Understanding defineConfig in Vite: Static vs Dynamic Configuration

If you're working with Vite and you've stumbled across the defineConfig function in the Vite configuration file, you might’ve noticed that there are two common ways to write it. At first glance, it might look like just a matter of style, but there’s actually more going on beneath the surface.

In this article, we’ll go through what defineConfig does, the two primary ways you can use it, when you should use one over the other, and a few advanced considerations that can help you write cleaner, more powerful configuration files.

What Is defineConfig?

The defineConfig helper is provided by Vite to define the configuration object with proper type inference and editor support—especially when you're using TypeScript. It also works just as well in JavaScript projects.

It’s imported from Vite like this:

import { defineConfig } from 'vite';

Then, you can use it in either of these forms.

Option 1: Static Configuration

export default defineConfig({
  plugins: [react(), tailwindcss()],
  base: "/my-project/",
});

This is the most common and simplest form. You’re just passing a plain object to defineConfig, which Vite will use as-is.

Use this when:

• You don't need environment-specific configuration.
• Your project settings are fixed.
• You're not referencing .env files directly inside vite.config.js.

This is the most readable and cleanest approach when you don’t need to introduce any logic.

Option 2: Dynamic Configuration (Using a Function)

export default defineConfig(({ mode }) => {
  const env = loadEnv(mode, process.cwd(), '');
  const base = env.VITE_BASE_PATH;

  return {
    plugins: [react(), tailwindcss()],
    ...(base ? { base } : {}),
  };
});

This form accepts a function that receives an object containing:

• mode: the current mode (development, production, etc.)
• command: either 'serve' or 'build'
• ssrBuild: boolean indicating if it's an SSR build

The biggest advantage here is that you can call loadEnv() to access values from .env files based on the mode.

Use this when:

• You need different settings per environment (e.g., dev vs production).
• You want to reference or transform values from .env files.
• You want to inject values conditionally (like using a custom base path only in production).

Using loadEnv() Properly

The function loadEnv(mode, root, prefix) is provided by Vite to load environment variables from .env files. The third argument (prefix) is crucial.

By default, Vite only exposes variables that start with VITE_ to your Vite config and client-side code. So make sure your variables are named like VITE_BASE_PATH, not just BASE_PATH.

const env = loadEnv(mode, process.cwd(), ''); // Load all variables

If you want to be specific:

const env = loadEnv(mode, process.cwd(), 'VITE_'); // Only loads VITE_ variables

Which One Should You Use?

• Start with static config if your app is simple and doesn't rely on different environments.
• Switch to dynamic config only when you need it—typically when referencing .env files or having logic that depends on mode or command.

Additional Considerations

1. Access to command

The function form also gives you access to command, which is either 'serve' or 'build'. This is useful when you want to do something like this:

export default defineConfig(({ command }) => ({
  base: command === 'serve' ? '/' : '/production-subpath/',
}));

This lets you use a different base in development vs production without relying on environment variables.

2. Avoid Logic in Static Config

While you technically could write:

const base = process.env.NODE_ENV === 'production' ? '/prod/' : '/';
export default defineConfig({
  base,
});

This is not recommended, especially in Vite, because:

• Vite caches config aggressively.
• Using loadEnv() ensures you’re working with the right .env.mode file for the current command.

3. Customizing resolve.alias, define, etc.

You can also use the function form to dynamically define things like:

define: {
  __APP_VERSION__: JSON.stringify(env.VITE_APP_VERSION),
}

This lets you embed environment data into your code during build time.

4. Client-Side Access to Env Variables

Remember: only variables starting with VITE_ are available to your client-side code. You can access them via import.meta.env.

console.log(import.meta.env.VITE_BASE_PATH);

Finally

Vite gives you a lot of flexibility with its config file, and knowing the difference between static and dynamic defineConfig usage is key to writing scalable and maintainable frontend builds.

Use static config as your default—and move to dynamic config only when your project demands it. This approach helps keep your config clean, performant, and easy to debug.

If you're still unsure when to switch to dynamic mode or how to structure .env files properly, feel free to comment below. There’s more to explore—like cascading .env priorities, advanced plugin configuration, or even multi-project Vite workspaces.