Using Solr from JavaScript clients is so straightforward that it deserves a special mention. In fact, it is so straightforward that there is no client API. You don’t need to install any packages or configure anything.
HTTP requests can be sent to Solr using the standard XMLHttpRequest mechanism.
By default, Solr sends JavaScript Object Notation (JSON) responses, which are easily interpreted in JavaScript. You don’t need to add anything to the request URL to have responses sent as JSON.
For more information and an excellent example, read the SolJSON page on the Solr Wiki: