---
title: alias | Config
outline: deep
---

# alias

- **Type:** `Record<string, string> | Array<{ find: string | RegExp, replacement: string, customResolver?: ResolverFunction | ResolverObject }>`

Define custom aliases when running inside tests. They will be merged with aliases from `resolve.alias`.

::: warning
Vitest uses Vite SSR primitives to run tests which has [certain pitfalls](https://vitejs.dev/guide/ssr.html#ssr-externals).

1. Aliases affect only modules imported directly with an `import` keyword by an [inlined](/config/server#server-deps-inline) module (all source code is inlined by default).
2. Vitest does not support aliasing `require` calls.
3. If you are aliasing an external dependency (e.g., `react` -> `preact`), you may want to alias the actual `node_modules` packages instead to make it work for externalized dependencies. Both [Yarn](https://classic.yarnpkg.com/en/docs/cli/add/#toc-yarn-add-alias) and [pnpm](https://pnpm.io/aliases/) support aliasing via the `npm:` prefix.
:::
