Converting Lua code into JSON, via JS

Lua, the other javascript

It’s amazing there’s another language older than javascript, with more metaprogramming abilities, yet employs a relatively simple syntax that is so incredibly identical that one could convert an existing javascript parser into parsing lua in just a few mere hours.

Esprima is a javascript parsing infrastructure that converts JS code into an AST which can be transformed and manipulated. I was tasked with converting lua code into a JSON object and thought this might be an interesting and quick way to get it done.

Code conversion

I forked esprima so that one could pass in lua code as a string and would return a json object converted from a lua table object. I was surprised how simple it was to convert esprima to become a lua parser. The author did a great job of maintaining code quality and readability.

Here’s a small snippet of my code where given a string of lua code, it will locate table creation and return the table as a json object. One could extend this concept to provide a more featureful lua code editor in-browser.

var esprima = require('esprima');

function jsonExtractor(source) {
  var root = esprima.parse(source);
  var array = []
  function traverse(node, obj, key) {
    var child, visited = false;

    if (node.type === 'TableAssignmentExpression') {
      var name =;
      if (node.right.type === 'Literal') {
        obj[name] = node.right.value;
      } else if (node.right.elements) {
        if (name) {
          visited = true;
          obj[name] = traverse(node.right.elements, {})

    if (!visited) {
      for (key in node) {
        if (node.hasOwnProperty(key)) {
          child = node[key];
          if (typeof child === 'object' && child !== null) {
            if (key === 'elements') {
              var x = traverse(child, {}, key);
              if (!x.frames) {
            } else {
              traverse(child, obj, key);

    return obj;
  return array;

Published: August 09 2012

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