index.js

var _     = require('lodash');
var gutil = require('gulp-util');
var map   = require('map-stream');
var pp    = require('preprocess');
var path  = require('path');

module.exports = function (options) {
  var opts    = _.merge({}, options);
  var context = _.merge({}, process.env, opts.context);

  function ppStream(file, callback) {
    var contents, extension;

    // TODO: support streaming files
    if (file.isNull()) return callback(null, file); // pass along
    if (file.isStream()) return callback(new Error("gulp-preprocess: Streaming not supported"));

    context.src = file.path;
    context.srcDir = path.dirname(file.path);
    context.NODE_ENV = context.NODE_ENV || 'development';

    contents = file.contents.toString('utf8');
    contents = pp.preprocess(contents, context, getExtension(context.src));
    file.contents = new Buffer(contents);

    callback(null, file);
  }

  return map(ppStream);
};

function getExtension(filename) {
  var ext = path.extname(filename||'').split('.');
  return ext[ext.length - 1];
}