Hello,
I was trying to make the search highlight (enabled in the Processors tab) work with JSON API. I did many searches in the Drupal community, also in this module page to find a patch/solution for this, but I didn't found.
So, I started to debug the module code to try to find a solution for this, and I found in the code the cause of this.
In line 108 of the indexResource.php it retrieves all the Result Items, that comes with the extra_data and data excerpt (if enabled in the Highlight Processor). But in the next line, it uses an array_map to get the original object from the retrieved Result Items. In this step, the extra_data and the excerpts are excluded, because they don't are part of the original entity. So, when the response is rendered, it comes without extra_data and data_excerpts.
Needs work
1.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
MR #9 attempts to attach the excerpt to individual entities, however it does not properly work as the field value (excerpt) is cached and subsequent searches on another keywords would return the same (outdated) excerpt.