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Immutable JSON

30 July 2014

Facebook just released immutable-js a persistent data structure library for JavaScript. Given the performance benefit immutable data can bring to React, it's little surprise Facebook is investing in them. However the question remains how to deliver immutable data to clients given the prevalence of JSON.

In this post I'm going to demonstrate consuming plain JSON with transit-js and producing immutable-js values instead of JavaScript objects and arrays.

If you have Node.js installed you can follow the code presented in the post easily on your machine. Create a directory on your machine and add the following package.json file to it:

    "name": "immutable-json",
    "version": "0.1.0",
    "dependencies": {
        "immutable": "2.0.3",
        "transit-js": "0.8.670"

Then run the following at the command line to install the dependencies:

npm install

Create a JavaScript file and put the following requires at the top:

var Immutable = require("immutable"),
    transit   = require("transit-js");

transit-js exposes two low-level options arrayBuilder and mapBuilder for constructing readers. This allows readers to interpret the meaning of the Transit array and map encodings.

We can customize a reader to return Immutable.Vector and Immutable.Map like so:

var rdr = transit.reader("json", {
    arrayBuilder: {
        init: function(node) { return Immutable.Vector().asMutable(); },
        add: function(ret, val, node) { return ret.push(val); },
        finalize: function(ret, node) { return ret.asImmutable(); },
        fromArray: function(arr, node) { return Immutable.Vector.from(arr); }
    mapBuilder: {
        init: function(node) { return Immutable.Map().asMutable(); },
        add: function(ret, key, val, node) { return ret.set(key, val);  },
        finalize: function(ret, node) { return ret.asImmutable(); }

Note that the builder methods get the original JSON node as contextual information. By default transit-js builds the values incrementally. transit-js can also build values at once from an array as in the case of Immutable.Vector. Sadly this can't be done for Immutable.Map yet. transit-js maps and ClojureScript both have an array map type for maps with less than or equal to 8 keys and it is a significant performance enhancement in time and space.

We can now read JSON objects and arrays into immutable maps and vectors:"[1,2,3]"); // Vector [ 1, 2, 3 ]'{"foo":"bar"}'); // Map { foo: "bar" }

For writing we need to make write handlers. This is also pretty straightforward:

var VectorHandler = transit.makeWriteHandler({
    tag: function(v) { return "array"; },
    rep: function(v) { return v; },
    stringRep: function(v) { return null; }

var MapHandler = transit.makeWriteHandler({
    tag: function(v) { return "map"; },
    rep: function(v) { return v; },
    stringRep: function(v) { return null; }

var wrtr = transit.writer("json-verbose", {
       Immutable.Vector, VectorHandler,
       Immutable.Map, MapHandler

And now we can roundtrip:

wrtr.write("[1,2,3]")); // [1,2,3]
wrtr.write('{"foo":"bar"}')); // {"foo":"bar"}

If you're excited about getting immutable data from your server into your React client application, transit-js presents a pretty good story even if you marshal plain JSON. However, if you change your backend to emit Transit JSON you will see a fairly significant performance boost. In the future if Immutable provides more efficient means to construct values as well as exposing an array map type, you will be able to deserialize immutable values nearly as fast or faster than you can JSON.parse plain JSON data in many modern browsers.