Example of babel plugin to aid rails sprocket tie-in.

get_asset_url.js

var fs = require('fs');
var path = require('path');
var md5itCfg = require('../grunt/config/cl_md5it');

module.exports = function(babel) {
  var t = babel.types;

  var md5sFile = md5itCfg.options.manifestFile;
  var md5s, cache = {};

  try {
    md5s = JSON.parse(fs.readFileSync(md5sFile, 'utf8'))
  } catch (e) {
    md5s = {};
  }

  function getUrl(path) {
    if (!cache[path]) {
      if (!md5s[path]) {
        cache[path] = path;
      } else {
        cache[path] = md5s[path][0].fullUrl;
      }
    }

    return cache[path];
  }

  return new babel.Transformer('get_asset_url', {
    CallExpression: {
      enter: function(node, parent) {
        if (t.isIdentifier(node.callee, {name: 'get_asset_url'})) {
          var url = getUrl(node.arguments[0].value);
          return t.expressionStatement(t.literal(url));
        }
      },
    },
  });
};