So You're Writing Your Own Transpile-To-Javascript Language?

Are you mad?

Or maybe you are just thinking about it, but writing transpilers requires a few specialist tools. Elsewhere I talk about the toolchain in LuvvieScript in great detail, but here I will talk about some of the tools I use to build and test that toolchain.

Lets define terms first - what do I mean by transpiling? We all know what compiling is - taking a high-level format (a programme in a human readable language, like Ruby or C or the mighty Fortran) doing some monkey business on it and outputting a form of that programme in a lower-level (maybe .elf or Java Byte Code or IRL) which a computer can run.

Well transpiling is a bit like that on its side - instead of converting a programme down the complexity stack you are translating it sideways - more like Swedish-into-Swahili.

LuvvieScript is Erlang-transpiled-to-Javascript - so we have a lot of syntax tools already. The Erlang language is defined, the Javascript language is defined, there are parsers and lexers and a whole lot of good stuff on both sides of the house.

On the Erlang side there is:

On the Javascript side we have:

These are pretty complex, but well documented tools, of the sort you will have to master if you want to write your own transpiler.

But what I really want to dig into is the actual day-to-day grunt work - what do you actually need to do and how do you actually do it.

With LuvvieScript it not quite the straight journey. First we compile down a bit: from Erlang to Core Erlang to a Core Erlang Abstract Syntax tree. Then we transpile sideways from the Core Erlang AST to the Javascript AST that the nice people at Mozilla have defined. Finally we build up from that Javascript AST to your actual Javascript.

The actual core of the work I am doing is the little sidie-ways bit between the AST’s.

Its a bit like digging a tunnel through a mountain. On the West is the Vale D’Erlang, on the right StrathJavascript. I get a fragment of Erlang and think ‘what Javascript pattern is going to implement that. I then write that Javascript and compile to the Javascript JSON API.

Then I push through the Erlang down my tunnel from the west getting a native Erlang data structure that represents the Erlang AST. I then prod my toolchain to actually emit the JSON representation of that and compare it to the one I am expecting to get - I am looking to see if the West and East tunnels are perfectly aligned - which of course they never are.

So there three tools that help here. The first is a json pretty printer. My Erlang libraries produce json that looks like this:


A simple json pretty printer is a godsend - I use a Python not-quite-one-liner:

#!/usr/bin/env python
Convert JSON data to human-readable form.

(Reads from stdin and writes to stdout)

import sys
import json

print json.dumps(json.loads(, indent=4)

Suddenly, my json is not quite so impenetrable:

    "body": [
            "kind": "var",
            "declarations": [
                    "init": {
                        "type": "ObjectExpression",
                        "properties": []
                    "type": "VariableDeclarator",
                    "id": {
                        "type": "Identifier",
                        "name": "exports"

This brings me to my next tool. JSON is notoriously hard on the scroll button, particularly highly-nested JSON like Abstract Syntax Trees. It is not unusual for a simple JSON AST expression so split over a hundred lines. An editor that can fold out the embedded bits is critical - enter Sublime Text:

Folding JSON In The Sublime Text Editor

There is still one little problem - the json you are producing on one side of the house is often not formatted the same way as that produced on the other side. Old fashioned diff is hard to get to work even with purty json - what we need is a json-diff tool.

Lets see this in action. Here is the simple erlang test function I am trying to get passing:



simple_fn() ->

not_exported_fn() ->

I write a fragment of my target output in the Esprima Online Parser:

Esprima Online Parser Generating Javascript Abstract Syntax Tree

I can now compare a fragment from my transpiler with what it ought to be:

Comparing Abstract Syntax Trees With JSON-Diff

The tool shows one of the typical hard-to-spot-by-inspection bugs - returning an object instead of a list of objects:

JSON-Diff tool showing differences

That’s it folks, a quick run through of the day-to-day work of developing a something-to-javascript transpiler.

If you are interested in this Erlang-to-javascript transpiler you can read about the toolchain and mission and, mebbies, install it and get started.


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