Flag

This library aims to offer a best-in-class interface for working with feature flags in TypeScript-based React applications.

npm install flag

Getting Started

flag works by creating bindings at runtime so that context providers, components, and hooks are all strictly typed together. createFlags builds these bindings without requiring an data.

// flags.ts

import { createFlags } from "flag";

export type MyFlags = {
  features: {
    useMyCoolNewThing: boolean;
  };
  config: {
    apiUrl: string;
  };
  cool: number;
  dude: number;
  coolAndDude: number;
  largeCoolAndDude: boolean;
};

export const { FlagBackendProvider, Flag, useFlag } = createFlags<MyFlags>();

React Bindings

FlagBackendProvider

Returned as part of createFlags<T>().

This React component provides a Backend<T> (see below) as a data source for Flag and useFlag.

Props	Type	Required	Description
backend	Types.Backend<T>	true	The data source for flags
children	ReactNode	true	React children

// index.tsx

import React from "react";
import ReactDOM from "react-dom";
import { NullBackend } from "flag";
import { App } from "./app";
import { FlagBackendProvider } from "./flags";

const backend = new NullBackend();

const root = ReactDOM.createRoot(document.querySelector("#app"));

root.render(
  <FlagBackendProvider backend={backend}>
    <App />
  </FlagBackendProvider>
);

useFlag

Returned as part of createFlags<T>().

A hook to fetch a single flag. Requires a valid key path and a default value. The key path must terminate at a string, boolean or number and the default value must be of the same type that it terminates. Forcing a default to be provided will minimize the change of a runtime error occurring.

Args	Type	Required	Description
keyPath	Types.KeyPath<T> | Types.KeyPathString<T>	true	A valid key path of T to a string, boolean or number
defaultValue	GetValueFromKeyPath<T, KP>	true	A fallback in case it is not available in the Backend<T>

// my-component.tsx

import { useFlag } from "./flags";

const MyComponent = () => {
  /**
   * The key path can be either an array or string of keys joined by `.`
   * It _must_ terminate at a string, boolean or number type.
   */
  const apiUrl = useFlag(["config", "apiUrl"], "https://example.com");
  const apiUrl2 = useFlag("config.apiUrl", "https://example.com");

  return <div>The API url is "{apiUrl}"</div>;
};

Flag

Returned as part of createFlags<T>().

Renders a some UI based on whether a flag is false or not. (It's a glorified if statement 😬).

Args	Type	Required	Description
keyPath	Types.KeyPath<T> | Types.KeyPathString<T>	true	A valid key path of T to a string, boolean or number
defaultValue	GetValueFromKeyPath<T, KP>	true	A fallback in case it is not available in the Backend<T>
render	(flags: T) => ReactNode	true	Function that returns a ReactNode
fallback	() => ReactNode	false	Function that returns a ReactNode

<Flag
  name="features.useMyCoolNewThing"
  defaultValue={false}
  render={() => <div>Rendered on truthy</div>}
  fallback={() => <div>Rendered on falsy</div>}
/>

Backends

FlagBackendProvider requires that you pass a Backend<T> which is responsible for retreiving flags to your application.

flag bundles with several useful backends, but you can also roll your own.

StaticBackend<T>

Accepts a JSON object that matches the partial shape of your flags. It can be nested, but shouldn't use arrays.

import React from "react";
import ReactDOM from "react-dom";
import { StaticBackend } from "flag";
import { FlagBackendProvider, MyFlags } from "./flags";
import { App } from "./app";

const backend = new StaticBackend<MyFlags>({
  features: {
    useMyCoolNewThing: false,
  },
  config: {
    apiUrl: "https://example.com",
  },
  cool: 100,
  dude: 200,
  coolAndDude: 300,
  largeCoolAndDude: 600,
});

const root = ReactDOM.createRoot(document.querySelector("#app"));

root.render(
  <FlagBackendProvider backend={backend}>
    <App />
  </FlagBackendProvider>
);

If a partial object is provided and a requested key is missing, it will fallback to the provided default. That is,

const backend = new StaticBackend<MyFlags>({
  features: {},
});

// ...

const SomeScreen = () => {
  const newThing = useFlag("features.useMyCoolNewThing", true);
  // => will always be `true`

  // ...
};

ComputedBackend<T>

Similar to StaticBackend but if a function is used as a value, it will pass in the object T as an argument. (Yes, this means you can end up with a stack overflow if you're not careful.) Useful when you need to have composite flags outside of the React render loop.

