# Function Handler (Custom Handler) As an API gateway, the Request Handler is the most important part of a Zuplo project. This document shows how you can build a custom handler - this is often used by developers building BFF (backend-for-frontend), doing orchestration or custom traffic management. A request handler is a module with an export that fulfills the following type definition (typescript). ```ts export type RequestHandler = ( request: ZuploRequest, context: ZuploContext, ) => Promise; ``` An example implementation is provided in the default module template (when you add a new module): ```ts import { ZuploRequest, ZuploContext } from "@zuplo/runtime"; export default async function (request: ZuploRequest, context: ZuploContext) { return "What zup?"; } ``` This is a request handler that receives a request (of type `ZuploRequest` - more on [this interface here](../programmable-api/zuplo-request.mdx)) and returns a response of type `string`. You can return any type from a Request Handler and Zuplo will automatically serialize the response to JSON and add a `content-type` header to your response of `application/json`. This makes it easy to build simple JSON APIs. ## Parameters & Query strings You can read parameters specified on the route path as follows: Route = `/foos/:foo/bars/:bar` This route has two parameters `foo` and `bar`. They can be accessed in the request handler on the `request.params` object: ```ts // GET root/foos/123/bars/car export default async function (request: ZuploRequest, context: ZuploContext) { return request.params.foo + request.params.bar; } // returns 123car ``` You can read **query strings** as follows: URL = `/foos?productId=xkcd&carId=1234` ```ts // GET /foos?productId=xkcd&carId=1234 export default async function (request: ZuploRequest, context: ZuploContext) { return request.query.productId + request.query.carId; } // returns xkcd1234 ``` ## Reading the body The `ZuploRequest` object inherits from the web standards `Request` object. You can read about it on MDN here: [https://developer.mozilla.org/en-US/docs/Web/API/Request](https://developer.mozilla.org/en-US/docs/Web/API/Request) The `Request` type has three properties that allow access to the incoming body of the request. The `body` property is of type [ReadableStream](https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream) and is the most efficient way to reuse a body in an outgoing request. If you want to read the body you have two options: - `await request.text()` - this method reads the full body into a string. - `await request.json()` - this method reads the body and performs a `JSON.parse()` to read the body into an object in memory. Use only if you’re confident the body is well-formed JSON (consider pre-validation with the [Validation Policy](../policies/request-validation-inbound.mdx)). ## Response Class If you want more control over the response you can return an instance of the `Response` class. The `Response` class is also from web standards - you can read about it on MDN here: [https://developer.mozilla.org/en-US/docs/Web/API/Response](https://developer.mozilla.org/en-US/docs/Web/API/Response) Here's an example ```ts import { ZuploRequest, ZuploContext } from "@zuplo/runtime"; export default async function (request: ZuploRequest, context: ZuploContext) { const response = new Response(`What zup?`, { status: 200, headers: { "content-type": "text/html", }, }); return response; } ```