Sergiy Dzysyak

This plugin provides simple REST API for the popular search <a href="https://wordpress.org/" title="Your favorite blogging software" rel="ugc">WordPress</a> search engine – <a href="https://wordpress.org/plugins/relevanssi/" title="A Better Search" rel="ugc">Relevanssi</a>.
As far as this plugin provides API for the Relevanssi plugin, it should be installed.
Key features
<ul>
<li>Search through posts of a certain type. By default all types.</li>
<li>Results in pagination and optional.</li>
<li>Sets X-WP-Total header with a total number of records, the same way as the default search API does.</li>
<li>Sets X-WP-TotalPages header with a total number of pages, the same way as the default search API does.</li>
<li>Multilingual websites support. Both WPML and Polylang are supported, but not tested well, so let me know if you will find any problems.</li>
<li>Taxonomy filters are supported now. Some features may be missed, so feel free to report them.</li>
<li>Ordering option added. It is also possible to order by meta_key/meta_value/meta_value_num. </li>
</ul>
Brief usage examples
<ul>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=query</li>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=query&per_page=5</li>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=query&per_page=5&page=2</li>
</ul>
Define post type:
<ul>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=query&per_page=5&page=2&type=post</li>
</ul>
Filter by taxonomy/taxonomies:
<ul>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=test&tax_query[0][taxonomy]=category&tax_query[0][field]=id&tax_query[0][terms]=3</li>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=test&tax_query[relation]=AND&tax_query[0][taxonomy]=category&tax_query[0][field]=id&tax_query[0][terms]=3&tax_query[1][taxonomy]=category&tax_query[1][field]=id&tax_query[1][terms]=2</li>
</ul>
Exclude category via taxonomies:
<ul>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=test&tax_query[0][taxonomy]=category&tax_query[0][field]=id&tax_query[0][terms]=3&tax_query[0][operator]=NOT IN</li>
</ul>
For multilingual websites (WPML & Polylang):
<ul>
<li>
https://[your domain]/wp-json/relevanssi/v1/search?keyword=query&lng=en
<ul>
<li>Results in order:</li>
</ul>
</li>
<li>
https://[your domain]/wp-json/relevanssi/v1/search?keyword=test&type=post&orderby=modified&order=DESC
</li>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=test&type=post&orderby=modified&order=ASC</li>
<li>https://[your domain]/wp-json/relevanssi/v1/search?keyword=test&type=post&meta_key=some_key&orderby=meta_value|meta_value_num&order=ASC</li>
</ul>
Demo website
You can try the plugin on our demo website http://demo.erlycoder.com/demo1/. For example, you can try the following request:
Basic: 
<a href="http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test" rel="nofollow ugc">http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test</a>
Order posts by modification time: 
<a href="http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&type=post&orderby=modified&order=DESC" rel="nofollow ugc">http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&type=post&orderby=modified&order=DESC</a> 
<a href="http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&type=post&orderby=modified&order=ASC" rel="nofollow ugc">http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&type=post&orderby=modified&order=ASC</a>
Filter posts by taxonomy (one single category): 
<a href="http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&tax_query[0][taxonomy]=category&tax_query[0][field]=id&tax_query[0][terms]=3" rel="nofollow ugc">http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&tax_query[0][taxonomy]=category&tax_query[0][field]=id&tax_query[0][terms]=3</a>
Filter posts by taxonomy (exclude category): 
[http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&tax_query[0][taxonomy]=category&tax_query[0][field]=id&tax_query[0][terms]=3&tax_query[0][operator]=NOT IN](http://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&tax_query[0][taxonomy]=category&tax_query[0][field]=id&tax_query[0][terms]=3&tax_query[0][operator]=NOT IN)

REST API for Relevanssi

Nice and simple plugin

The plugin provides a REST API endpoint for the Relevanssi search plugin.

jojaba

This plugin is meant to add the word autocomplete in search input field by using the index stored in Relevanssi database. So you have to <a href="https://wordpress.org/plugins/relevanssi/" rel="ugc">install first Relevanssi</a> (free or premium version) to have it to work.
The JS script used to accomplish the autocompletion work has been developed by <a href="http://lea.verou.me/" rel="nofollow ugc">Lea Verou</a> and is called <a href="https://leaverou.github.io/awesomplete" title="Go to the Awesomplete homepage" rel="nofollow ugc">Awesomplete</a>. It is very simple to set up, lightweight and accessible.
<h4>List of functionalities</h4>
<ul>
<li>Settings available: styling, min characters typed to show the autocompletion box, max suggestions in autocompletion box.</li>
<li>Get the Relevanssi Indexed words to have the autcompletion list</li>
<li>Fast and lightweight</li>
<li>English and french languages included</li>
</ul>

Autocomplete For Relevanssi

Relevanssi author here. Please do not use this plugin in its current form. It's dangerous for performance and may cause server crashes. It will also slow down page loads in a rather terrifying manner.

Dangerous, do not use

Autocompletion functionality for WordPress search input when Relevanssi plugin is installed.

REST API for Relevanssi vs Autocomplete For Relevanssi