> ## Documentation Index
> Fetch the complete documentation index at: https://doc.lucidworks.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom JavaScript Stages for Query Pipelines

export const LwTemplate = ({title = "Key questions to get you started", icon = "sparkles", cta = "Powered by Agent Studio", linkHref = "https://lucidworks.com/demo/?utm_source=docs&utm_medium=referral&utm_campaign=docs_cta_ai"}) => {
  const [isLoaded, setIsLoaded] = useState(false);
  useEffect(() => {
    const timer = setTimeout(() => {
      setIsLoaded(true);
    }, 500);
    return () => clearTimeout(timer);
  }, []);
  return <div className="lw-template-container">
      <Card title={title} icon={icon}>
        {isLoaded && <span dangerouslySetInnerHTML={{
    __html: `<lw-template id="a029c1a9-28be-427e-b0e1-5d918920246a"></lw-template
            >`
  }} />}
        <Link href={linkHref} className="agent-studio-link text-left text-gray-600 gap-2 dark:text-gray-400 text-sm font-medium flex flex-row items-center hover:text-primary dark:hover:text-primary-light group-hover:text-primary group-hover:dark:text-primary-light">Powered by Lucidworks Agent Studio</Link>
      </Card>
    </div>;
};

[localhost link]: http://localhost:3000/docs/4/fusion-server/concepts/querying/pipelines/custom-javascript-query-stages

[mintlify link]: https://doc.lucidworks.com/docs/4/fusion-server/concepts/querying/pipelines/custom-javascript-query-stages

[old doc.lw link]: https://doc.lucidworks.com/fusion/5.9/170

The JavaScript Query stage allows you to write custom processing logic using JavaScript to manipulate search requests and responses. The first time that the pipeline is run, Fusion compiles the JavaScript program into Java bytecode using the JDK’s JavaScript engine.

The JavaScript Query stage allows you to run JavaScript functions over search requests and responses by manipulating variables called "request" and "response" which are Request objects and Response objects, respectively.

<LwTemplate />

## JavaScript Stages Variables and Parameters

[JavaScript](http://en.wikipedia.org/wiki/JavaScript) is a lightweight scripting language.
The JavaScript in a JavaScript stage is standard [ECMAScript](http://en.wikipedia.org/wiki/ECMAScript).
What a JavaScript program can do depends on the container in which it runs.
For a JavaScript Query stage, the container is a Fusion query pipeline.
The following global pipeline variables are available:

| Name                      | Type                                                                                                                                                     | Description                                                                                                                                                                                                                                                                              |
| ------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `request`                 | [Request](https://javadoc.lucidworks.com/fusion-pipeline-javadocs/4.2/com/lucidworks/apollo/pipeline/query/Request.html)                                 | The Solr query information.                                                                                                                                                                                                                                                              |
| `response`                | [Response](https://javadoc.lucidworks.com/fusion-pipeline-javadocs/4.2/com/lucidworks/apollo/pipeline/query/Response.html)                               | The Solr response information.                                                                                                                                                                                                                                                           |
| `ctx`                     | [Context](https://javadoc.lucidworks.com/fusion-pipeline-javadocs/4.2/com/lucidworks/apollo/pipeline/Context.html)                                       | A reference to the container which holds a map over the pipeline properties. Used to update or modify this information for downstream pipeline stages.                                                                                                                                   |
| `collection`              | String                                                                                                                                                   | The name of the Fusion collection being indexed or queried.                                                                                                                                                                                                                              |
| `solrServer`              | [BufferingSolrServer](https://javadoc.lucidworks.com/fusion-pipeline-javadocs/4.2/com/lucidworks/apollo/component/BufferingSolrServer)                   | The Solr server instance that manages the pipeline’s default Fusion collection. All indexing and query requests are done by calls to methods on this object. See [SolrClient](https://lucene.apache.org/solr/5_2_1/solr-solrj/org/apache/solr/client/solrj/SolrClient.html) for details. |
| `solrServerFactory`       | [SolrClientFactory](https://javadoc.lucidworks.com/fusion-pipeline-javadocs/4.2/com/lucidworks/apollo/component/SolrClientFactory.html)                  | The SolrCluster server used for lookups by collection name which returns a Solr server instance for a that collection. For example:  `var productsSolr = solrServerFactory.getSolrServer("products");`.                                                                                  |
| `QueryRequestAndResponse` | [QueryRequestAndResponse](https://javadoc.lucidworks.com/fusion-pipeline-javadocs/4.2/com/lucidworks/apollo/pipeline/query/QueryRequestAndResponse.html) | Returns list of info as a string.                                                                                                                                                                                                                                                        |

### Syntax Variants

JavaScript stages are written using ***function syntax***, which passes variables as function parameters.

#### Function Syntax

```js wrap  theme={"dark"}
function(request,response) {
   request.addParam("foo", "bar");
}
```

<Tip>
  **Important**

  *Function syntax* is used for the examples in this document.
</Tip>

### Global variable `logger`

The global variable named `logger` writes messages to the log file of the server running the pipeline.
This variable is truly global and does not need to be declared as part of the function parameter list.

There are 5 methods available, which each take either a single argument (the string message to log) or two arguments (the string message and an exception to log).
The five methods are, "debug", "info", "warn", and "error".

## See also

* [Fusion Pipeline Javadocs](https://javadoc.lucidworks.com/fusion-pipeline-javadocs/4.2/)
* [JavaScript Query Stage](/docs/4/fusion-server/reference/pipeline-stages/query/javascript-query-stage)
* [Custom JavaScript Query Stage Examples](/docs/4/fusion-server/reference/pipeline-stages/query/example-custom-javascript-stages)
* [Custom JavaScript Stages Variables and Parameters](/docs/4/fusion-server/reference/query-ops/custom-javascript-stages-variables-and-parameters)