# resolve.exports [![CI](https://github.com/lukeed/resolve.exports/workflows/CI/badge.svg)](https://github.com/lukeed/resolve.exports/actions) [![licenses](https://licenses.dev/b/npm/resolve.exports)](https://licenses.dev/npm/resolve.exports) [![codecov](https://codecov.io/gh/lukeed/resolve.exports/branch/master/graph/badge.svg?token=4P7d4Omw2h)](https://codecov.io/gh/lukeed/resolve.exports)

> A tiny (952b), correct, general-purpose, and configurable `"exports"` and `"imports"` resolver without file-system reliance

***Why?***

Hopefully, this module may serve as a reference point (and/or be used directly) so that the varying tools and bundlers within the ecosystem can share a common approach with one another **as well as** with the native Node.js implementation.

With the push for ESM, we must be _very_ careful and avoid fragmentation. If we, as a community, begin propagating different _dialects_ of the resolution algorithm, then we're headed for deep trouble. It will make supporting (and using) `"exports"` nearly impossible, which may force its abandonment and along with it, its benefits.

Let's have nice things.

## Install

```sh
$ npm install resolve.exports
```

## Usage

> Please see [`/test/`](/test) for examples.

```js
import * as resolve from 'resolve.exports';

// package.json contents
const pkg = {
  "name": "foobar",
  "module": "dist/module.mjs",
  "main": "dist/require.js",
  "imports": {
    "#hash": {
      "import": {
        "browser": "./hash/web.mjs",
        "node": "./hash/node.mjs",
      },
      "default": "./hash/detect.js"
    }
  },
  "exports": {
    ".": {
      "import": "./dist/module.mjs",
      "require": "./dist/require.js"
    },
    "./lite": {
      "worker": {
        "browser": "./lite/worker.browser.js",
        "node": "./lite/worker.node.js"
      },
      "import": "./lite/module.mjs",
      "require": "./lite/require.js"
    }
  }
};

// ---
// Exports
// ---

// entry: "foobar" === "." === default
// conditions: ["default", "import", "node"]
resolve.exports(pkg);
resolve.exports(pkg, '.');
resolve.exports(pkg, 'foobar');
//=> ["./dist/module.mjs"]

// entry: "foobar/lite" === "./lite"
// conditions: ["default", "import", "node"]
resolve.exports(pkg, 'foobar/lite');
resolve.exports(pkg, './lite');
//=> ["./lite/module.mjs"]

// Enable `require` condition
// conditions: ["default", "require", "node"]
resolve.exports(pkg, 'foobar', { require: true }); //=> ["./dist/require.js"]
resolve.exports(pkg, './lite', { require: true }); //=> ["./lite/require.js"]

// Throws "Missing <entry> specifier in <name> package" Error
resolve.exports(pkg, 'foobar/hello');
resolve.exports(pkg, './hello/world');

// Add custom condition(s)
// conditions: ["default", "worker", "import", "node"]
resolve.exports(pkg, 'foobar/lite', {
  conditions: ['worker']
}); //=> ["./lite/worker.node.js"]

// Toggle "browser" condition
// conditions: ["default", "worker", "import", "browser"]
resolve.exports(pkg, 'foobar/lite', {
  conditions: ['worker'],
  browser: true
}); //=> ["./lite/worker.browser.js"]

// Disable non-"default" condition activate
// NOTE: breaks from Node.js default behavior
// conditions: ["default", "custom"]
resolve.exports(pkg, 'foobar/lite', {
  conditions: ['custom'],
  unsafe: true,
});
//=> Error: No known conditions for "./lite" specifier in "foobar" package

// ---
// Imports
// ---

// conditions: ["default", "import", "node"]
resolve.imports(pkg, '#hash');
resolve.imports(pkg, 'foobar/#hash');
//=> ["./hash/node.mjs"]

// conditions: ["default", "import", "browser"]
resolve.imports(pkg, '#hash', { browser: true });
resolve.imports(pkg, 'foobar/#hash');
//=> ["./hash/web.mjs"]

// conditions: ["default"]
resolve.imports(pkg, '#hash', { unsafe: true });
resolve.imports(pkg, 'foobar/#hash');
//=> ["./hash/detect.mjs"]

resolve.imports(pkg, '#hello/world');
resolve.imports(pkg, 'foobar/#hello/world');
//=> Error: Missing "#hello/world" specifier in "foobar" package

// ---
// Legacy
// ---

// prefer "module" > "main" (default)
resolve.legacy(pkg); //=> "dist/module.mjs"

// customize fields order
resolve.legacy(pkg, {
  fields: ['main', 'module']
}); //=> "dist/require.js"
```

## API

The [`resolve()`](#resolvepkg-entry-options), [`exports()`](#exportspkg-entry-options), and [`imports()`](#importspkg-target-options) functions share similar API signatures:

```ts
export function resolve(pkg: Package, entry?: string, options?: Options): string[] | undefined;
export function exports(pkg: Package, entry?: string, options?: Options): string[] | undefined;
export function imports(pkg: Package, target: string, options?: Options): string[] | undefined;
//                                         ^ not optional!
```

All three:
* accept a `package.json` file's contents as a JSON object
* accept a target/entry identifier
* may accept an [Options](#options) object
* return `string[]`, `string`, or `undefined`

The only difference is that `imports()` must accept a target identifier as there can be no inferred default.

