Walk a tree with Promises

gistfile1.txt

// Walk a tree using Promises, where the act of fetching a node is asynchronous.
//
// This version visits the nodes in the expected order and returns a promise that
// is resolved when the last node is visited.  (If you're here from the StackOverflow
// post, the original error was a missing return statement in the call to map())

"use strict";

var family_tree = {
    "pops" : { name: "pops", children: ["chuck", "george", "pete"]},
    "chuck" : { name: "chuck", children: ["emma", "frank"] },
    "george" : { name: "george", children: [] },
    "pete" : { name: "pete", children: ["greg", "andrea"] },
    "emma" : { name: "emma", children: [] },
    "frank" : { name: "frank", children: [] },
    "greg" : { name: "greg", children: [] },
    "andrea" : { name: "andrea", children: [] },
}

function nameOf(node) { return node.name; }
function childrenNamesOf(node) { return node.children; }

// Return a promise to return a node.  Uses a timeout to emulate
// a web lookup (or other asynchronous operation).
function getNodeAsync(node_name) {
    return new Promise(function(resolve, reject) {
        var node = family_tree[node_name];
        if (node === undefined) {
            reject('cannot find node named ' + node_name);
        } else {
            setTimeout(function() { resolve(node); }, 500);
        }
    });
};

function walkTree(node_name, visit_fn) {
    return getNodeAsync(node_name)
        .then(function(node) {
            visit_fn(node);
            var child_names = childrenNamesOf(node);
            var promises = child_names.map(function(child_name) {
                return walkTree(child_name, visit_fn);
            });
            return Promise.all(promises);
        });
};

var nodes = [];

walkTree("pops", function(node) { console.log('visiting ' + nameOf(node));
                                  nodes.push(node);
                                 })
    .then(function() { console.log('found ' + nodes.length + ' nodes.') })

// Expected output is:
//
// visiting pops
// visiting chuck
// visiting emma
// visiting frank
// visiting george
// visiting pete
// visiting greg
// visiting andrea
// found 8 nodes.