gistfile1.hrl

-module(test).

-export([json/0]).

json() ->
    Id = <<"abc123">>,
    Name = undefined,
    User = [],
    User2 = [{<<"id">>, Id} | User],
    User3 = [{<<"name">>, Name} | User2],
    Struct = {struct, [{<<"user">>, {struct, lists:reverse(User3)}}]},
    iolist_to_binary(mochijson2:encode(Struct)).

<<"{"user":{"id":"abc123","name":"undefined"}}">>

json() ->
    Id = <<"abc123">>,
    Name = undefined,
    User = [],
    User3 = filter({<<"name">>, Name}, User2 = filter({<<"id">>, Id}, User)),
    Struct = {struct, [{<<"user">>, {struct, lists:reverse(User3)}}]},
    iolist_to_binary(mochijson2:encode(Struct)).

 filter({Tag, undefined}, Old) -> Old;
 filter({Tag, Val}, Old) -> [Tag, Val | Old].