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What is the difference/purpose of 'content' and 'elements' keys, in preprocess?

jp flag
artfulrobot

I'm using hook_preprocess_node and looking at $variables. I can see there's two similar keys: content and elements. I see that there's a comment in the core code:

  // Helpful $content variable for templates.

next to code that copies all elements keys into content... I'm unsure why it's helpful, how it's different, and when one would use elements or content to achieve a particular goal.

This is a general question, but my task at hand is that I have two separate taxonomy term reference fields and I wish to combine them, alphabetically by term name, into one list for presentation.

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4uk4 avatar Drupal
cn flag
4uk4

elements is the variable name defined in theme_hook() to hold the renderable elements (for the node this is the entity build array). Preprocess is then responsible to prepare variables for rendering, like content.

If your goal is to modify the template output then check first which variables are actually printed. In a node template it doesn't make sense to change elements in preprocess, any changes you make will be discarded. You can however do this in the build process, for example in hook_entity_view_alter().

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jp flag
artfulrobot
So the *node* module generates a render array with all the node's data in render arrays under `'elements'`. Then `template_preprocess_node()` sets defaults for the node, which is where the whole *copy `elements` to `content`* comes from. So this means `elements` is only left there "just in case" (?) but that in terms of continuing the render process, `content` is what now matters. Is that about right?
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4uk4 avatar
cn flag
4uk4
Yes, but in your case you might want to change the build array before it reaches the preprocess hook. This looks more like you want to alter the build array, not to theme the output.
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jp flag
artfulrobot
Thanks, I've started [another question](https://drupal.stackexchange.com/questions/309299/render-two-taxonomy-fields-in-one-alphabetical-list/309300#309300) for that part of the task, to keep things clearer.
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