When do Template.currentData() and template.data differ in value?

Question

I know one is a reactive source, while the other is not. But I thought they would always give the same value.

Then I found the following code in Telescope's source:

    var newTerms = Template.currentData().terms; // ⚡ reactive ⚡
    if (!_.isEqual(newTerms, instance.data.terms)) {
      instance.postsLimit.set(instance.data.terms.limit || Settings.get('postsPerPage', 10));
    }

link: https://github.com/TelescopeJS/Telescope/blob/master/packages/telescope-posts/lib/client/templates/posts_list/posts_list_controller.js#L33

So it seems these two values can sometimes differ. When?

Answer 1

According to Meteor's documentation, about template.data:

This property provides access to the data context at the top level of the template. It is updated each time the template is re-rendered. Access is read-only and non-reactive.

Since we know that the current data context is reactive, hence can change without the template being re-rendered (which is what makes reactivity look all nice and smooth on Blaze), this if statement is written to check if the "real" current terms (which are stored in the reactive Template.currentData()) have changed compared to the "previous" terms we had the last time the current template was rendered. (stored in the non-reactive template.data)

To wrap it up, what this autorun does is:

1. Anytime the current data context changes...
2. Fetch the terms from said data context
3. Compare these terms to the ones stored in template.data when the template was rendered
4. If they differ, that means the terms have changed (duh): reset the post limit.