> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt.

# Environment Variables

Modern.js provides support for environment variables, including built-in environment variables and custom environment variables.

## Built-in Environment Variables

### ASSET\_PREFIX

The current path prefix of the asset, which is a read-only environment variable.

### NODE\_ENV

The current execution environment and is a **read-only** environment variable whose have different values under different execution commands:

- `production`: Default value when running `modern build` or `modern serve`.
- `development`: Default value when running `modern dev`, also the default value in other cases.

### MODERN\_ENV

Set the current execution environment manually. In addition to the values in the NODE\_ENV, custom environment names are supported here, such as `staging`, `boe`, etc.

:::tip
MODERN\_ENV priority is higher than NODE\_ENV.

:::

### MODERN\_TARGET

When using `@modern-js/runtime`, Modern.js will automatically inject `MODERN_TARGET` to distinguish between SSR and CSR environments.

You can use `process.env.MODERN_TARGET` to determine the environment and execute appropriate code.

```ts title="App.tsx"
function App() {
  if (process.env.MODERN_TARGET === 'browser') {
    console.log(window.innerHeight);
  }
}
```

After the development build, you can see that the SSR and CSR bundles as follows:

```js title="dist/bundles/main.js"
// SSR bundles
function App() {
  if (false) {
  }
}
```

```js title="dist/static/main.js"
// CSR bundles
function App() {
  if (true) {
    console.log(window.innerHeight);
  }
}
```

This can provide different outputs for different environments to ensure that the bundle size is minimized. It can also be convenient to deal with some side effects for different environments.

:::note Dead Code Elimination
In production environment, minimizers such as Terser and SWC will analyze the code and remove dead code to reduce the bundle size. This process is called "Dead Code Elimination" (DCE).

For example, the code inside the `if (false)` statement will be removed, while the code inside the `if (true)` will be preserved.

If you do not use `process.env.MODERN_TARGET` as described above, the code minimizer may not be able to analyze the dead code, resulting in an increased bundle size.

:::

## Custom Environment Variables

You can specify custom environment variables in both `shell` and `.env` files.

### Via `shell`

Add custom environment variables before the command:

```shell
REACT_APP_FOO=123 BAR=456 pnpm run dev
```

### Via `.env` file

Create a `.env` file in the project root and add custom environment variables, which are added to the Node.js process by default, for example:

```bash
REACT_APP_FOO=123
BAR=456
```

The `.env` file follows the following loading rules:

- `.env`: default.
- `.env.{ MODERN_ENV | NODE_ENV }`: Overrides `.env` for a specific environment.

When you need to use different config according to the environment, you can define environment variables in the `.env` file corresponding to the environment name, and manually set the execution environment when starting the project.

For example, when starting a project with the following command, the `.env` and `.env.staging` will load:

```shell
MODERN_ENV=staging pnpm run dev
```

## Using Environment Variables

### Convention Names

`NODE_ENV` can be used directly in front-end code. In addition, custom environment variables starting with `MODERN_` can also be used directly in code.

For Example:

```js
if (process.env.NODE_ENV === 'development') {
  // do something
}
```

After executing the `pnpm run dev`, you can see the following bundle:

```js
if (true) {
  // do something
}
```

In custom HTML templates, you can also use such environment variables directly. For example, in `config/html/head.html`:

```html
<meta name="test" content="<process.env.NODE_ENV>" />
```

### Any Other Names

If you need to use environment variables with any other names in your code, you can configure them in [`source.globalVars`](/configure/app/source/global-vars.md). For example:

```ts title="modern.config.ts"
export default defineConfig({
  source: {
    globalVars: {
      'process.env.VERSION': process.env.VERSION,
    },
  },
});
```

At this point, the `process.env.VERSION` in the code will be replaced by the value of `VERSION` in the environment variables.

:::note
`source.globalVars` also supports replacing other expressions or strings with specified values, not limited to environment variables.

:::

## Use Global Replacement

In addition to environment variables, Modern.js also supports replacing variables in code with other values or expressions, which can be used to distinguish between development environment and production environment in code.

For example, converts the expression `TWO` to `1 + 1`:

```ts
export default {
  source: {
    define: {
      TWO: '1 + 1',
    },
  },
};
```

```ts
const foo = TWO;

// ⬇️ Turn into being...
const foo = 1 + 1;
```

In most cases, `source.globalVars` is already sufficient to replace variables. But the values passed in by `source.globalVars` will be serialized by JSON by default. So it cannot be replaced like `1 + 1` in the example above, at this time, we need to use [`source.define`](/configure/app/source/define.md).