index.js

var babel  = require("babel-core");
var mkdirp = require("mkdirp");
var path   = require("path");
var url    = require("url");
var fs     = require("fs");
var _      = require("lodash");

module.exports = function (opts) {
  opts = _.defaults(opts || {}, {
    options: {},
    dest:    "cache",
    src:     "assets",
    ignore:  false
  });

  opts.ignore = babel.util.regexify(opts.ignore);

  var cache = Object.create(null);

  return function (req, res, next) {
    if (opts.ignore.test(req.url)) return next();
    if (!babel.util.canCompile(req.url)) return next();

    var pathname = path.normalize(url.parse(req.url).pathname);
    var dest = path.join(opts.dest, pathname);
    var src  = path.join(opts.src, pathname);
    var srcStat;

    var send = function (data) {
      res.setHeader("Content-Type", "application/javascript");
      res.end(data);
    };

    var write = function (transformed) {
      mkdirp(path.dirname(dest), function (err) {
        if (err) return next(err);
        fs.writeFile(dest, transformed, function (err) {
          if (err) {
            next(err);
          } else {
            cache[pathname] = +srcStat.mtime;
            send(transformed);
          }
        });
      })
    };

    var compile = function () {
      var transformOpts = _.clone(opts.options);
      transformOpts.sourceFileName = "babel:///" + src.replace(opts.src, "");
      babel.transformFile(src, transformOpts, function (err, result) {
        if (err) {
          next(err);
        } else {
          write(result.code);
        }
      });
    };

    var tryCache = function () {
      fs.readFile(dest, function (err, data) {
        if (err && err.code === "ENOENT") {
          compile();
        } else if (err) {
          next(err);
        } else {
          send(data);
        }
      });
    };

    fs.stat(src, function (err, stat) {
      srcStat = stat;
      if (err && err.code === "ENOENT") {
        next();
      } else if (err) {
        next(err);
      } else if (cache[pathname] === +stat.mtime) {
        tryCache();
      } else {
        compile();
      }
    });
  };
};