object.keys.deep.js

function keys(object, deep) {
    'use strict';

    var keys = [];
    
    function isObject(obj) {
        var type = typeof obj;
        return type === 'function' || type === 'object' && !!obj;
    } 

    function resolveKeys(obj, result) {
        for (var key in obj) {
            if (obj.hasOwnProperty(key)) {

                if (deep && isObject(obj[key])) {
                    return resolveKeys(obj[key], result);
                }

                result.push(key);   
            }       
        }

        return result;
    }

    return resolveKeys(object, keys);
}

keys(JSON.parse('{ "a": 1, "b": 2, "c": { "d": 1 } }'), true);