CORS preflight (HTTP `options` method)

[danielweck]

Currently, a number of HTTP routes / URL paths defined in r2-streamer-js invoke the setResponseCORS() utility function in order to populate HTTP responses with adequate CORS headers:

r2-streamer-js/src/http/server.ts

Lines 259 to 271 in c1f3ac0

	public setResponseCORS(res: express.Response) {
	res.setHeader("Access-Control-Allow-Origin",
	"*");
	res.setHeader("Access-Control-Allow-Methods",
	"GET, HEAD, OPTIONS"); // POST, DELETE, PUT, PATCH
	res.setHeader("Access-Control-Allow-Headers",
	"Content-Type, Content-Length, Accept-Ranges, Content-Range, Range, Link, Transfer-Encoding");
	res.setHeader("Access-Control-Expose-Headers",
	"Content-Type, Content-Length, Accept-Ranges, Content-Range, Range, Link, Transfer-Encoding");
	}

( https://github.com/readium/r2-streamer-js/blob/develop/src/http/server.ts#L259-L271 )

For example, for serving the ReadiumWebPublicationManifest (JSON):

r2-streamer-js/src/http/server-manifestjson.ts

Lines 344 to 345 in c1f3ac0

	server.setResponseCORS(res);
	res.set("Content-Type", "application/webpub+json; charset=utf-8");

( https://github.com/readium/r2-streamer-js/blob/develop/src/http/server-manifestjson.ts#L344-L345 )

However, what about HTTP options for CORS? How is this useful? What are the use-cases?

Code example:

public expressOptions(paths: string[], func: express.Handler) {
    this.expressApp.options(paths, func);
}

...added to:

r2-streamer-js/src/http/server.ts

Lines 136 to 142 in c1f3ac0

	public expressUse(pathf: string, func: express.Handler) {
	this.expressApp.use(pathf, func);
	}
	public expressGet(paths: string[], func: express.Handler) {
	this.expressApp.get(paths, func);
	}

( https://github.com/readium/r2-streamer-js/blob/develop/src/http/server.ts#L136-L142 )

...and:

_server.expressOptions(["/URL_ROUTE_PATH"],
    (req: express.Request, res: express.Response) => {

    res.setHeader("Access-Control-Allow-Origin", "*");
    res.setHeader("Access-Control-Allow-Credentials", true);
    res.setHeader("Access-Control-Allow-Methods", req.headers["access-control-request-method"]);
    res.setHeader("Access-Control-Allow-Headers", req.headers["access-control-request-headers"]);
    res.status(200).end();
});

Inspired from NYPL's customizations:

https://github.com/NYPL-Simplified/opds-web-client/blob/1600781c267e241e5deb4094007e50df79eb6236/packages/server/index.js#L73
( https://github.com/NYPL-Simplified/opds-web-client/blob/master/packages/server/index.js#L73 )

CC @aslagle