attempting to port a function from SICP lecture video.

sum-fringe.js

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

function first(arr) {
    return arr[0];
}

function rest(arr) {
    return arr.slice(1);
}

function add(a,b) {
    return a + b;
}

function sumFringe(tree) {
    if ( ! tree.length ) {
        return ( typeof tree == "number") ? tree : 0;
    } else if ( tree.length == 1 ) {
        return first(tree);
    } else {
        return add( sumFringe(first(tree)), sumFringe(rest(tree)));
    }
}

console.log(sumFringe(tree));