Count elements in a binary search tree

count_binary_search_tree.js

// add an element to a tree
var add_elem = function(tree, value) {
    if(tree === null) {
        return {
            data: value,
            left: null,
            right: null
        };
    }
    if(value <= tree.data) {
        return {
            data: tree.data,
            left: add_elem(tree.left, value),
            right: tree.right
        };
    } else {
        return {
            data: tree.data,
            left: tree.left,
            right: add_elem(tree.right, value)
        };
    }
};

// create a tree from a list of elements
var create_tree = function(lst) {
    var i;
    var curr = null;
    for(i = 0; i < lst.length; i++) {
        curr = add_elem(curr, lst[i]);
    }
    return curr;
};

// count binary search tree recursively
var count = function(tree) {
    counter = 0;
    if (tree === null){
        return counter;
    }
    if (tree.left !== null || tree.right !== null){
        counter++;
    }    
    return 1 + count(tree.left) + count(tree.right);
};