fix: undefined fallback fetch on watch update with pages dev

Subsequent Pages Functions worker builds due to file changes were
not replacing `__FALLBACK_SERVICE_FETCH__` leading to
`ReferenceError`s. This change switches to using `esbuild`'s built-in
`define` option, instead of `esbuild-plugin-replace`, removing an
additional dependency.

packages/wrangler/package.json

@@ -37,7 +37,6 @@
   ],
   "dependencies": {
     "esbuild": "0.14.1",
-    "esbuild-plugin-replace": "^1.1.0",
     "miniflare": "2.0.0",
     "path-to-regexp": "^6.2.0",
     "semiver": "^1.1.0"

packages/wrangler/pages/functions/buildWorker.ts

@@ -1,6 +1,5 @@
 import path from "path";
 import { build } from "esbuild";
-import { replace } from "esbuild-plugin-replace";
 
 type Options = {
   routesModule: string;
@@ -34,12 +33,10 @@ export function buildWorker({
     sourcemap,
     watch,
     allowOverwrite: true,
+    define: {
+      __FALLBACK_SERVICE__: JSON.stringify(fallbackService),
+    },
     plugins: [
-      replace({
-        __FALLBACK_SERVICE_FETCH__: fallbackService
-          ? `env.${fallbackService}.fetch`
-          : "fetch",
-      }),
       {
         name: "wrangler notifier and monitor",
         setup(pluginBuild) {

packages/wrangler/pages/functions/template-worker.ts

@@ -29,6 +29,8 @@ type RouteHandler = {
 
 // inject `routes` via ESBuild
 declare const routes: RouteHandler[];
+// define `__FALLBACK_SERVICE__` via ESBuild
+declare const __FALLBACK_SERVICE__: string;
 
 // expect an ASSETS fetcher binding pointing to the asset-server stage
 type Env = {
@@ -89,8 +91,11 @@ function* executeRequest(request: Request, env: Env) {
 
   // Finally, yield to the fallback service (`env.ASSETS.fetch` in Pages' case)
   return {
-    // @ts-expect-error we don't define a type for the special global __FALLBACK_SERVICE_FETCH__ since we replace it when we're building anyway
-    handler: () => __FALLBACK_SERVICE_FETCH__(request.url, request),
+    handler: () =>
+      __FALLBACK_SERVICE__
+        ? // @ts-expect-error expecting __FALLBACK_SERVICE__ to be the name of a service binding, so fetch should be defined
+          env[__FALLBACK_SERVICE__].fetch(request)
+        : fetch(request),
     params: {} as Params,
   };
 }