With Underscore, how do I recursively flatten an array of objects?

Question

I have a tree/traversable object that looks like this:

                var data = {children: [
                {
                name: 'foo',
                url: 'http://foo',
                children: [
                    {
                    name: 'bar',
                    url: 'http://bar',
                    children: []
                }
                ]
            },
            {
                name: 'baz',
                url: 'http://baz',
                children: []
            },
            {
                name: 'biff',
                children: []
            }
            ]};

What I need to do is be able to flatten the data into a single dimensional list:

var flattenedData = [{name: 'foo', url: 'http://foo'}, {name: 'bar', url: 'http://bar'}, {name: 'baz', url: 'http://baz'}, {name: 'biff'}];

Currently, I've created a recursive helper function to walk the data structure and push the results onto an array. I'd like to do this more functionally if possible. Something like:

var flattenedData = _.chain(data.children).flatten().filter(function(item){//real filtering; return item;}).value();

The problem is, flattening doesn't seem to flatten an array of objects, just simple arrays. I could be wrong.

How would I perform this task in a more functional way without traversing the tree in a helper function?

Answer 1

I don't see what you mean by "flatten doesn't seem to flatten an array of objects, just simple arrays" - how so? It most definitely can flatten any kind of array. Anyway, a simple one-pass solution using reduce :

Object.prototype.flatten = function () {
    if (this instanceof Array) {
        return this.reduce(function(a, b) {
            return a.concat(b.flatten());
        }, []);
    }

    var res = [];

    // this is the example condition
    if (this.url !== undefined) {
        res.push(this);
    }

    return res.concat(this.children.flatten());
};

var flat_data = data.flatten();
print(flat_data);

By the way, it would be advisable to replace Object with your custom data type you want to flatten, so that you don't mess up the global prototype for all objects.