Sprockets > How to specify a binding for Erb evaluation / rendering? - ruby-on-rails-3

Question

I've spent a lot of time digging into sprockets' and tit's source code, trying to figure out how to pass variables / bindings to the Erb evaluation context. Here's what I'm trying to do: I need to serve a JS file whose contents change on a per-request basis. The portions that change depend on data stored in the DB, hence the need to route requests through the Rails app and the need to pass variables / bindings. On top of that the JS file uses the require directives to insert other JS files, hence the need to use sprockets.
Here's the code snippet that isn't working:
Controller file:
def ever_changing_js
#foobars = Foobar.all
MyApp::Application.assets.instance_eval do
def foobars
#foobars
end
end
render :text => MyApp::Application.assets.find_asset('ever_changing.js').to_s, :content_type => "application/javascript"
end
ever_changing.js:
//= require file1.js
//= require file2.js
// Some code that uses #foobars
How can I get this done? Any help would be appreciated.

Answer 1

JavaScript files should be completely static; Sprockets is not meant to do what you are trying to do.
Any data that changes on a per-request basis should be written to a <script> tag at the bottom of the template you are rendering.
app/assets/javascripts/user.js
(function(exports) {
function User(name) {
this.name = name;
}
User.prototype.speak() {
console.log(this.name + ' says, "Hello!"');
};
exports.User = User;
})(this);
app/views/users/show.html.erb
...
<%= javascript_include_tag('user') %>
<script>
(function() {
var user = new User(<%= #user.name %>);
$('#speak-button').click(function() {
user.speak();
});
})();
</script>
</html>
If you can give more context around your specific use case, I can give a more specific example.

Answer 2

I am trying to accomplish the same thing you are. I see a couple problems with your controller code snippet. Rather than doing an instance_eval on the Sprockets::Environment, you should class_eval the context_class, as shown in the Sprockets::Context documentation.
MyApp::Application.assets.context_class.class_eval do
def foobars
#foobars
end
end
Then foobars will be available to your ERb template.
As a sidenote, you can do
render js: MyApp::Application.assets.find_asset('ever_changing.js').to_s
instead of setting the content type yourself.