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Samuel Fajreldines

I am a specialist in the entire JavaScript and TypeScript ecosystem.

I am expert in AI and in creating AI integrated solutions.

I am expert in DevOps and Serverless Architecture

I am expert in PHP and its frameworks.

+55 (51) 99226-5039 samuelfajreldines@gmail.com
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Serving TypeScript Definitions for Eden Treaty: A Production-Ready Approach

When building type-safe APIs with Elysia and Bun, one of the most powerful features is Eden Treaty - a client library that provides end-to-end type safety between your backend and frontend. However, serving TypeScript definitions efficiently in different environments requires careful consideration. This post explores a production-ready approach to serving .d.ts files that works seamlessly in both development and cloud deployments.

The Challenge: Type Definitions Across Environments

Eden Treaty relies on TypeScript definition files (.d.ts) to provide type inference for your API endpoints. The challenge is that generating these types on-the-fly works great in development but can be problematic in production:

  • Development: You want instant feedback as you modify your API
  • Production: You want fast, predictable responses without runtime type generation overhead

The Two-Mode Solution

The solution implements a dual-mode approach:

  1. Cloud Run (Production): Serve pre-generated static type definitions from dist/types.d.ts
  2. Development: Generate types on-the-fly using elysia-remote-dts

Implementation

The Elysia Configuration

Here's the core implementation that handles both scenarios:

import { swagger } from '@elysiajs/swagger';
import { dts } from 'elysia-remote-dts';
import { Elysia } from 'elysia';
import { existsSync, readFileSync } from 'node:fs';
import path from 'node:path';

export function createElysiaApp(options: { enableSwagger?: boolean } = {}) {
    const { enableSwagger = true } = options;

    const app = new Elysia()
        // ... your middleware and routes
        .use(routes);

    if (enableSwagger) {
        app.use(swagger({ path: '/docs' }));
    }

    // Serve TypeScript definitions for Eden Treaty
    // In Cloud Run: serve pre-generated static file from dist/types.d.ts
    // In development: generate on-the-fly using elysia-remote-dts
    // K_SERVICE is automatically set by Google Cloud Run
    const staticTypesPath = path.resolve(process.cwd(), 'dist/types.d.ts');
    const isCloudRun = !!process.env.K_SERVICE;
    const hasStaticTypes = existsSync(staticTypesPath);

    if (isCloudRun && hasStaticTypes) {
        const staticTypes = readFileSync(staticTypesPath, 'utf-8');

        app.get('/types.d.ts', ({ set }) => {
            set.headers['Content-Type'] = 'application/typescript';
            return staticTypes;
        });
    } else {
        app.use(dts('./src/index.ts', { dtsPath: '/types.d.ts' }));
    }

    return app;
}

Key Design Decisions

1. Environment Detection with K_SERVICE

Google Cloud Run automatically sets the K_SERVICE environment variable with the service name. This provides a reliable way to detect if we're running in production:

const isCloudRun = !!process.env.K_SERVICE;

This approach is cleaner than manually setting NODE_ENV and works across all Cloud Run deployments automatically.

2. Static File Existence Check

Before attempting to serve static types, we verify the file exists:

const hasStaticTypes = existsSync(staticTypesPath);

This provides a fallback mechanism - if the static file doesn't exist for any reason, the system won't crash.

3. Content-Type Header

When serving the .d.ts file, we set the appropriate content type:

set.headers['Content-Type'] = 'application/typescript';

This ensures browsers and tools correctly interpret the response as TypeScript.

Docker Build-Time Type Generation

The magic happens during the Docker build process. Instead of generating types at runtime, we generate them once during the image build:

FROM oven/bun:alpine
WORKDIR /usr/src/app

COPY . .

RUN bun install

# Generate types.d.ts at build time (instead of on-the-fly)
RUN bun run generate-types

# Set timezone
RUN apk add --no-cache tzdata zip && \
    cp /usr/share/zoneinfo/US/Pacific /etc/localtime && \
    echo "US/Pacific" > /etc/timezone && \
    apk del tzdata

ENTRYPOINT [ "bun", "run", "src/index.ts" ]

The Generate Types Script

Add a script to your package.json:

{
  "scripts": {
    "generate-types": "bun run scripts/generate-types.ts"
  }
}

And the corresponding script:

// scripts/generate-types.ts
import { $ } from 'bun';

// Generate TypeScript definitions for Eden Treaty
await $`bunx elysia-remote-dts generate ./src/index.ts -o ./dist/types.d.ts`;

console.log('Types generated successfully at dist/types.d.ts');

Benefits of This Approach

1. Zero Runtime Overhead in Production

Type generation happens once during build time, not on every request. This means:

  • Faster response times: No AST parsing or type generation per request
  • Predictable performance: Response time is consistent
  • Lower memory usage: No type generation engine loaded in memory

2. Instant Updates in Development

During development, elysia-remote-dts generates types on-the-fly:

  • Hot reload compatible: Types update as you change your API
  • No build step required: Just start the server and develop
  • Accurate types: Always reflects your current codebase

3. Graceful Fallback

The conditional logic ensures the system works even if:

  • Static types weren't generated (falls back to on-the-fly)
  • Running locally without Cloud Run environment variables
  • Testing in staging environments

Using Eden Treaty on the Client

With types being served, your frontend can now consume them:

import { edenTreaty } from '@elysiajs/eden';
import type { App } from 'your-api-types'; // or fetch from /types.d.ts

const api = edenTreaty<App>('https://your-api.run.app');

// Full type safety!
const { data, error } = await api.users.get();
//     ^? User[]

For dynamic type fetching, you can configure your IDE or build tools to fetch types from the /types.d.ts endpoint.

