Reliably add/replace extensions of select import specifiers or just rewrite them entirely!

---

babel-plugin-transform-rewrite-imports

This Babel plugin (1) reliably adds extensions to import and export specifiers that do not already have one, (2) selectively replaces extensions of specifiers that do, and (3) can rewrite whole specifiers in intricate ways using simple yet powerful replacement maps.

All TypeScript and JavaScript flavors are supported depending on how Babel is configured in the project.

For example, something like this:

import { item1, type item2 } from '==> pretty specifier #1 <==';

export { here as there } from '==> pretty specifier #2 <==';

const elsewhere = `==> pretty specifier #${getRandomNumber()} <==`;

jest.mock('==> pretty specifier #3 <==');

const x = require('==> pretty specifier #4 <==');

export async function myFunction() {
  return (await import(elsewhere, { with: { type: 'json' } })).name;
}

export type MyType = {
  typeOnlyImport: typeof import('==> pretty specifier #5 <==').Type;
};

Can be easily transformed into something like this:

import { item1, type item2 } from '../../../specifier-1.js';

export { here as there } from '../specifier-2.js';

const elsewhere = `==> specifier #${getRandomNumber()} <==`;

jest.mock('../../../packages/a-different-monorepo-package/src/specifier-3.js');

const x = require('@specifier/four');

const __injected_dynamic_rewrite = function () {
  /*...*/
};

export async function myFunction() {
  return (
    await import(__injected_dynamic_rewrite(elsewhere), {
      with: { type: 'json' }
    })
  ).name;
}

export type MyType = {
  typeOnlyImport: typeof import('../../../../node_modules/s-5/dist/src/lib.d.ts').Type;
};

The transform-rewrite-imports plugin comes in handy in situations like transpiling TypeScript source with extensionless imports to ESM, or changing alias paths in TypeScript declaration (i.e. .d.ts) files into relative paths suitable for publishing. It does this more reliably and efficiently than prior art.

---

• Install
• Comparison with Prior Art
• Usage
  • appendExtension
  • recognizedExtensions
  • replaceExtensions
  • requireLikeFunctions
  • injectDynamicRewriter
• Advanced Usage
  • Rewriting Dynamic Imports and Requires with Non-Literal Arguments
• Comprehensive Logging
• Examples
  • Real-World Examples
• Appendix
  • Published Package Details
  • License
• Contributing and Support
  • Contributors

Install

npm install --save-dev babel-plugin-transform-rewrite-imports

And integrate the following snippet into your Babel configuration:

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        // See below for configuration instructions and examples
      }
    ]
  ]
};

Finally, run Babel through your toolchain (Webpack, Jest, etc) or manually:

npx babel src --out-dir dist

Comparison with Prior Art

The transform-rewrite-imports plugin effectively combines the functionality of the following:

• babel-plugin-module-resolver

  The module-resolver plugin inspired quite a bit of the functionality of transform-rewrite-imports, such as transforming require-like functions, and offers some similar features like RegExp-based aliasing and support for substitution functions. However, transform-rewrite-imports is capable of more complex replacements (like handling intricate file extension changes) with support for a wider variety of specifier types while surfacing a simpler interface.

• babel-plugin-add-import-extension

  transform-rewrite-imports started off as a fork of add-import-extension; transform-rewrite-imports functions more consistently and includes support for transforming require, require-like, static import(), and arbitrary dynamic import() statements, replacing multiple extensions using complex logic, and reliably ignoring extensions that should not be replaced.

• babel-plugin-replace-import-extension

  transform-rewrite-imports is similar in intent to replace-import-extension. However, rewriting extensions is only a small fraction of what transform-rewrite-imports can do. And while both extensions support transforming dynamic imports, transform-rewrite-imports results in more efficient code in production environments due to (1) avoiding injecting dynamic code into the AST except as the very last resort and (2) only injecting dynamic code once and caching it globally (at the file level) rather than filling the file with repeated functions.

• babel-plugin-transform-rename-import

  With its last update published over 6 years ago, transform-rename-import can also replace import specifiers, though transform-rewrite-imports offers a powerful superset of replacer functionality, including optionally performing replacements of arbitrary dynamic imports at runtime and appending extensions to specifiers that would otherwise not have one.

• tsconfig-replace-paths / tsconfig-paths / tscpaths

  tsconfig-replace-paths and its predecessors/alternatives tsconfig-paths and tscpaths are extremely useful for transpiling TypeScript projects, as they handle alias- and path- resolving use cases without additional configuration; transform-rewrite-imports, on the other hand, must be fed path/alias information from tsconfig.json manually.

  Unfortunately, tsconfig-paths is not a Babel plugin and requires patching your runtime while the others do not support all the latest TypeScript/Babel AST features (like TsImportType) and therefore fail to consistently and correctly transform certain files (especially certain .d.ts files).

