feat: async pathRewrite (#397)

Allow the path rewriter function to be async

README.md

@@ -199,6 +199,14 @@ Providing an alternative way to decide which requests should be proxied; In case
 
   // custom rewriting
   pathRewrite: function (path, req) { return path.replace('/api', '/base/api') }
+
+  // custom rewriting, returning Promise
+  pathRewrite: async function (path, req) {
+    var should_add_something = await httpRequestToDecideSomething(path);
+    if (should_add_something) path += "something";
+    return path;
+  }
+
   ```
 
 - **option.router**: object/function, re-target `option.target` for specific requests.

src/http-proxy-middleware.ts

@@ -116,7 +116,7 @@ export class HttpProxyMiddleware {
     // 1. option.router
     // 2. option.pathRewrite
     await this.applyRouter(req, newProxyOptions);
-    this.applyPathRewrite(req, this.pathRewriter);
+    await this.applyPathRewrite(req, this.pathRewriter);
 
     // debug logging for both http(s) and websockets
     if (this.proxyOptions.logLevel === 'debug') {
@@ -157,9 +157,9 @@ export class HttpProxyMiddleware {
   };
 
   // rewrite path
-  private applyPathRewrite = (req: IRequest, pathRewriter) => {
+  private applyPathRewrite = async (req: IRequest, pathRewriter) => {
     if (pathRewriter) {
-      const path = pathRewriter(req.url, req);
+      const path = await pathRewriter(req.url, req);
 
       if (typeof path === 'string') {
         req.url = path;

src/types.ts

@@ -18,7 +18,8 @@ export type Filter =
 export interface Options extends httpProxy.ServerOptions {
   pathRewrite?:
     | { [regexp: string]: string }
-    | ((path: string, req: IRequest) => string);
+    | ((path: string, req: IRequest) => string)
+    | ((path: string, req: IRequest) => Promise<string>);
   router?:
     | { [hostOrPath: string]: string }
     | ((req: IRequest) => string)

test/types.spec.ts

@@ -58,6 +58,11 @@ describe('http-proxy-middleware TypeScript Types', () => {
         options = { pathRewrite: (path, req) => '/path' };
         expect(options).toBeDefined();
       });
+
+      it('should have pathRewrite Type with async function', () => {
+        options = { pathRewrite: async (path, req) => '/path' };
+        expect(options).toBeDefined();
+      });
     });
 
     describe('router', () => {

test/unit/path-rewriter.spec.ts

@@ -102,6 +102,44 @@ describe('Path rewriting', () => {
 
       expect(rewriter(rewriteFn)).toBe('/123/789');
     });
+
+    // Same tests as the above three, but async
+
+    it('is async and should return unmodified path', () => {
+      const rewriteFn = async path => {
+        const promise = new Promise((resolve, reject) => {
+          resolve(path);
+        });
+        const changed = await promise;
+        return changed;
+      };
+
+      expect(rewriter(rewriteFn)).resolves.toBe('/123/456');
+    });
+
+    it('is async and should return alternative path', () => {
+      const rewriteFn = async path => {
+        const promise = new Promise((resolve, reject) => {
+          resolve('/foo/bar');
+        });
+        const changed = await promise;
+        return changed;
+      };
+
+      expect(rewriter(rewriteFn)).resolves.toBe('/foo/bar');
+    });
+
+    it('is async and should return replaced path', () => {
+      const rewriteFn = async path => {
+        const promise = new Promise((resolve, reject) => {
+          resolve(path.replace('/456', '/789'));
+        });
+        const changed = await promise;
+        return changed;
+      };
+
+      expect(rewriter(rewriteFn)).resolves.toBe('/123/789');
+    });
   });
 
   describe('Invalid configuration', () => {
@@ -136,5 +174,10 @@ describe('Path rewriting', () => {
       // tslint:disable-next-line: no-empty
       expect(badFn(() => {})).not.toThrowError(Error);
     });
+
+    it('should not throw when async function config is provided', () => {
+      // tslint:disable-next-line: no-empty
+      expect(badFn(async () => {})).not.toThrowError(Error);
+    });
   });
 });