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Grain is an async framework for node.js template languages

Grain Templating System

All template languages have 5 parts in common.

  • Static content
  • Parameters
  • Dynamic content
  • Loops/Conditionals
  • Asynchronous parts

Ok, maybe the last one isn't that common, but in a NodeJS world where nothing blocks it's required for many use cases. Partial templates require usually that another external resource get loaded and compiled. If this resource is loaded over some IO then it's an asynchronous operation. Also it would be nice to be able to stream content to the browser as information is known.

Grain is both a spec for interfaces between template languages and a helper library to help write compilers that follow the spec. Included is a sample template language called Corn that implements the spec using grain.

Inversion of control

One way to handle the async nature of retrieving data is to pre-calculate all the data that's needed for a template to render, and then as the last step pass it to the template function which returns the output text in a single sync function call. This works and is very simple from the point of view of the author of the template language, but it's a lot of burden on the person using the template language.

My proposed method is to pass the template function an hash of values and data providing functions. The template language will call the data providers as it needs data. If they are async functions, then it will hold that place in the template and defer rendering of that part till the callback comes back with the data. Then the template will output it's result after the pending function calls have finished.

Template language compiler module interface

All grain engines need to comply with this simple interface to allow interoperability between frameworks and projects.


The module itself is a function that takes in template source code as text and returns compiled function that renders that template on demand.

function compile(text) -> function fn(locals, callback|stream)
module.exports = compile

The returned function accepts two arguments, they are locals and then either a callback function or a stream instance. The properties of locals are available within the template as local variables, usually using with.


If a callback is provided it's interface will be:

function callback(err, result) { ... }

Where err will contain an instance of Error if an exception happens while rendering, otherwise, err will be falsy and result will contain the final output of the entire template.


If a stream if given instead, it will emit("data", chunk) as chunks of output are finished. It will emit('end') when done, and emit('error', err) where there is an exception.


The compile function will act as if it's a curried function and accept the locals and callback|stream parameters directly.

function callback(text, locals, callback|stream)


The module itself will also have a helpers object that gets mixed into every locals parameter. A simple way to implement this is on the first line of the generated code in fn do:

locals.__proto__ = module.exports.helpers;

Sample code

Suppose this simple template language where @foo is the variable foo and @bar() calls the data provider bar() and inserts the result inline.

I would require my compiler like this (corn is included as a sample):

var compiler = require('grain/corn');

Now let's suppose this simple template:

var template = "Hello @planet, my name is @name() and I am @age() years old.";

With this sample data:

var data = {
  // Value
  planet: "world",
  // Async getter
  name: function name(callback) {
    process.nextTick(function () {
      callback(null, "Tim Caswell");
  // sync getter
  age: function age() {
    return 28;

To compile the template it's simply:

var fn = compiler(template);

And then to render it do:

fn(data, function (err, text) {
  if (err) {
    console.log("ERROR " + err.stack);
  console.log("OUTPUT " + text);

Or to render with a stream:

var stream = new process.EventEmitter();
fn(data, stream);
stream.addListener('data', function (data) {
  console.log("DATA " + JSON.stringify(data.toString()));
stream.addListener('end', function () {
stream.addListener('error', function (err) {
  console.log("ERROR " + err.stack);

Also you can compile and render it in one shot:

compiler(template, data, callback);

If I wanted a helper that did partials, I would add it like this:

var fs = require('fs');
compiler.helpers = {
  partial: function (filename, data, callback) {
    fs.readFile(filename, 'utf8', function (err, text) {
      if (err) { callback(err); return; }
      compiler(text, data, callback);

And then it would be available within the template as partial() without me having to pass it into the data hash every time.