Dynamic Feature Flagging with TypeScript Conditional Exports

In modern software development, enabling or disabling features dynamically based on environment or configuration is a common requirement. This challenge focuses on leveraging TypeScript's conditional export capabilities to achieve this without resorting to runtime checks that can clutter your code and impact performance. You will create a system where different implementations of a service are exported based on a build-time flag.

Problem Description

Your task is to implement a feature flagging system using TypeScript's conditional export syntax. You will have two potential implementations for a simple "Greeter" service: one that returns a generic greeting and another that returns a personalized greeting. The choice of which implementation to export should be determined by a build-time condition, simulating a feature flag.

Key Requirements:

• Create two distinct implementations for a Greeter interface.
• Use TypeScript's conditional export syntax (export const ... within conditional blocks) to export the appropriate implementation based on a build-time variable.
• The build-time variable should be accessible within your TypeScript code during compilation.
• Ensure that only one implementation is available in the final bundled JavaScript, depending on the build configuration.

Expected Behavior:

When compiled with one condition, the application should be able to import and use the generic greeter. When compiled with a different condition, it should import and use the personalized greeter.

Edge Cases to Consider:

• What happens if the build condition is not met by any export? (TypeScript should ideally catch this during compilation, but consider how your code structure might prevent it.)

Examples

Example 1: Feature Enabled (e.g., FEATURE_A_ENABLED=true)

Assuming a build setup that defines FEATURE_A_ENABLED as true, and the Greeter interface and implementations are structured to export the personalized greeting conditionally:

// File: greeter.ts
interface Greeter {
  greet(name: string): string;
}

class GenericGreeter implements Greeter {
  greet(name: string): string {
    return `Hello, ${name}!`;
  }
}

class PersonalizedGreeter implements Greeter {
  greet(name: string): string {
    return `Welcome, dear ${name}! It's great to have you.`;
  }
}

// Assume FEATURE_A_ENABLED is true during compilation
export const greeter: Greeter = __feature_a_enabled__ ? new PersonalizedGreeter() : new GenericGreeter();

// File: main.ts (in a separate project or bundled output)
import { greeter } from './greeter';

const message = greeter.greet("Alice");
console.log(message);

Output:

Welcome, dear Alice! It's great to have you.

Explanation:

During compilation, the __feature_a_enabled__ condition evaluates to true. Therefore, the new PersonalizedGreeter() is assigned to the greeter export. The GenericGreeter implementation is effectively pruned from the output bundle by the TypeScript compiler and bundler.

Example 2: Feature Disabled (e.g., FEATURE_A_ENABLED=false)

Assuming the same greeter.ts file but compiled with FEATURE_A_ENABLED as false:

// File: main.ts (in a separate project or bundled output)
import { greeter } from './greeter';

const message = greeter.greet("Bob");
console.log(message);

Output:

Hello, Bob!

Explanation:

During compilation, the __feature_a_enabled__ condition evaluates to false. Therefore, the new GenericGreeter() is assigned to the greeter export. The PersonalizedGreeter implementation is pruned from the output bundle.

Constraints

• The conditional export must be handled at build time. Runtime checks within the greeter.ts file that rely on environment variables set after compilation are not allowed.
• You must use TypeScript's conditional export syntax.
• The solution should be written in TypeScript.
• The output JavaScript should contain only the necessary code for the selected export, minimizing bundle size.

Notes

• To simulate the build-time variable, you will likely need to use a build tool like Webpack, Rollup, or esbuild with appropriate plugins or configuration. For instance, you might use DefinePlugin in Webpack or define in Rollup to replace a placeholder like __feature_a_enabled__ with true or false during compilation.
• Consider how your build tool can perform dead code elimination (tree-shaking) effectively when using this pattern. TypeScript's const and export keywords are crucial for enabling this.
• Think about naming your conditional variables clearly to reflect the feature they control.