feat: improve flag override API for testing (#548)

roncohen · web-flow · commit e9920bc98e3f · 2026-03-13T12:35:02.000Z
diff --git a/.changeset/solid-doodles-glow.md b/.changeset/solid-doodles-glow.md
@@ -0,0 +1,5 @@
+---
+"@reflag/node-sdk": minor
+---
+
+improve flag override API for testing
diff --git a/packages/node-sdk/README.md b/packages/node-sdk/README.md
@@ -498,14 +498,32 @@ reflagClient.initialize().then(() => {
 
 ## Testing
 
-When writing tests that cover code with flags, you can toggle flags on/off programmatically to test the different behavior.
+When writing tests that cover code with flags, you can toggle flags on/off programmatically to test different behavior. For tests, you will often want to run the client in offline mode and provide flag overrides directly through the client options.
 
 `reflag.ts`:
 
 ```typescript
 import { ReflagClient } from "@reflag/node-sdk";
 
-export const reflag = new ReflagClient();
+export const reflag = new ReflagClient({
+  offline: true,
+});
+```
+
+You can then set base overrides for a test run by passing `flagOverrides` in the constructor, replacing them later with `setFlagOverrides()`, or clearing them with `clearFlagOverrides()`:
+
+```typescript
+// pass directly in the constructor
+const client = new ReflagClient({
+  offline: true,
+  flagOverrides: { myFlag: true },
+});
+
+// or replace the base overrides at a later time
+client.setFlagOverrides({ myFlag: false });
+
+// clear only the base overrides
+client.clearFlagOverrides();
 ```
 
 `app.test.ts`:
@@ -520,9 +538,9 @@ afterEach(() => {
 
 describe("API Tests", () => {
   it("should return 200 for the root endpoint", async () => {
-    reflag.flagOverrides = {
+    reflag.setFlagOverrides({
       "show-todo": true,
-    };
+    });
 
     const response = await request(app).get("/");
     expect(response.status).toBe(200);
@@ -531,11 +549,61 @@ describe("API Tests", () => {
 });
 ```
 
-See more on flag overrides in the section below.
+`pushFlagOverrides()` serves a different purpose: it adds a temporary layer on top of the base overrides and returns a remove function that removes only that layer. This is useful for nested tests:
+
+```typescript
+export const flag = function (name: string, enabled: boolean): void {
+  let remove: (() => void) | undefined;
+
+  beforeEach(function () {
+    remove = reflagClient.pushFlagOverrides({ [name]: enabled });
+  });
+
+  afterEach(function () {
+    remove?.();
+    remove = undefined;
+  });
+};
+
+describe("foo", () => {
+  describe("with new search ranking enabled", () => {
+    flag("search-ranking-v2", true);
+
+    describe("with summaries enabled", () => {
+      flag("smart-summaries", true);
+
+      // ...
+    });
+  });
+});
+```
+
+The precedence is:
+
+1. Base overrides from the constructor or `setFlagOverrides()`
+2. Temporary layers added by `pushFlagOverrides()`
+
+If the same flag is set in both places, the pushed override wins until its remove function is called.
+
+`pushFlagOverrides()` also accepts a function if the temporary override depends on the evaluation context:
+
+```typescript
+const remove = client.pushFlagOverrides((context) => ({
+  "smart-summaries": context.user?.id === "qa-user",
+}));
+
+// ...
+
+remove();
+```
 
 ## Flag Overrides
 
-Flag overrides allow you to override flags and their configurations locally. This is particularly useful for development and testing. You can specify overrides in three ways:
+Flag overrides allow you to override flags and their configurations locally. This is particularly useful when testing changes locally, for example when running your app and clicking around to verify behavior before deploying your changes.
+
+For automated tests, see the [Testing](#testing) section above.
+
+When testing locally during development, you also have these additional ways to provide overrides:
 
 1. Through environment variables:
 
@@ -563,20 +631,6 @@ REFLAG_FLAGS_DISABLED=flag3,flag4
 }
 ```
 
