Question: I have an existing React.js application and I want to integrate it with Sitecore to provide CMS features.

Is it possible to integrate the existing React.js App with `Sitecore 10.2` with `Headless` support? Do I need to go with Next.js and rewrite React app?

Answer: 
This can be achieved by overwriting the `sxacontent` field in Solr document.

1. Create a class by inheriting `AggregatedContent` class and overriding `ComputedFieldValue`
2. Inside override method, access `ComputedFiedValue` from base class which is nothing but `sxacontent` field value from Solr
3. Write a condition for a specific item or items from the specific template and replace the existing `sxacontent` field value with the necessary field values only or add the extra field content to the existing `sxacontent` field value as per your requirement.
4. Outside the condition, return the `sxacontent` field value grabbed from the base class as it is. So that it will not impact your changes to the other items content.
5. Create a patch file and patch it instead of existing `sxacontent` computed field value,

`<field fieldName="sxacontent" patch:instead="*[@fieldName='sxacontent']" returnType="textCollection" type="your class, namespace"/>`

deploy the code and patch file to webroot

6. Rebuild the indexes

Refer to my blog on [overwriting Sxacontent](https://sitecoreseries.blogspot.com/2022/09/overwriting-sxa-content-field-in-solr.html) field for more details.
I think you might be able to get somewhere using a custom `SearchQueryToken`. I wrote a [blog post](https://ggullentops.blogspot.com/2019/01/extending-sxa-search-query-tokens.html) on this topic some years ago (including the code). In my case I had to specify a field that had to match to a value on the current item. In your case that field would have to match the entry from the searchbox but I assume you will have access to that value so you can create the necessary query.

Note that although the code in the post is tested and works, I haven't tested this in your scenario and access to the search term is an assumption but I believe it's definitely worth trying.

The code itself is a bit much to copy here, but in short you need to create a processor with a Process method that loops over all the `SearchStringModel` objects. This object is a piece of the query and you need to transform that model to a new one that can be used by the search engine.

Once you have created your custom querytoken you can use it to define the scope as it will appear in the query designer with the ootb sxa ones:
[![enter image description here](https://i.stack.imgur.com/1mY5F.png)](https://i.stack.imgur.com/1mY5F.png)