Recreating core.async with ES6 Generators, JSON Mask

2013-11-12 00:00:00 +0000 by Alex R. Young

Recreating core.async with ES6 Generators

Recreating core.async with ES6 Generators is a post by Andrey Popp inspired by Clojure's core.async. He uses Browserify with a transpiler module that allows ES6 generators to be used in browsers:

function *listen(el, evType) {
  while (true)
    yield function(cb) {
      var fire = function(ev) {
        el.removeEventListener(evType, fire);
        cb(null, ev);
      el.addEventListener(evType, fire);

The tutorial goes on to use JSONP to fetch data from the Wikipedia API. The full source is available as a gist: andreypopp / index.html.


JSON Mask (GitHub: nemtsov / json-mask, License: MIT, npm: json-mask) by Yuriy Nemtsov is a module that provides a DSL for filtering JSON. It works in browsers, includes unit tests, and there's also Express middleware that can help you filter responses in your Express applications.

One use-case is when you have an HTTP service that returns {"name": "mary", "age": 25} (for example), and you have two clients: (1) that needs all of that information, and (2) that just needs the name.

Now, if the server uses JSON Mask, it would accept a ?fields= query-string. The (2)nd client would then add the following query-string to the request: ?fields=name; after which the server would respond with {"name": "mary"}, and filter out the rest of the information.

This saves bandwidth and improves performance; especially on large objects.

Although trivial in the simplest cases, the task of filtering objects becomes a difficult one quickly. This is why JSON Mask is actually a tiny (and highly optimized) language that is loosely based on XPath, and has support for filtering parts of objects, arrays, wild-card filtering and combinations of the three. So, here's a less trivial example and usage (note how p/a/b is filtered out):