See below for further API descriptions.

> **Note:** There is also a [Legacy Resolver API](#legacy-resolver)

---

### resolve(pkg, entry?, options?)
Returns: `string[]` or `undefined`

A convenience helper which automatically reroutes to [`exports()`](#exportspkg-entry-options) or [`imports()`](#importspkg-target-options) depending on the `entry` value.

When unspecified, `entry` defaults to the `"."` identifier, which means that `exports()` will be invoked.

```js
import * as r from 'resolve.exports';

let pkg = {
  name: 'foobar',
  // ...
};

r.resolve(pkg);
//~> r.exports(pkg, '.');

r.resolve(pkg, 'foobar');
//~> r.exports(pkg, '.');

r.resolve(pkg, 'foobar/subpath');
//~> r.exports(pkg, './subpath');

r.resolve(pkg, '#hash/md5');
//~> r.imports(pkg, '#hash/md5');

r.resolve(pkg, 'foobar/#hash/md5');
//~> r.imports(pkg, '#hash/md5');
```

### exports(pkg, entry?, options?)
Returns: `string[]` or `undefined`

Traverse the `"exports"` within the contents of a `package.json` file. <br>
If the contents _does not_ contain an `"exports"` map, then `undefined` will be returned.

Successful resolutions will always result in a `string` or `string[]` value. This will be the value of the resolved mapping itself – which means that the output is a relative file path.

This function may throw an Error if:

* the requested `entry` cannot be resolved (aka, not defined in the `"exports"` map)
* an `entry` _is_ defined but no known conditions were matched (see [`options.conditions`](#optionsconditions))

#### pkg
Type: `object` <br>
Required: `true`

The `package.json` contents.

#### entry
Type: `string` <br>
Required: `false` <br>
Default: `.` (aka, root)

The desired target entry, or the original `import` path.

When `entry` _is not_ a relative path (aka, does not start with `'.'`), then `entry` is given the `'./'` prefix.

When `entry` begins with the package name (determined via the `pkg.name` value), then `entry` is truncated and made relative.

When `entry` is already relative, it is accepted as is.

***Examples***

Assume we have a module named "foobar" and whose `pkg` contains `"name": "foobar"`.

| `entry` value | treated as | reason |
|-|-|-|
| `null` / `undefined` | `'.'` | default |
| `'.'` | `'.'` | value was relative |
| `'foobar'` | `'.'` | value was `pkg.name` |
| `'foobar/lite'` | `'./lite'` | value had `pkg.name` prefix |
| `'./lite'` | `'./lite'` | value was relative |
| `'lite'` | `'./lite'` | value was not relative & did not have `pkg.name` prefix |


### imports(pkg, target, options?)
Returns: `string[]` or `undefined`

Traverse the `"imports"` within the contents of a `package.json` file. <br>
If the contents _does not_ contain an `"imports"` map, then `undefined` will be returned.

Successful resolutions will always result in a `string` or `string[]` value. This will be the value of the resolved mapping itself – which means that the output is a relative file path.

This function may throw an Error if:

* the requested `target` cannot be resolved (aka, not defined in the `"imports"` map)
* an `target` _is_ defined but no known conditions were matched (see [`options.conditions`](#optionsconditions))

#### pkg
Type: `object` <br>
Required: `true`

The `package.json` contents.

#### target
Type: `string` <br>
Required: `true`

The target import identifier; for example, `#hash` or `#hash/md5`.

Import specifiers _must_ begin with the `#` character, as required by the resolution specification. However, if `target` begins with the package name (determined by the `pkg.name` value), then `resolve.exports` will trim it from the `target` identifier. For example, `"foobar/#hash/md5"` will be treated as `"#hash/md5"` for the `"foobar"` package.

## Options

The [`resolve()`](#resolvepkg-entry-options), [`imports()`](#importspkg-target-options), and [`exports()`](#exportspkg-entry-options) functions share these options. All properties are optional and you are not required to pass an `options` argument.

Collectively, the `options` are used to assemble a list of [conditions](https://nodejs.org/docs/latest-v18.x/api/packages.html#conditional-exports) that should be activated while resolving your