  By mapping a project's tsconfig.json paths value to a replacement map transform-rewrite-imports can understand, it becomes possible to ditch tsconfig-replace-paths etc and reduce dependency count. Here's an example using transform-rewrite-imports to replace these plugins (and babel-plugin-module-resolver) for transforming both sources and type definitions. Essentially, this Babel configuration file maps the project's tsconfig.json paths into a replaceExtensions replacement map.

Usage

By default this plugin does not affect Babel's output. You must explicitly configure this extension before any specifier will be rewritten.

More information on the available options can be found in the docs:

{
  appendExtension?: string | Callback<string | undefined>;
  recognizedExtensions?: string[];
  replaceExtensions?: Record<string, string | Callback<string>>;
  requireLikeFunctions?: string[];
  injectDynamicRewriter?: 'never' | 'only-if-necessary';
  silent?: boolean;
  verbose?: boolean;
}

appendExtension

To append an extension to all relative import specifiers that do not already have a recognized extension, use appendExtension:

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        appendExtension: '.mjs'
      }
    ]
  ]
};

Tip

Only relative import specifiers (that start with ./ or ../) will be considered for appendExtension. This means bare specifiers (e.g. built-in packages and packages imported from node_modules) and absolute specifiers will never be affected by appendExtension.

recognizedExtensions

What is and is not considered a "recognized extension" is determined by recognizedExtensions:

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        appendExtension: '.mjs',
        recognizedExtensions: ['.js']
      }
    ]
  ]
};

That is: import specifiers that end with an extension included in recognizedExtensions will never have appendExtension appended to them. All other imports, including those with a . in the file name (e.g. component.module.style.ts), may be rewritten.

recognizedExtensions is set to ['.js', '.jsx', '.mjs', '.cjs', '.json'] by default.

If the value of appendExtension is not included in recognizedExtensions, then imports that already end in appendExtension will have appendExtension appended to them (e.g. index.ts is rewritten as index.ts.ts when appendExtension: '.ts' and recognizedExtensions is its default value). If this behavior is undesired, ensure appendExtension is included in recognizedExtensions.

Warning

Note that specifying a custom value for recognizedExtensions overwrites the default value entirely. To extend rather than overwrite, you can import the default value from the package itself:

const {
  defaultRecognizedExtensions
} = require('babel-plugin-transform-rewrite-imports');

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        appendExtension: '.mjs',
        recognizedExtensions: [...defaultRecognizedExtensions, '.ts']
      }
    ]
  ]
};

replaceExtensions

You can also replace one or more existing extensions in specifiers using a replacement map:

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        replaceExtensions: {
          // Replacements are evaluated **in order**, stopping on the first match.
          // That means if the following two keys were listed in reverse order,
          // .node.js would become .node.mjs instead of .cjs
          '.node.js': '.cjs',
          '.js': '.mjs'
        }
      }
    ]
  ]
};

These configurations can be combined to rewrite many imports at once. For instance, if you wanted to replace certain extensions and append only when no recognized or listed extension is specified:

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        appendExtension: '.mjs',
        replaceExtensions: {
          '.node.js': '.cjs',
          // Since .js is in recognizedExtensions by default, file.js would normally
          // be ignored. However, since .js is mapped to .mjs in the
          // replaceExtensions map, file.js becomes file.mjs
          '.js': '.mjs'
        }
      }
    ]
  ]
};

Since specifier comparisons are made using String.endsWith without splitting on directory separators or any other characters, appendExtension and replaceExtensions accept any suffix, not just those that begin with .; additionally, replaceExtensions accepts regular expressions. This allows you to partially or entirely rewrite a specifier rather than just its extension:

const {
  defaultRecognizedExtensions
} = require('babel-plugin-transform-rewrite-imports');

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        appendExtension: '.mjs',
        // Add .css to recognizedExtensions so .mjs isn't automatically appended
        recognizedExtensions: [...defaultRecognizedExtensions, '.css'],
        replaceExtensions: {
          '.node.js': '.cjs',
          '.js': '.mjs',

          // The following key replaces the entire specifier when matched
          '^package$': `${__dirname}/package.json`,
          // If .css wasn't in recognizedExtensions, my-utils/src/file.less would
          // become my-utils/src/file.css.mjs instead of my-utils/src/file.css
          '(.+?)\\.less$': '$1.css'
        }
      }
    ]
  ]
};

Tip

If a key of replaceExtensions begins with ^ or ends with $, it is considered a regular expression instead of an extension. Regular expression replacements support substitutions of capturing groups as well (e.g. $1, $2, etc).

replaceExtensions is evaluated and replacements made before appendExtension is appended to specifiers with unrecognized or missing extensions. This means an extensionless import specifier could be rewritten by replaceExtensions to have a recognized extension, which would then be ignored instead of having appendExtension appended to it.

requireLikeFunctions

When it comes to deciding what is and is not a specifier, transform-rewrite-imports will always scan ImportDeclaration, ExportAllDeclaration, ExportNamedDeclaration, TSImportType, and dynamic import CallExpressions for specifiers.

