep, x]);
    }, [arr[0]]);
}

which allows you to write code like:

var tags = item.tags.map(function(tag, i) {
    return <Tag key={i} tag={item.tags[i]} />;
};
tags = intersperse(tags, ", ");

Simply

{tags.map((tag, i) => <span key={i}>
    {i > 0 && ", "}
    <Tag tag={tag} />
</span>)}

In React 16 it can be done even more simpler:

{tags.map((tag, i) => [
    i > 0 && ", ",
    <Tag key={i} tag={tag} />
])}