tree_builder.js

function TreeBuilder() {

    var makeEmptyNode = function() {
        return {
            "_items": []
        };
    };

    var root = makeEmptyNode();

    this.addNode = function (node) {
        var current = root;
        node.path.map(function (pathItem) {
            if (!current.hasOwnProperty(pathItem)) {
                current[pathItem] = makeEmptyNode();
            }
            current = current[pathItem];
        });
        current._items.push(node);
    };

    this.getTree = function() {
        return root;
    }
}

var nodes = [
    {
        "id": 6022788483583,
        "name": "Frequent International Travelers",
        "description": "People who have traveled abroad more than once in the past six months",
        "type": "behaviors",
        "audience_size": 189974052,
        "path": ["Behaviors", "Travel"]
    },
    {
        "id": 6006521036425,
        "name": "Leisure travelers",
        "description": "26",
        "type": "behaviors",
        "audience_size": 38439600,
        "path": ["Behaviors", "Travel"]
    }
];

var builder = new TreeBuilder();
nodes.map(function (node) {
    builder.addNode(node);
});

console.log(JSON.stringify(builder.getTree()));