# Auto-install

> Bun's automatic package installation feature for standalone script execution

If Bun finds no `node_modules` directory in the working directory or higher, it abandons Node.js-style module resolution in favor of the **Bun module resolution algorithm**.

Under Bun-style module resolution, Bun auto-installs every imported package on the fly into a [global module cache](/pm/global-cache) during execution. [`bun install`](/pm/cli/install) uses the same cache.

```ts index.ts icon="/icons/typescript.svg"
import { foo } from "foo"; // install `latest` version

foo();
```

The first time you run this script, Bun auto-installs `"foo"` and caches it. Later runs use the cached version.

---

## Version resolution

Bun determines which version to install as follows:

1. Check for a `bun.lock` file in the project root. If it exists, use the version specified in the lockfile.
2. Otherwise, scan up the tree for a `package.json` that includes `"foo"` as a dependency. If found, use the specified semver version or version range.
3. Otherwise, use `latest`.

---

## Cache behavior

Once Bun determines a version or version range, it:

1. Checks the module cache for a compatible version. If one exists, uses it.
2. When resolving `latest`, checks if `package@latest` was downloaded and cached in the last _24 hours_. If so, uses it.
3. Otherwise, downloads and installs the appropriate version from the `npm` registry.

---

## Installation

Bun installs and caches packages into `<cache>/<pkg>@<version>`, so Bun can cache multiple versions of the same package at once. It also creates a symlink under `<cache>/<pkg>/<version>` to speed up looking up all cached versions of a package.

---

## Version specifiers

To bypass version resolution entirely, specify a version or version range directly in your import statement.

```ts index.ts icon="/icons/typescript.svg"
import { z } from "zod@3.0.0"; // specific version
import { z } from "zod@next"; // npm tag
import { z } from "zod@^3.20.0"; // semver range
```

---

## Benefits

- **Space efficiency** — Each version of a dependency exists in only one place on disk. This saves space and time compared to redundant per-project installations.
- **Portability** — Your source file is _self-contained_, so sharing scripts and gists doesn't mean zipping up a directory of code and config files. With version specifiers in `import` statements, even a `package.json` isn't necessary.
- **Convenience** — You don't need to run `npm install` or `bun install` before running a file or script with `bun run`.
- **Backwards compatibility** — Because Bun still respects the versions specified in `package.json` if one exists, you can switch to Bun-style resolution with a single command: `rm -rf node_modules`.

---

## Limitations

- No Intellisense. TypeScript auto-completion in IDEs relies on type declaration files inside `node_modules`. We are investigating solutions to this.
- No [patch-package](https://github.com/ds300/patch-package) support

---

## FAQ

<AccordionGroup>

    <Accordion title="How is this different from what pnpm does?">

    With pnpm, you have to run `pnpm install`, which creates a `node_modules` folder of symlinks for the runtime to resolve. By contrast, Bun resolves dependencies on the fly when you run a file; there's no need to run any `install` command ahead of time. Bun also doesn't create a `node_modules` folder.

    </Accordion>

    <Accordion title="How is this different from what Yarn Plug'N'Play does?">
    With Yarn, you must run `yarn install` before you run a script. By contrast, Bun resolves dependencies on the fly when you run a file; there's no need to run any `install` command ahead of time.

    Yarn Plug'N'Play also uses zip files to store dependencies. This makes dependency loading [slower at runtime](https://twitter.com/jarredsumner/status/1458207919636287490), as random access reads on zip files tend to be slower than the equivalent disk lookup.
    </Accordion>

    <Accordion title="How is this different from what Deno does?">

    Deno requires an `npm:` specifier before each npm `import`, lacks support for import maps through `compilerOptions.paths` in `tsconfig.json`, and has incomplete support for `package.json` settings. Unlike Deno, Bun does not currently support URL imports.
    </Accordion>

</AccordionGroup>