Dynamic Properties

posted 7 years ago

Was reading through code on github, and came across this little gem of a package merge-descriptors by @jonathanong.

The package is just a few lines long, but opened up a world of ideas for me.

function (dest, src) {
  Object.getOwnPropertyNames(src).forEach(function (name) {
    var descriptor = Object.getOwnPropertyDescriptor(src, name)
    Object.defineProperty(dest, name, descriptor)

  return dest

The rest of the article will go into depth about why this package is awesome.

Define Properties

ECMAScript 5 added the ability to define detailed properties. The default behavior of setting a property on an object can be replicated with more explicit syntax.

var person = {};

person.name = 'AJ';

Object.defineProperty(person, 'name', {
  writeable: true,
  configurable: true, 
  enumerable: true, 
  value: 'AJ' 

Writable means the property value can be changed via assignment.

person.name = 'Not AJ';

Configurable means it can be deleted, changed, and the property settings can be changed as well.

delete person.name;
Object.defineProperty(person, 'name', { /* changes */ });

Enumerable means that the property shows up when iterating.

for (var key in person) {
  console.log(key, person[key]);

You can get the property settings for any property on any object.

Object.getOwnPropertyDescriptor(person, 'name');

That's just default behavior though.

Getters and Setters

Properties can also have custom getters and setters. This allows for cool things like lazy attributes. For example, only parsing a request body if necessary.

function Request (bodyString) {
  this.bodyString = bodyString;

Object.defineProperty(Request.prototype, 'bodyJSON', {
  get: function () {
    if (this._bodyJSON === undefined) {
      this._bodyJSON = JSON.parse(this.bodyString);
    return this._bodyJSON;

var req = new Request('{ "name": "AJ" }');
req.bodyJSON.name;  // 'AJ'

For a setter, we can make sure the request method is always all caps.

Object.defineProperty(Request.prototype, 'method', {
  writeable: true,
  enumerable: true,
  set: function (method) {
    this._method = method.toUpperCase();
  get: function () {
    return this._method;

req.method = 'post';
req.method; // 'POST'


The above code doesn't look so good. Fortunately there's nicer syntax available.

var mixins = {};

var table = {
  ok: 200,
  created: 201,
  accepted: 202,
  // ...

// reverse lookups too
Object.keys(table).forEach(function (key) {
  table[ table[key] ] = key;

mixins.HumanStatus = {
  get status () {
    return table[this.statusCode];

  set status (type) {
    this.statusCode = table[type];

Now suppose we called the function written by Jonathan at the very beginning of the article function mixin.

function Response () {}
mixin(Response.prototype, mixins.HumanStatus);
var res = new Response;

res.statusCode = 200;
res.status; // 'ok'

res.status = 'created';
res.statusCode; // 201

Nifty Idea

I haven't looked too deeply into javascript ORMs, but if I were writing one, I would provide then-able associations through getters. Basically augmented promises.

For example, consider a find static method that returns a promise. If you call then on the promise, the user is eventually returned.

var userPromise = User.find(id);

userPromise.then(function (user) {
  // user has been fetched from db

But you could also add a getter to the user promise to return a new promise for associated posts.

var postsPromise = userPromise.posts;

postsPromise.then(function (posts) {
  // posts belonging to user fetched from db

Cool I guess. How about some methods on that new posts promise for chainable querying?

postsPromise.order({ created_at: -1 }).limit(10).then(function (recentPosts) {
  // recent posts fetched from db

If you use a web framework that resolves promises for you implicitly (like Koa) then your asynchronous code could actually look and feel like synchronous code.

function *() {
  var user = User.find(id);
  var posts = user.posts.limit(5);
  yield { user: user, posts: posts, count: posts.count }; 

So, yeah, dynamic properties!