generator-iterator

function* iterTree (tree) {
    if (Array.isArray(tree)) {
        for (var i = tree.length - 1; i >= 0; i--) {
            yield* iterTree(tree[i])
        }
    } else {
        yield tree
    }
}
const tree = [ 'a', ['b', 'c'], ['d', 'e'] ];
for(let x of iterTree(tree)) {
    console.log(x);
}