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

$ npm install resolve.exports

Usage

Please see /test/ for examples.

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(), exports(), and imports() functions share similar API signatures:

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 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

resolve(pkg, entry?, options?)

Returns: string[] or undefined

A convenience helper which automatically reroutes to exports() or imports() depending on the entry value.

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

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.
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)

pkg

Type: object
Required: true

The package.json contents.

entry

Type: string
Required: false
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".

imports(pkg, target, options?)

Returns: string[] or undefined

Traverse the "imports" within the contents of a package.json file.
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)

pkg

Type: object
Required: true

The package.json contents.

target

Type: string
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(), imports(), and exports() 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 that should be activated while resolving your target(s).

Note: Although the Node.js documentation primarily showcases conditions alongside "exports" usage, they also apply to "imports" maps too. (example)

options.require

Type: boolean
Default: false

When truthy, the "require" field is added to the list of allowed/known conditions.
Otherwise the "import" field is added instead.

options.browser

Type: boolean
Default: false

When truthy, the "browser" field is added to the list of allowed/known conditions.
Otherwise the "node" field is added instead.

options.conditions

Type: string[]
Default: []

A list of additional/custom conditions that should be accepted when seen.

Important: The order specified within options.conditions does not matter.
The matching order/priority is always determined by the "exports" map's key order.

For example, you may choose to accept a "production" condition in certain environments. Given the following pkg content:

const pkg = {
  // package.json ...
  "exports": {
    "worker": "./$worker.js",
    "require": "./$require.js",
    "production": "./$production.js",
    "import": "./$import.mjs",
  }
};

resolve.exports(pkg, '.');
// Conditions: ["default", "import", "node"]
//=> ["./$import.mjs"]

resolve.exports(pkg, '.', {
  conditions: ['production']
});
// Conditions: ["default", "production", "import", "node"]
//=> ["./$production.js"]

resolve.exports(pkg, '.', {
  conditions: ['production'],
  require: true,
});
// Conditions: ["default", "production", "require", "node"]
//=> ["./$require.js"]

resolve.exports(pkg, '.', {
  conditions: ['production', 'worker'],
  require: true,
});
// Conditions: ["default", "production", "worker", "require", "node"]
//=> ["./$worker.js"]

resolve.exports(pkg, '.', {
  conditions: ['production', 'worker']
});
// Conditions: ["default", "production", "worker", "import", "node"]
//=> ["./$worker.js"]

options.unsafe

Type: boolean
Default: false

Important: You probably do not want this option!
It will break out of Node's default resolution conditions.

When enabled, this option will ignore all other options except options.conditions. This is because, when enabled, options.unsafe does not assume or provide any default conditions except the "default" condition.

resolve.exports(pkg, '.');
//=> Conditions: ["default", "import", "node"]

resolve.exports(pkg, '.', { unsafe: true });
//=> Conditions: ["default"]

resolve.exports(pkg, '.', { unsafe: true, require: true, browser: true });
//=> Conditions: ["default"]

In other words, this means that trying to use options.require or options.browser alongside options.unsafe will have no effect. In order to enable these conditions, you must provide them manually into the options.conditions list:

resolve.exports(pkg, '.', {
  unsafe: true,
  conditions: ["require"]
});
//=> Conditions: ["default", "require"]

resolve.exports(pkg, '.', {
  unsafe: true,
  conditions: ["browser", "require", "custom123"]
});
//=> Conditions: ["default", "browser", "require", "custom123"]

Legacy Resolver

Also included is a "legacy" method for resolving non-"exports" package fields. This may be used as a fallback method when for when no "exports" mapping is defined. In other words, it's completely optional (and tree-shakeable).

legacy(pkg, options?)

Returns: string or undefined

You may customize the field priority via options.fields.

When a field is found, its value is returned as written.
When no fields were found, undefined is returned. If you wish to mimic Node.js behavior, you can assume this means 'index.js' – but this module does not make that assumption for you.

options.browser

Type: boolean or string
Default: false

When truthy, ensures that the 'browser' field is part of the acceptable fields list.

Important: If your custom options.fields value includes 'browser', then your order is respected.
Otherwise, when truthy, options.browser will move 'browser' to the front of the list, making it the top priority.

When true and "browser" is an object, then legacy() will return the the entire "browser" object.

You may also pass a string value, which will be treated as an import/file path. When this is the case and "browser" is an object, then legacy() may return:

• false – if the package author decided a file should be ignored; or
• your options.browser string value – but made relative, if not already

See the `"browser" field specification for more information.

options.fields

Type: string[]
Default: ['module', 'main']

A list of fields to accept. The order of the array determines the priority/importance of each field, with the most important fields at the beginning of the list.

By default, the legacy() method will accept any "module" and/or "main" fields if they are defined. However, if both fields are defined, then "module" will be returned.

import { legacy } from 'resolve.exports';

// package.json
const pkg = {
  "name": "...",
  "worker": "worker.js",
  "module": "module.mjs",
  "browser": "browser.js",
  "main": "main.js",
};

legacy(pkg);
// fields = [module, main]
//=> "module.mjs"

legacy(pkg, { browser: true });
// fields = [browser, module, main]
//=> "browser.mjs"

legacy(pkg, {
  fields: ['missing', 'worker', 'module', 'main']
});
// fields = [missing, worker, module, main]
//=> "worker.js"

legacy(pkg, {
  fields: ['missing', 'worker', 'module', 'main'],
  browser: true,
});
// fields = [browser, missing, worker, module, main]
//=> "browser.js"

legacy(pkg, {
  fields: ['module', 'browser', 'main'],
  browser: true,
});
// fields = [module, browser, main]
//=> "module.mjs"

License

MIT © Luke Edwards