In-Source Testing & Zero-Overhead Production Dead Code Elimination

In-source testing lets you write unit tests directly inside the same file as your implementation code, similar to Rust’s #[cfg(test)] modules.

This is especially effective for utility functions, math helpers, and data parsing algorithms where co-locating tests improves developer velocity.

// src/utils/math.ts
export function add(a: number, b: number): number {
  return a + b;
}

// In-source test block (only executed when running Vitest)
if (import.meta.vitest) {
  const { it, expect } = import.meta.vitest;

  it('adds two numbers correctly', () => {
    expect(add(2, 3)).toBe(5);
  });
}

Production Dead Code Elimination

To ensure test blocks are stripped from production bundle output, configure Vite’s define replacement:

// vite.config.ts
export default defineConfig({
  define: {
    'import.meta.vitest': 'undefined',
  },
  test: {
    includeSource: ['src/**/*.{js,ts}'],
  },
});

When building for production (vite build), esbuild replaces import.meta.vitest with undefined. The dead-code elimination (tree-shaking) pass removes the if (false) block completely, resulting in zero bytes added to production output.