In-Source Testing

Vitest provides a way to run tests within your source code along side the implementation, similar to Rust's module tests.

This makes the tests share the same closure as the implementations and able to test against private states without exporting. Meanwhile, it also brings a closer feedback loop for development.

Setup

To get started, put a if (import.meta.vitest) block at the end of your source file and write some tests inside it. For example:

// the implementation
export function add(...args: number[]) {
  return args.reduce((a, b) => a + b, 0)
}

// in-source test suites
if (import.meta.vitest) {
  const { it, expect } = import.meta.vitest
  it('add', () => {
    expect(add()).toBe(0)
    expect(add(1)).toBe(1)
    expect(add(1, 2, 3)).toBe(6)
  })
}

Update the includeSource config for Vitest to grab the files under src/:

import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    includeSource: ['src/**/*.{js,ts}'], 
  },
})

Then you can start to test!

$ npx vitest

Production Build

For the production build, you will need to set the define options in your config file, letting the bundler do the dead code elimination. For example, in Vite

/// <reference types="vitest/config" />

import { defineConfig } from 'vite'

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

Other Bundlers

import { defineConfig } from 'rolldown/config'

export default defineConfig({
  transform: {
    define: { 
      'import.meta.vitest': 'undefined', 
    }, 
  },
})

Learn more: Rolldown

import replace from '@rollup/plugin-replace'

export default {
  plugins: [
    replace({ 
      'import.meta.vitest': 'undefined', 
    }) 
  ],
  // other options
}

Learn more: Rollup

import { defineBuildConfig } from 'unbuild'

export default defineBuildConfig({
  replace: { 
    'import.meta.vitest': 'undefined', 
  }, 
  // other options
})

Learn more: unbuild

const webpack = require('webpack')

module.exports = {
  plugins: [
    new webpack.DefinePlugin({ 
      'import.meta.vitest': 'undefined', 
    })
  ],
}

Learn more: webpack

TypeScript

To get TypeScript support for import.meta.vitest, add vitest/importMeta to your tsconfig.json:

{
  "compilerOptions": {
    "types": [
      "vitest/importMeta"
    ]
  }
}

Reference to examples/in-source-test for the full example.

Notes

This feature could be useful for:

• Unit testing for small-scoped functions or utilities
• Prototyping
• Inline Assertion

It's recommended to use separate test files instead for more complex tests like components or E2E testing.