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

# source.define

- **Type:** `Record<string, unknown>`
- **Default:**

```ts
const defaultDefine = {
  'process.env.MODERN_TARGET': '"browser"',
};
```

Replaces variables in your code with other values or expressions at compile time. This can be useful for allowing different behavior between development builds and production builds.

:::info
The usage of this configuration item is exactly the same as that of Rsbuild. For detailed information, please refer to [Rsbuild - source.define](https://v2.rsbuild.dev/config/source/define).
:::