Class <Interface> Handler


Requests are fulfilled by handlers. A handler receives the request context as well as a next function with which to pass along a request to the next handler if it so chooses.

A handler may be any object with a request method. This allows both stateful and non-stateful handlers to be utilized.

If a handler calls next, it receives a Future which resolves to a StructuredDocument that it can then compose how it sees fit with its own response.

type NextFn<P> = (req: RequestInfo) => Future<P>;

interface Handler {
 async request<T>(context: RequestContext, next: NextFn<P>): T;

RequestContext contains a readonly version of the RequestInfo as well as a few methods for building up the StructuredDocument and Future that will be part of the response.

interface RequestContext<T> {
 readonly request: RequestInfo;

 setStream(stream: ReadableStream | Promise<ReadableStream>): void;
 setResponse(response: Response | ResponseInfo): void;

A basic fetch handler with support for streaming content updates while the download is still underway might look like the following, where we use response.clone() to tee the ReadableStream into two streams.

A more efficient handler might read from the response stream, building up the response content before passing along the chunk downstream.

const FetchHandler = {
 async request(context) {
   const response = await fetch(context.request);

   return response.json();

Stream Currying

RequestManager.request and next differ from fetch in one crucial detail in that the outer Promise resolves only once the response stream has been processed.

For context, it helps to understand a few of the use-cases that RequestManager is intended to allow.

  • to manage and return streaming content (such as video files)
  • to fulfill a request from multiple sources or by splitting one request into multiple requests
  • for instance one API call for a user and another for the user's friends
  • or e.g. fulfilling part of the request from one source (one API, in-memory, localStorage, IndexedDB etc.) and the rest from another source (a different API, a WebWorker, etc.)
  • to coalesce multiple requests
  • to decorate a request with additional info
  • e.g. an Auth handler that ensures the correct tokens or headers or cookies are attached.

await fetch(<req>) resolves at the moment headers are received. This allows for the body of the request to be processed as a stream by application code while chunks are still being received by the browser.

When an app chooses to await response.json() what occurs is the browser reads the stream to completion and then returns the result. Additionally, this stream may only be read once.

The RequestManager preserves this ability to subscribe to and utilize the stream by either the application or the handler – thereby delivering the full power and flexibility of native APIs – without restricting developers in ways that lead to complicated workarounds.

Each handler may call setStream only once, but may do so at any time until the promise that the handler returns has resolved. The associated promise returned by calling future.getStream will resolve with the stream set by setStream if that method is called, or null if that method has not been called by the time that the handler's request method has resolved.

Handlers that do not create a stream of their own, but which call next, should defensively pipe the stream forward. While this is not required (see automatic currying below) it is better to do so in most cases as otherwise the stream may not become available to downstream handlers or the application until the upstream handler has fully read it.


Handlers that either call next multiple times or otherwise have reason to create multiple fetch requests should either choose to return no stream, meaningfully combine the streams, or select a single prioritized stream.

Of course, any handler may choose to read and handle the stream, and return either no stream or a different stream in the process.

Automatic Currying of Stream and Response

In order to simplify the common case for handlers which decorate a request, if next is called only a single time and setResponse was never called by the handler, the response set by the next handler in the chain will be applied to that handler's outcome. For instance, this makes the following pattern possible return (await next(<req>)).content;.

Similarly, if next is called only a single time and neither setStream nor getStream was called, we automatically curry the stream from the future returned by next onto the future returned by the handler.

Finally, if the return value of a handler is a Future, we curry content and errors as well, thus enabling the simplest form return next(<req>).

In the case of the Future being returned, Stream proxying is automatic and immediate and does not wait for the Future to resolve.

Handler Order

Request handlers are registered by configuring the manager via use

const manager = new RequestManager();

manager.use([Handler1, Handler2]);

Handlers will be invoked in the order they are registered ("fifo", first-in first-out), and may only be registered up until the first request is made. It is recommended but not required to register all handlers at one time in order to ensure explicitly visible handler ordering.




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