For call expressions specifically, requireLikeFunctions is used to determine which additional function calls will have their first arguments scanned for specifiers. By default, requireLikeFunctions is set to:

[
  'require',
  'require.resolve',
  'System.import',
  'jest.genMockFromModule',
  'jest.mock',
  'jest.unmock',
  'jest.doMock',
  'jest.dontMock',
  'jest.requireActual'
];

Tip

Similar to defaultRequireLikeFunctions, these defaults are exported under the name defaultRequireLikeFunctions.

This means call expressions like require(...), jest.mock(...), etc will be treated the same way as import(...), where the first parameter is considered a specifier. You are free to tweak this functionality to suit your environment.

injectDynamicRewriter

By default, a dynamic rewriter is injected only if necessary. Set this to "never" to ensure a dynamic rewriter is never injected into the the output regardless of any perceived necessity.

Refer to the Advanced Usage section for more details.

Advanced Usage

replaceExtensions and appendExtension both accept function callbacks as values everywhere strings are accepted. This can be used to provide advanced replacement logic.

These callback functions have the following signatures:

type AppendExtensionCallback = (context: {
  specifier: string;
  capturingGroups: never[];
  filepath: string;
}) => string | undefined;

type ReplaceExtensionsCallback = (context: {
  specifier: string;
  capturingGroups: string[];
  filepath: string;
}) => string;

Where specifier is the import/export specifier being rewritten, capturingGroups is a simple string array of capturing groups returned by String.prototype.match(), and filepath is the absolute path to the original input file being transformed by Babel. capturingGroups will always be an empty array except when it appears within a function value of a replaceExtensions entry that has a regular expression key.

When provided as the value of appendExtension, a string containing an extension should be returned (including leading dot). When provided as the value of a replaceExtensions entry, a string containing the full specifier should be returned. When returning a full specifier, capturing group substitutions (e.g. $1, $2, etc) within the returned string will be honored.

Further, in the case of appendExtension, note that specifier, if its basename is . or .. or if it ends in a directory separator (e.g. /), will have "/index" appended to the end before the callback is invoked. However, if the callback returns undefined (and the specifier was not matched in replaceExtensions), the specifier will not be modified in any way.

By way of example (see the output of this example here):

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        // If the specifier ends with "/no-ext", do not append any extension
        appendExtension: ({ specifier }) => {
          return specifier.endsWith('/no-ext') ||
            specifier.endsWith('..') ||
            specifier === './another-thing'
            ? undefined
            : '.mjs';
        },
        replaceExtensions: {
          // Rewrite imports of packages in a monorepo to use their actual names
          //         v capturing group #1: capturingGroups[1]
          '^packages/([^/]+)(/.+)?': ({ specifier, capturingGroups }) => {
            //              ^ capturing group #2: capturingGroups[2]
            if (
              specifier === 'packages/root' ||
              specifier.startsWith('packages/root/')
            ) {
              return `./monorepo-js${capturingGroups[2] ?? '/'}`;
            } else if (
              !capturingGroups[2] ||
              capturingGroups[2].startsWith('/src/index')
            ) {
              return `@monorepo/$1`;
            } else if (capturingGroups[2].startsWith('/package.json')) {
              return `@monorepo/$1$2`;
            } else {
              return `@monorepo/$1/dist$2`;
            }
          }
        }
      }
    ]
  ]
};

Rewriting Dynamic Imports and Requires with Non-Literal Arguments

When transforming dynamic imports and require statements that do not have a string literal as the first argument, and injectDynamicRewriter is not set to 'never', the options passed to this plugin are transpiled and injected into the resulting AST.

Caution

This means you take a slight performance hit when you do arbitrary dynamic imports that cannot be statically analyzed (e.g. require(getMd5Hash() + '.txt')). These types of dynamic imports are usually code smell, but this library is built to accommodate them regardless.

However, if you are NOT doing arbitrary dynamic imports, which are dynamic imports where the first argument is not a string literal, then this section is of no relevance to you since nothing extra will be injected into the AST.

Therefore, to be safe, callback functions must not reference variables outside of their immediate scope.