-1. Programmatically through the client options:
-
-You can use a simple `Record<string, boolean>` and pass it either in the constructor or by setting `client.flagOverrides`:
-
-```typescript
-// pass directly in the constructor
-const client = new ReflagClient({ flagOverrides: { myFlag: true } });
-// or set on the client at a later time
-client.flagOverrides = { myFlag: false };
-
-// clear flag overrides. Same as setting to {}.
-client.clearFlagOverrides();
-```
-
 To get dynamic overrides, use a function which takes a context and returns a boolean or an object with the shape of `{isEnabled, config}`:
 
 ```typescript
diff --git a/packages/node-sdk/examples/express/app.test.ts b/packages/node-sdk/examples/express/app.test.ts
@@ -1,14 +1,32 @@
 import request from "supertest";
 import app, { todos } from "./app";
-import { beforeEach, describe, it, expect, beforeAll } from "vitest";
+import { afterEach, beforeAll, beforeEach, describe, expect, it } from "vitest";
 
 import reflag from "./reflag";
 
+function flag(name: string, enabled: boolean): void {
+  let remove: (() => void) | undefined;
+
+  beforeEach(() => {
+    remove = reflag.pushFlagOverrides({ [name]: enabled });
+  });
+
+  afterEach(() => {
+    remove?.();
+    remove = undefined;
+  });
+}
+
 beforeAll(async () => await reflag.initialize());
+
 beforeEach(() => {
-  reflag.featureOverrides = {
+  reflag.setFlagOverrides({
     "show-todos": true,
-  };
+  });
+});
+
+afterEach(() => {
+  reflag.clearFlagOverrides();
 });
 
 describe("API Tests", () => {
@@ -24,12 +42,13 @@ describe("API Tests", () => {
     expect(response.body).toEqual({ todos });
   });
 
-  it("should return no todos when list is disabled", async () => {
-    reflag.featureOverrides = () => ({
-      "show-todos": false,
+  describe("with show-todos temporarily disabled", () => {
+    flag("show-todos", false);
+
+    it("should return no todos", async () => {
+      const response = await request(app).get("/todos");
+      expect(response.status).toBe(200);
+      expect(response.body).toEqual({ todos: [] });
     });
-    const response = await request(app).get("/todos");
-    expect(response.status).toBe(200);
-    expect(response.body).toEqual({ todos: [] });
   });
 });
diff --git a/packages/node-sdk/examples/express/app.ts b/packages/node-sdk/examples/express/app.ts
@@ -1,6 +1,6 @@
 import reflag from "./reflag";
 import express from "express";
-import { BoundReflagClient } from "../src";
+import type { BoundReflagClient } from "../../src";
 
 // Augment the Express types to include the `reflagUser` property on the `res.locals` object
 // This will allow us to access the ReflagClient instance in our route handlers
diff --git a/packages/node-sdk/examples/express/bucket.ts b/packages/node-sdk/examples/express/bucket.ts
@@ -1,11 +1,11 @@
-import { ReflagClient, Context, FlagOverrides } from "../../";
+import { ReflagClient, Context, FlagOverrides } from "../../src";
 
 type CreateConfigPayload = {
   minimumLength: number;
 };
 
 // Extending the Flags interface to define the available features
-declare module "../../types" {
+declare module "../../src/types" {
   interface Flags {
     "show-todos": boolean;
     "create-todos": {
@@ -18,7 +18,7 @@ declare module "../../types" {
   }
 }
 
-let featureOverrides = (_: Context): FlagOverrides => {
+const flagOverrides = (_: Context): FlagOverrides => {
   return {
     "create-todos": {
       isEnabled: true,
@@ -39,5 +39,5 @@ let featureOverrides = (_: Context): FlagOverrides => {
 export default new ReflagClient({
   // Optional: Set a logger to log debug information, errors, etc.
   logger: console,
-  featureOverrides, // Optional: Set feature overrides
+  flagOverrides, // Optional: Set flag overrides
 });
diff --git a/packages/node-sdk/examples/express/reflag.ts b/packages/node-sdk/examples/express/reflag.ts
@@ -0,0 +1 @@
+export { default } from "./bucket";
diff --git a/packages/node-sdk/src/client.ts b/packages/node-sdk/src/client.ts
diff --git a/packages/node-sdk/src/types.ts b/packages/node-sdk/src/types.ts
diff --git a/packages/node-sdk/test/client.test.ts b/packages/node-sdk/test/client.test.ts

-Original file line number
+Diff line change
@@ @@ -0,0 +1,5 @@ @@
 +---
 +"@reflag/node-sdk": minor
 +---
++
 +improve flag override API for testing