original.js

filterMenuItems = function (menuItems, level, parentId) {
    var childLevel = level + 1;

    // filter out admin-only items if needed
    if (!Users.is.admin(Meteor.user())) {
        menuItems = _.reject(menuItems, function (item) {
            return item.adminOnly;
        });
    }

    menuItems = _.filter(menuItems, function (item) {
        if (level === 0) {
            // if this is the root level, return elements with no parentId
            return typeof item.parentId === "undefined";
        } else {
            // else, return elements with the correct parentId
            return item.parentId === parentId;
        }
    });

    // decorate child item with their level
    menuItems = _.map(menuItems, function (item) {
        item.level = childLevel;
        return item;
    });

    return menuItems;
};

ramda_version.js

var _ = require('ramda');
// Utils
trace = _.curry(function (tag, x) {
    console.log(tag, x);
    return x;
});

var byAdminOnly = function (isAdmin, item) {
    trace("admin", isAdmin());
    return _.propEq('adminOnly', isAdmin(), item);
};
var filterAdminOnly = _.filter(byAdminOnly);


var isRootLevel = function (level) {
    return level == 0;
};

var itemsWithoutParent = function (item) {
    return typeof item.parentId === "undefined";
};

var itemsForSpecificParent = _.propEq('parentId');

var filterByParent = function (conditions, items) {
    return _.filter(_.ifElse(isRootLevel(conditions.level), itemsWithoutParent, itemsForSpecificParent(conditions.parentId)));
};

function _filterMenuItems(conditions) {
    return _.compose(filterByParent(conditions),filterAdminOnly(conditions));
}
function filterMenuItems(menuItems, level, parentId, isAdmin) {
    if (!isAdmin) {
        isAdmin = function () {
            Users.is.admin(Meteor.user())
        };
    }
    return _filterMenuItems({
        level: level, parentId: parentId, isAdmin: isAdmin
    })(menuItems);
}

module.exports = {
    filterMenuItems: filterMenuItems,
    filterAdminOnly: filterAdminOnly,
    filterByParent: filterByParent
};

ramda_version_spec.js

var expect = require("chai").expect;
var _ = require('ramda');
var filters = require("../ramda_version.js");
var filterMenuItems = filters.filterMenuItems;
var filterAdminOnly = filters.filterAdminOnly;
var filterByParent = filters.filterByParent;

describe("filterItems", function () {
    var expectedItems = function (expected, actual) {
        expected = _.append(expected, []);
        expect(actual).to.deep.include.members(expected);
    };

    it("returns empty array for empty array", function () {
        expect(filterMenuItems([],0, 0)).to.be.empty;
    });

    it("filters admin only items for non admin", function () {
        items = filterAdminOnly(_.always(false),[{adminOnly: true}, {adminOnly: false}]);

        expectedItems({adminOnly: false}, items);
    });
    it("shows admin only items for admin", function () {
        items = filterAdminOnly(_.always(true), [{adminOnly: true}, {adminOnly: false}]);

        expectedItems({adminOnly: true}, items);
    });

    it("filter items from child level for root level", function () {
        items = filterByParent([{parentId: 1}, {id: 'no_parent'}], {level: 0});

        expectedItems({id: 'no_parent'}, items);
    })
});

what is the purpose of setting up the -parentId and item.level? Why your menuItems don't have those properties set up by default?

Don't know ask telescope developers :D , I'm just refactoring it

can you provide a test for this?

where is filterItemsBasedOnParent?

Can you inline this:
if (!isAdmin) {
isAdmin = function () {
Users.is.admin(Meteor.user())
};
}

into isAdmin: Users.is.admin(Meteor.user())

added specs and fixed name

i can't because it's ability to override isAdmin function. filterMenuItems is unpure version maybe I should be testing from _filterMenuItems instead.
I'm trying to keep the old interface but separate all unpure parts from _filterMenuItems