Add first draft (untested).

src/index.ts

@@ -1,2 +1,103 @@
-import { Request } from "node";
-export function handler(req: Request) {}
+import { Request, Response } from "express";
+import { parseRangeHeader, RangeParserError, Range } from "./parseRangeHeader";
+import { Stream } from "stream";
+
+/**
+ * @type {function (Request): Promise<Content>}
+ */
+export type ContentProvider = (req: Request) => Promise<Content>;
+
+export class ContentDoesNotExistError extends Error {}
+
+export interface Logger {
+  debug(message: string, extra?: any): void;
+}
+
+export type Content = {
+  /**
+   * Returns a readable stream based on the provided range (optional).
+   * @param {Range} range The start-end range of stream data.
+   * @returns {Stream} A readable stream
+   */
+  getStream(range?: Range): Stream;
+  /**
+   * Total size of the content
+   */
+  readonly totalSize: number;
+  /**
+   * Mime type to be sent in Content-Type header
+   */
+  readonly mimeType: string;
+  /**
+   * File name to be sent in Content-Disposition header
+   */
+  readonly fileName: string;
+};
+
+const getHeader = (name: string, req: Request) => req.headers[name];
+const getRangeHeader = getHeader.bind(null, "range");
+const setHeader = (name: string, value: string, res: Response) => res.setHeader(name, value);
+const setContentTypeHeader = setHeader.bind(null, "Content-Type");
+const setContentLengthHeader = setHeader.bind(null, "Content-Length");
+const setAcceptRangesHeader = setHeader.bind(null, "Accept-Ranges", "bytes");
+const setContentRangeHeader = (range: Range | null, size: number, res: Response) =>
+  setHeader("Content-Range", `bytes ${range ? `${range.start}-${range.end}` : "*"}/${size}`, res);
+const setContentDispositionHeader = (fileName: string, res: Response) =>
+  setHeader("Content-Disposition", `attachment; filename="${fileName}"`, res);
+const setCacheControlHeaderNoCache = setHeader.bind(null, "Cache-Control", "no-cache");
+
+export function create(contentProvider: ContentProvider, logger: Logger) {
+  return async function handler(req: Request, res: Response) {
+    let content;
+    try {
+      content = await contentProvider(req);
+    } catch (error) {
+      logger.debug("ContentProvider threw exception: ", error);
+      if (error instanceof ContentDoesNotExistError) {
+        return res.status(400).send(error.message);
+      }
+
+      return res.sendStatus(500);
+    }
+
+    let { getStream, mimeType, fileName, totalSize } = content;
+
+    const rangeHeader = getRangeHeader(req);
+    let range;
+    try {
+      range = parseRangeHeader(rangeHeader, totalSize);
+    } catch (error) {
+      logger.debug(`parseRangeHeader error: `, error);
+      if (error instanceof RangeParserError) {
+        setContentRangeHeader(null, totalSize, res);
+
+        return res
+          .send(error.message)
+          .status(416)
+          .end();
+      }
+
+      return res.sendStatus(500);
+    }
+
+    let { start, end } = range;
+
+    setContentTypeHeader(mimeType, res);
+    setContentDispositionHeader(fileName, res);
+    setAcceptRangesHeader(res);
+
+    // If range is not specified, or the file is empty, return the full stream
+    if (range === null) {
+      setContentLengthHeader(totalSize, res);
+      return getStream().pipe(res);
+    }
+
+    setContentRangeHeader(range, totalSize, res);
+    setContentLengthHeader(start === end ? 0 : end - start + 1);
+    setCacheControlHeaderNoCache(res);
+
+    // Return 206 Partial Content status
+    res.status(206);
+    getStream(range).pipe(res);
+  };
+}