http-conditional-requests

HTTP Conditional Requests middleware for Fetch API.

What

Middleware for HTTP Conditional Requests.

It conditionally processes a HTTP request based on a precondition.

It compliant with RFC 9110, 13. Conditional Requests.

Middleware

For a definition of Universal HTTP middleware, see the http-middleware project.

Usage

Middleware factory is exported by default.

To evaluate precondition, you need to provide a function to retrieve the selected representation.

The following example evaluates the If-None-Match precondition and controls the handler.

import conditionalRequests from "https://deno.land/x/http_conditional_requests@$VERSION/mod.ts";
import { assertEquals } from "https://deno.land/std/testing/asserts.ts";
import { assertSpyCalls, spy } from "https://deno.land/std/testing/mock.ts";

const selectedRepresentation = new Response("<body>", {
  headers: { etag: "<etag>" },
});
const selectRepresentation = spy(() => selectedRepresentation);
const middleware = conditionalRequests(selectRepresentation);
const conditionalRequest = new Request("<uri>", {
  headers: { "if-none-match": "<etag>" },
});
const handler = spy(() => selectedRepresentation);

const response = await middleware(conditionalRequest, handler);

assertSpyCalls(handler, 0);
assertSpyCalls(selectRepresentation, 1);
assertEquals(response.status, 304);

Preconditions

RFC 9110, 13.1. Preconditions compliant and supports the following precondition:

If-Match
If-None-Match
If-Modified-Since
If-Unmodified-Since

If multiple precondition headers are present, precondition is processed according to precedence.

Effects

Middleware will effect following:

HTTP response status
- 304 (Not Modified)
- 412 (Precondition Failed)

Conditions

Middleware will execute only if the following conditions are met:

The precondition header exists
- If-Match
  - The ETag header exist
- If-None-Match
  - The ETag header exist
- If-Modified-Since
  - The Last-Modified header exist
- If-Unmodified-Since
  - The Last-Modified header exist

License

Released under the MIT license