cors-middleware

HTTP cross-origin resource sharing(CORS) middleware.

Compliant with Fetch living standard, 3.2. CORS protocol.

Terms

CORS request

A request with Origin header

CORS preflight request

A CORS request and satisfies the following:

• Method is Options
• Includes Access-Control-Request-Method header
• Includes Access-Control-Request-Headers header

Middleware

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

CORS request

Add a CORS header to the response in the downstream.

No special action is taken in response to CORS preflight requests. Use preflight for that.

import {
  cors,
  type Handler,
} from "https://deno.land/x/cors_middleware@$VERSION/mod.ts";
import { assert } from "https://deno.land/std/testing/asserts.ts";

const middleware = cors();
const corsRequest = new Request("test:", {
  headers: { origin: "<origin>" },
});
declare const handler: Handler;

const response = await middleware(corsRequest, handler);

assert(response.headers.has("access-control-allow-origin"));

yield:

Access-Control-Allow-Origin: *
Vary: Origin

CORS request options

cors accept following options:

Name	Type	Description
allowOrigins	* | (string | RegExp )[]	Allowed origin list.
allowCredentials	true | "true"	Access-Control-Allow-Credentials
exposeHeaders	string[]	Access-Control-Expose-Headers

AllowOrigins

allowOrigins is * or a list of allowed origins.

The default is *.

Asterisk

If *, Access-Control-Allow-Origin(*) will add to the response.

List

The list may consist of strings or regular expression objects.

The middleware compares Origin header and each element of the allowOrigins.

If matched, Access-Control-Allow-Origin(Origin header) will add to the response.

If no match, Access-Control-Allow-Origin(null) will add to the response.

import {
  cors,
} from "https://deno.land/x/cors_middleware@$VERSION/middleware.ts";
import { assert } from "https://deno.land/std/testing/asserts.ts";

const middleware = cors({
  allowOrigins: ["https://test.example", /^https:\/\/cdn\..*/],
});

yield:

Access-Control-Allow-Origin: <Origin>
Vary: Origin

AllowCredentials

The allowCredentials value will serialize and added to the response as Access-Control-Allow-Credentials header.

import {
  cors,
} from "https://deno.land/x/cors_middleware@$VERSION/middleware.ts";

const middleware = cors({ allowCredentials: true });

yield:

Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
Vary: Origin

ExposeHeaders

The value of exposeHeaders will serialize and added to the response as an Access-Control-Expose-Headers header.

However, if the request is a CORS preflight request, it is not added.

import {
  cors,
} from "https://deno.land/x/cors_middleware@$VERSION/middleware.ts";

const middleware = cors({ exposeHeaders: ["x-test"] });

yield:

Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: x-test
Vary: Origin

CORS options serialization error

Each option will serialize.

If serialization fails, it throws an error as follows:

• Elements of exposeHeaders are not <field-name format

import {
  cors,
} from "https://deno.land/x/cors_middleware@$VERSION/middleware.ts";
import { assertThrows } from "https://deno.land/std/testing/asserts.ts";

assertThrows(() => cors({ exposeHeaders: ["<invalid:field-name>"] }));

Effects

Middleware will make changes to the following elements of the HTTP message.

• HTTP Headers
  • Access-Control-Allow-Origin
  • Access-Control-Allow-Credentials
  • Access-Control-Expose-Headers
  • Vary

CORS preflight request

Create CORS preflight request handler.

import {
  type Handler,
  preflight,
} from "https://deno.land/x/cors_middleware@$VERSION/mod.ts";
import { assert } from "https://deno.land/std/testing/asserts.ts";
import { assertSpyCalls, spy } from "https://deno.land/std/testing/mock.ts";

const corsPreflightRequest = new Request("test:", {
  method: "OPTIONS",
  headers: {
    origin: "<origin>",
    "access-control-request-method": "POST",
    "access-control-request-headers": "content-type",
  },
});

declare const handler: Handler;
const next = spy(handler);
const handlePreflight = preflight();
const response = await handlePreflight(corsPreflightRequest, next);

assertSpyCalls(next, 0);
assert(response.status === 204);
assert(response.headers.has("access-control-allow-origin"));
assert(response.headers.has("access-control-allow-methods"));
assert(response.headers.has("access-control-allow-headers"));
assert(response.headers.has("vary"));

yield:

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: POST
Access-Control-Allow-Headers: content-type
Vary: origin, access-control-request-method, access-control-request-headers

If the request is not a CORS preflight request, next will execute.

CORS preflight options

preflight accept following options:

Name	Type	Description
allowMethods	string[]	Access-Control-Allow-Methods
allowHeaders	string[]	Access-Control-Allow-Headers
maxAge	number	Access-Control-Max-Age
status	200 | 204	Preflight response status code.

and CORS request options without exposeHeaders.

AllowMethods

The value of allowMethods will serialize and added to the response as an Access-Control-Allow-Methods header.

If not specified, Access-Control-Request-Method header will add as Access-Control-Allow-Methods header to the response.

import { preflight } from "https://deno.land/x/cors_middleware@$VERSION/middleware.ts";

const handler = preflight({ allowMethods: ["POST", "PUT"] });

yield:

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: POST, PUT
Access-Control-Allow-Headers: <Access-Control-Request-Headers>
Vary: origin, access-control-request-method, access-control-request-headers

AllowHeaders

The value of allowHeaders will serialize and added to the response as an Access-Control-Allow-Headers header.

If not specified, Access-Control-Request-Headers will add as Access-Control-Allow-Headers header to the response.

import { preflight } from "https://deno.land/x/cors_middleware@$VERSION/middleware.ts";

const handler = preflight({ allowHeaders: ["x-test1", "x-test2"] });

yield:

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: <Access-Control-Request-Method>
Access-Control-Allow-Headers: x-test1, x-test2
Vary: origin, access-control-request-method, access-control-request-headers

MaxAge

The value of maxAge will serialize and added to the response as an Access-Control-Max-Age header.

import { preflight } from "https://deno.land/x/cors_middleware@$VERSION/middleware.ts";

const handler = preflight({ maxAge: 86400 });

yield:

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: <Access-Control-Request-Method>
Access-Control-Allow-Headers: <Access-Control-Request-Headers>
Access-Control-Max-Age: 86400
Vary: origin, access-control-request-method, access-control-request-headers

Status

The default is 204 No Content.

You can change to 200 OK.

Serialization error

Throws an error if option has an invalid value.

This is following case:

• Elements of allowMethods are not <method> format
• Elements of allowHeaders are not <field-name> format
• maxAge is not non-negative integer

import {
  preflight,
} from "https://deno.land/x/cors_middleware@$VERSION/middleware.ts";
import { assertThrows } from "https://deno.land/std/testing/asserts.ts";

assertThrows(() => preflight({ allowMethods: ["<invalid:method>"] }));
assertThrows(() => preflight({ allowHeaders: ["<invalid:field-name>"] }));
assertThrows(() => preflight({ maxAge: NaN }));

API

All APIs can be found in the deno doc.

FAQ

Why are there two separate modules?

Because the two offer different functions. cors creates middleware to provide CORS headers.

On the other hand, preflight creates a handler for CORS preflight requests. (Although it is actually a middleware signature, since it transfers processing to subsequent requests other than CORS preflight requests.)

Mixing middleware with handler characteristics and middleware characteristics will create expressive constraints.

License

Copyright © 2023-present httpland.

Released under the MIT license