import React from "react";
import ReactDOM from "react-dom";
import { ComputedBackend } from "flag";
import { FlagBackendProvider, MyFlags } from "./flags";
import { App } from "./app";

const backend = new ComputedBackend<MyFlags>({
  features: {
    useMyCoolNewThing: false,
  },
  config: {
    apiUrl: "https://example.com",
  },
  cool: 100,
  dude: 200,
  coolAndDude: (flags) => flags.cool + flags.dude,
  largeCoolAndDude: (flags) => flags.coolAndDude * 2,
});

const root = ReactDOM.createRoot(document.querySelector("#app"));

root.render(
  <FlagBackendProvider backend={backend}>
    <App />
  </FlagBackendProvider>
);

AlwaysBackend

Given a partial mapping of { boolean: boolean; string: string; number: number; } will always yield the mapping value for a given type. If a type is missing from the mapping, it will fallback to the default value given to useFlag. Useful for testing.

import React from "react";
import ReactDOM from "react-dom";
import { AlwaysBackend } from "flag";
import { FlagBackendProvider } from "./flags";
import { App } from "./app";

const backend = new AlwaysBackend({
  boolean: false,
  string: "some string",
  number: 1000,
});

const root = ReactDOM.createRoot(document.querySelector("#app"));

root.render(
  <FlagBackendProvider backend={backend}>
    <App />
  </FlagBackendProvider>
);

NullBackend

A backend that always returns the default value. Useful for testing.

import React from "react";
import ReactDOM from "react-dom";
import { NullBackend } from "flag";
import { FlagBackendProvider } from "./flags";
import { App } from "./app";

const backend = new NullBackend();

const root = ReactDOM.createRoot(document.querySelector("#app"));

root.render(
  <FlagBackendProvider backend={backend}>
    <App />
  </FlagBackendProvider>
);

Rolling your own with AbstractBackend<T>

You can roll your own backend by extending a class off of AbstractBackend<T>. You need only implement the getSnapshot() (and optionally getServerSnapshot()) method.

import { AbstractBackend, Types } from "flag";

/**
 * `F` is the shape of your flags.
 */
export class MyBackend<F> extends AbstractBackend<F> {
  /**
   * `KP` is a valid key path (as an array).
   * `T` is the type of value associated the key path.
   */
  getSnapshot<KP extends Types.KeyPath<F>, T extends Types.GetValueFromKeyPath<F, KP>>(keyPath: KP, defaultValue: T): T {
    /**
     * `getSnapshot` must return `T`.
     */
    return defaultValue;
  }

  /**
   * OPTIONAL: you can override `getServerSnapshot` if you need different behavior for server rendering.
   * Defaults to `getSnapshot`.
   */
  override getServerSnapshot<KP extends Types.KeyPath<F>, T extends Types.GetValueFromKeyPath<F, KP>>(keyPath: KP, defaultValue: T): T {
    /**
     * Do something different than `getSnapshot`
     */
    return defaultValue;
  }
}

If you want your backend to work with React Suspense, you can create an async ref object by calling this.createAsyncRef() and using it in getSnapshot().

import { AbstractBackend, Types } from "flag";

export class MyBackend<F> extends AbstractBackend<F> {
  #data: Types.AsyncMutableRefObject<T>;

  constructor() {
    this.#data = this.createAsyncRef();

    fetch("/api-with-data")
      .then((res) => res.json())
      .then((data) => {
        this.#data.current = data;
      });
  }

  getSnapshot<KP extends Types.KeyPath<F>, T extends Types.GetValueFromKeyPath<F, KP>>(keyPath: KP, defaultValue: T): T {
    /**
     * Throws a promise if a current value has not yet been assigned.
     */
    const data = this.#data.current;

    return someGetterFn(data, keyPath);
  }
}

If your backend is asynchronous and you do not want to use suspense, you can imperatively call this.notify() in order to tell React to re-render.

import { AbstractBackend, Types } from "flag";

export class MyBackend<F> extends AbstractBackend<F> {
  #data: T | null = null;

  constructor() {
    this.#data = this.createAsyncRef();

    fetch("/api-with-data")
      .then((res) => res.json())
      .then((data) => {
        this.#data = data;
        this.notify();
      });
  }

  getSnapshot<KP extends Types.KeyPath<F>, T extends Types.GetValueFromKeyPath<F, KP>>(keyPath: KP, defaultValue: T): T {
    if (this.#data === null) {
      return defaultValue;
    }

    const data = this.#data;

    return someGetterFn(data, keyPath);
  }
}

Setting NODE_ENV

While in development, you should be sure to set process.env.NODE_ENV to "development" for useful warnings when possible. Tool kits like Remix, Next and CRA do this automatically for you.

License

MPL-2.0