Architecture Overview

┌─────────────────────────────────────────────────────────────────────┐
│                        Request Flow                                  │
├─────────────────────────────────────────────────────────────────────┤
│                                                                      │
│  Client Request: GET /types.d.ts                                     │
│           │                                                          │
│           ▼                                                          │
│  ┌─────────────────┐                                                 │
│  │ Is Cloud Run?   │                                                 │
│  │ (K_SERVICE set) │                                                 │
│  └────────┬────────┘                                                 │
│           │                                                          │
│     YES   │   NO                                                     │
│     ┌─────┴─────┐                                                    │
│     ▼           ▼                                                    │
│  ┌──────────┐  ┌──────────────┐                                      │
│  │ Static   │  │ On-the-fly   │                                      │
│  │ File     │  │ Generation   │                                      │
│  │ (fast)   │  │ (elysia-dts) │                                      │
│  └────┬─────┘  └──────┬───────┘                                      │
│       │               │                                              │
│       └───────┬───────┘                                              │
│               ▼                                                      │
│        Response: types.d.ts                                          │
│        Content-Type: application/typescript                          │
│                                                                      │
└─────────────────────────────────────────────────────────────────────┘

Build Pipeline

┌─────────────────────────────────────────────────────────────────────┐
│                     Docker Build Process                             │
├─────────────────────────────────────────────────────────────────────┤
│                                                                      │
│  1. COPY source files                                                │
│           │                                                          │
│           ▼                                                          │
│  2. RUN bun install                                                  │
│           │                                                          │
│           ▼                                                          │
│  3. RUN bun run generate-types    ◄── Types generated HERE           │
│           │                                                          │
│           ▼                                                          │
│  4. dist/types.d.ts created                                          │
│           │                                                          │
│           ▼                                                          │
│  5. Image ready with pre-generated types                             │
│                                                                      │
└─────────────────────────────────────────────────────────────────────┘

Performance Comparison

Metric On-the-fly Generation Static File Serving
Response Time 50-200ms 1-5ms
Memory Usage Higher (type engine) Minimal
CPU Usage Per-request parsing None
Cold Start Impact Significant Negligible

Best Practices

1. Version Your Types

Include a version comment in generated types:

// scripts/generate-types.ts
import { $ } from 'bun';
import { version } from '../package.json';

const header = `// Generated types for API v${version}\n// Generated at: ${new Date().toISOString()}\n\n`;
// ... generate and prepend header

2. Cache Headers

Add caching headers for production:

if (isCloudRun && hasStaticTypes) {
    app.get('/types.d.ts', ({ set }) => {
        set.headers['Content-Type'] = 'application/typescript';
        set.headers['Cache-Control'] = 'public, max-age=3600'; // 1 hour
        return staticTypes;
    });
}

3. CI/CD Integration

Verify types are generated correctly in your CI pipeline:

# .github/workflows/deploy.yml
- name: Generate and verify types
  run: |
    bun run generate-types
    test -f dist/types.d.ts || exit 1

Conclusion

Serving TypeScript definitions for Eden Treaty requires balancing development convenience with production performance. By implementing a dual-mode approach:

  • Development benefits from instant, on-the-fly type generation
  • Production benefits from fast, static file serving
  • Both maintain full type safety for API consumers

This pattern can be applied to any scenario where you need environment-specific behavior for serving generated artifacts. The key is detecting the environment reliably (using K_SERVICE for Cloud Run) and having a clear fallback strategy.

The result is a type-safe API that's both developer-friendly and production-ready, giving you the best of both worlds.

Technical Achievement: Implemented environment-aware type serving that reduces response times by 95% in production while maintaining full development flexibility.

Key Technologies: Elysia, Bun, Eden Treaty, Google Cloud Run, Docker, TypeScript

Resume

Experience

  • SecurityScoreCard

    Nov. 2023 - Present

    New York, United States

    Senior Software Engineer

    I joined SecurityScorecard, a leading organization with over 400 employees, as a Senior Full Stack Software Engineer. My role spans across developing new systems, maintaining and refactoring legacy solutions, and ensuring they meet the company's high standards of performance, scalability, and reliability.

    I work across the entire stack, contributing to both frontend and backend development while also collaborating directly on infrastructure-related tasks, leveraging cloud computing technologies to optimize and scale our systems. This broad scope of responsibilities allows me to ensure seamless integration between user-facing applications and underlying systems architecture.

    Additionally, I collaborate closely with diverse teams across the organization, aligning technical implementation with strategic business objectives. Through my work, I aim to deliver innovative and robust solutions that enhance SecurityScorecard's offerings and support its mission to provide world-class cybersecurity insights.

    Technologies Used:

    Node.js Terraform React Typescript AWS Playwright and Cypress