tsconfig-resolution

Problem

When running a TypeScript file with Bun from a different directory than where the tsconfig.json resides, Bun doesn’t pick up the TypeScript configuration. This causes emitDecoratorMetadata to not be applied, breaking NestJS dependency injection.

Symptoms

• Constructor-injected services are undefined
• Error: undefined is not an object (evaluating 'this.service.method')
• NestJS modules initialize successfully but commands/controllers fail
• Constructor is called before modules are initialized (visible in debug logs)

Reproduction

Monorepo Structure

project-root/
├── package.json           # workspaces: ["packages/*"]
├── packages/
│   └── my-cli/
│       ├── package.json   # name: "@project/my-cli"
│       ├── tsconfig.json  # emitDecoratorMetadata: true
│       └── src/
│           └── main.ts

What Fails

# From project root - FAILS
bun run packages/my-cli/src/main.ts
# DI is broken, injected services are undefined

What Works

# From within package directory - WORKS
cd packages/my-cli
bun run src/main.ts

# Using workspace filter - WORKS
bun run --filter @project/my-cli dev

# Building first, then running built output - WORKS
cd packages/my-cli
bun build src/main.ts --outdir dist --target bun
bun run dist/main.js

Root Cause

Bun’s TypeScript transpiler looks for tsconfig.json relative to the current working directory, not relative to the file being executed. When running from project root:

1. Bun looks for project-root/tsconfig.json
2. Doesn’t find emitDecoratorMetadata: true (root tsconfig may not have it)
3. Transpiles without decorator metadata
4. NestJS DI can’t resolve constructor parameter types
5. Services are injected as undefined

Solutions

Solution 1: Use Workspace Filter (Recommended)

In root package.json:

{
  "scripts": {
    "my-cli": "bun run --filter @project/my-cli dev"
  }
}

In packages/my-cli/package.json:

{
  "scripts": {
    "dev": "bun run src/main.ts"
  }
}

Usage:

bun run my-cli [args...]

Why it works: The --filter flag runs the script from within the package’s directory context, where Bun finds the correct tsconfig.json.

Solution 2: Build Before Running

# Build with Bun's bundler (preserves decorator metadata)
bun build src/main.ts --outdir dist --target bun --packages external

# Run built output
bun run dist/main.js

Why it works: The bundler properly processes TypeScript with the correct tsconfig, and the built JavaScript includes the decorator metadata.

Solution 3: Root tsconfig with Path Reference

Create project-root/tsconfig.json:

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

Note: This applies to ALL TypeScript files run from root, which may not be desirable.

Verification

Add debug logging to verify DI is working:

@Injectable()
export class MyCommand extends CommandRunner {
  constructor(private readonly service: MyService) {
    super();
    console.log(`DEBUG: service injected = ${!!service}`);
  }
}

Working output:

[Nest] ... [InstanceLoader] MyModule dependencies initialized
DEBUG: service injected = true

Broken output:

DEBUG: service injected = false
[Nest] ... [InstanceLoader] MyModule dependencies initialized

Note how in the broken case, the constructor runs before modules initialize.

Related

• Bun + NestJS compatibility notes
• GitHub Issue: nestjs/nest#13881 (CLI support)
• Bun docs: TypeScript configuration

Key Takeaway

Always run Bun TypeScript files from the directory containing the tsconfig.json, or use workspace filters to ensure correct configuration resolution.