Add mergeConfig option to opt out of implicit config object merging

README.md

@@ -203,7 +203,8 @@ Instantiates middleware. See an [example](https://github.com/firebase/superstati
 
 * `options` - Optional configuration:
   * `fallthrough` - When `false`, render a 404 page from within Superstatic rather than calling through to the next middleware. Defaults to `true`.
-  * `config` - A file path to your application's configuration file (see [Configuration](#configuration)) or an object containing your application's configuration. If an object is provided, it will be merged into existing config in a `superstatic.json`.
+  * `config` - A file path to your application's configuration file (see [Configuration](#configuration)) or an object containing your application's configuration. If an object is provided, it will be merged into existing config in a `superstatic.json` or `firebase.json` unless `mergeConfig` is set to `false`.
+  * `mergeConfig` - When `false`, the config object you provide is used as-is without being merged with any `superstatic.json` or `firebase.json` found on disk. Defaults to `true`. Only applies when `config` is a plain object.
   * `protect` - Adds HTTP basic auth. Example:  `username:password`
   * `env`- A file path your application's environment variables file or an object containing values that are made available at the urls `/__/env.json` and `/__/env.js`. See the documentation detail on [environment variables](http://docs.firebase.com/guides/environment-variables).
   * `cwd` - The current working directory to set as the root. Your application's `public` configuration option will be used relative to this.

src/loaders/config-file.js

@@ -27,22 +27,25 @@ const { isPlainObject } = require("../utils/objectutils");
 
 const CONFIG_FILE = ["superstatic.json", "firebase.json"];
 
-module.exports = function (filename) {
+module.exports = function (filename, mergeConfig = true) {
   if (typeof filename === "function") {
     return filename;
   }
 
   filename = filename ?? CONFIG_FILE;
 
   let configObject = {};
+  let configObjectProvided = false;
   let config = {};
 
   // From custom config data passed in
   try {
     configObject = JSON.parse(filename);
+    configObjectProvided = true;
   } catch {
     if (isPlainObject(filename)) {
       configObject = filename;
+      configObjectProvided = true;
       filename = CONFIG_FILE;
     }
   }
@@ -71,5 +74,5 @@ module.exports = function (filename) {
 
   // Passing an object as the config value merges
   // the config data
-  return { ...config, ...configObject };
+  return configObjectProvided && !mergeConfig ? configObject : { ...config, ...configObject };
 };

src/options.ts

@@ -3,6 +3,7 @@ import { Configuration } from "./config";
 
 export interface MiddlewareOptions {
   fallthrough?: boolean;
+  mergeConfig?: boolean;
   config?: string | Configuration;
   protect?: string;
   env?: string | Record<string, string>;

src/superstatic.js

@@ -51,7 +51,7 @@ const superstatic = function (spec = {}) {
 
   // Load data
   /** @type {Configuration} */
-  const config = (spec.config = loadConfigFile(spec.config));
+  const config = (spec.config = loadConfigFile(spec.config, spec.mergeConfig));
   config.errorPage = config.errorPage ?? "/404.html";
 
   // Set up provider

test/unit/loaders/config-file.spec.js

@@ -77,6 +77,48 @@ describe("loading config files", () => {
     done();
   });
 
+  describe("mergeConfig: false", () => {
+    it("returns object as-is without merging with superstatic.json", async () => {
+      await fs.writeFile(
+        "superstatic.json",
+        '{"firebase": "superstatic", "public": "./"}',
+        "utf-8",
+      );
+
+      const config = loadConfigFile({ public: "app" }, false);
+
+      expect(config).to.eql({ public: "app" });
+
+      await fs.rm("superstatic.json");
+    });
+
+    it("returns object as-is without merging with firebase.json", async () => {
+      await fs.writeFile(
+        "firebase.json",
+        '{"firebase": "example", "public": "./"}',
+        "utf-8",
+      );
+
+      const config = loadConfigFile({ public: "app" }, false);
+
+      expect(config).to.eql({ public: "app" });
+
+      await fs.rm("firebase.json");
+    });
+
+    it("returns file config as-is when mergeConfig is false and config is a file path", async () => {
+      await fs.writeFile(
+        ".tmp/myconfig.json",
+        '{"public": "app"}',
+        "utf-8",
+      );
+
+      const config = loadConfigFile(".tmp/myconfig.json", false);
+
+      expect(config).to.eql({ public: "app" });
+    });
+  });
+
   describe("extends the file config with the object passed", () => {
     it("superstatic.json", async () => {
       await fs.writeFile(