nest/vitest.config.mts

import { existsSync } from 'fs';
import type { Plugin } from 'vite';
import { defineConfig } from 'vitest/config';

/**
 * Resolves `.js` imports to `.ts` source files when both exist side-by-side.
 *
 * The NestJS monorepo keeps compiled `.js` output next to the `.ts` sources.
 * Without this plugin, Vite resolves `import '…/foo.js'` to the compiled JS
 * instead of the original TypeScript source.
 */
function resolveTypescriptSource(): Plugin {
  return {
    name: 'resolve-ts-source',
    enforce: 'pre',
    async resolveId(source, importer, options) {
      if (!importer) return null;
      const resolved = await this.resolve(source, importer, {
        ...options,
        skipSelf: true,
      });
      if (resolved && !resolved.external && resolved.id.endsWith('.js')) {
        const tsPath = resolved.id.replace(/\.js$/, '.ts');
        if (existsSync(tsPath)) return tsPath;
      }
      return null;
    },
  };
}

export default defineConfig({
  // Spec files are excluded from every tsconfig in the repo, so Oxc cannot pick
  // up `experimentalDecorators`/`emitDecoratorMetadata` from them and would emit
  // TC39 decorators (which reject parameter decorators). Configure them here.
  oxc: {
    decorator: {
      legacy: true,
      emitDecoratorMetadata: true,
    },
    assumptions: {
      setPublicClassFields: true,
    },
    typescript: {
      removeClassFieldsWithoutInitializer: true,
    },
  },
  test: {
    globals: true,
    root: './',
    include: ['packages/**/*.spec.ts'],
    alias: {
      '@nestjs/common': './packages/common',
      '@nestjs/core': './packages/core',
      '@nestjs/microservices': './packages/microservices',
      '@nestjs/websockets': './packages/websockets',
      '@nestjs/testing': './packages/testing',
      '@nestjs/platform-express': './packages/platform-express',
      '@nestjs/platform-ws': './packages/platform-ws',
      '@nestjs/platform-fastify': './packages/platform-fastify',
      '@nestjs/platform-socket.io': './packages/platform-socket.io',
    },
    setupFiles: ['reflect-metadata'],
  },
  plugins: [resolveTypescriptSource()],
});