Binary tree iterator implemented with lazy list

binary_tree_iterator_lazy.js

#!/usr/bin/env node

function empty() {
    return null
}

function singleton(e) {
    return function() {
        return {
            first : e,
            rest : null
        }
    }
}

function first(list) {
    return list().first
}

function rest(list) {
    return list().rest
}

function append(list1, list2) {
    if (null == list1) return list2
    if (null == list2) return list1

    return function() {
        return {
            first : first(list1), 
            rest : append(rest(list1), list2)
        }
    }
}

function make_binary_tree_iterator(node) {
    var left_it = (null == node.left ? null : make_binary_tree_iterator(node.left))
    var root_it = singleton(node)
    var right_it = (null == node.right ? null : make_binary_tree_iterator(node.right))

    return append(append(left_it, root_it), right_it)

}

//======== Test case ========
var tree = { 
    value : 1, 
    left : {
        value : 2,
        left : {
            value : 4,
            left : null,
            right : null
        },
        right : null
    },
    right : {
        value : 3,
        left : null,
        right : {
            value : 7,
            left : null,
            right : null
        }
    }
}

for (var it = make_binary_tree_iterator(tree); null != it; it = rest(it)) {
    console.log(first(it).value)
}