Good:

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        replaceExtensions: {
          '^packages/([^/]+)(/.+)?': ({ specifier, capturingGroups }) => {
            const myPkg = require('my-pkg');
            myPkg.doStuff(specifier, capturingGroups);
          }
        }
      }
    ]
  ]
};

Bad:

const myPkg = require('my-pkg');

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        replaceExtensions: {
          '^packages/([^/]+)(/.+)?': ({ specifier, capturingGroups }) => {
            myPkg.doStuff(specifier, capturingGroups);
          }
        }
      }
    ]
  ]
};

Technically, you can get away with violating this rule if you're sure you'll only ever use dynamic imports/require statements with string literal arguments.

Comprehensive Logging

Like Babel itself, this plugin leverages debug (via rejoinder) under the hood for log management. You can take advantage of this to peer into transform-rewrite-imports's innermost workings and deepest decision-making processes by activating the appropriate log level. For example, the following will enable all logging related to this plugin:

DEBUG='babel-plugin-transform-rewrite-imports:*' npx babel src --out-dir dist

Examples

With the following snippet integrated into your Babel configuration:

const {
  defaultRecognizedExtensions
} = require('babel-plugin-transform-rewrite-imports');

module.exports = {
  plugins: [
    [
      'babel-plugin-transform-rewrite-imports',
      {
        appendExtension: '.mjs',
        recognizedExtensions: [...defaultRecognizedExtensions, '.css'],
        replaceExtensions: {
          '.ts': '.mjs',
          '^package$': `${__dirname}/package.json`,
          '(.+?)\\.less$': '$1.css'
        }
      }
    ]
  ]
};

The following source:

/* file: src/index.ts */
import { name as pkgName } from 'package';
import { primary } from '.';
import { secondary } from '..';
import { tertiary } from '../..';
import dirImport from './some-dir/';
import jsConfig from './jsconfig.json';
import projectConfig from './project.config.cjs';
import { add, double } from './src/numbers';
import { curry } from './src/typed/curry.ts';
import styles from './src/less/styles.less';

// Note that, unless otherwise configured, @babel/preset-typescript deletes
// type-only imports. If you want to operate on type imports and/or .d.ts files,
// use @babel/syntax-typescript instead. See ./test/supports-type-only for
// an example.
import type * as AllTypes from './lib/something.mjs';

export { triple, quadruple } from './lib/num-utils';

// Note that, unless otherwise configured, @babel/preset-typescript deletes
// type-only exports. If you want to operate on type imports and/or .d.ts files,
// use @babel/syntax-typescript instead. See ./test/supports-type-only for
// an example.
export type { NamedType } from './lib/something';

const thing = await import('./thing');
const anotherThing = require('./another-thing');

const thing2 = await import(someFn(`./${someVariable}`) + '.json');
const anotherThing2 = require(someOtherVariable);

Is, depending on your other plugins/settings, transformed into something like:

/* file: dist/index.js */
const _rewrite = (importPath, options) => {
    ...
  },
  _rewrite_options = {
    appendExtension: '.mjs',
    recognizedExtensions: ['.js', '.jsx', '.mjs', '.cjs', '.json', '.css'],
    replaceExtensions: {
      '.ts': '.mjs',
      '^package$': '/absolute/path/to/project/package.json',
      '(.+?)\\.less$': '$1.css'
    }
  };

import { name as pkgName } from '/absolute/path/to/project/package.json';
import { primary } from './index.mjs';
import { secondary } from '../index.mjs';
import { tertiary } from '../../index.mjs';
import dirImport from './some-dir/index.mjs';
import jsConfig from './jsconfig.json';
import projectConfig from './project.config.cjs';
import { add, double } from './src/numbers.mjs';
import { curry } from './src/typed/curry.mjs';
import styles from './src/less/styles.css';

export { triple, quadruple } from './lib/num-utils.mjs';

const thing = await import('./thing.mjs');
const anotherThing = require('./another-thing.mjs');

// Require calls and dynamic imports with a non-string-literal first argument
// are transformed into function calls that dynamically return the rewritten
// string:

const thing2 = await import(
  _rewrite(someFn(`./${someVariable}`) + '.json', _rewrite_options)
);

const anotherThing2 = require(_rewrite(someOtherVariable, _rewrite_options));

Note

See the full output of this example here.

Real-World Examples

For some real-world examples of this Babel plugin in action, check out symbiote's babel.config.js file (which uses transform-rewrite-imports to replace both babel-plugin-module-resolver and tsconfig-replace-paths), unified-utils, this very repository, or just take a peek at the test cases.

Appendix

Further documentation can be found under docs/.

Published Package Details

This is a CJS2 package with statically-analyzable exports built by Babel for use in Node.js versions that are not end-of-life. For TypeScript users, this package supports both "Node10" and "Node16" module resolution strategies.

Expand details

That means both CJS2 (via require(...)) and ESM (via import { ... } from ... or await import(...)) source will load this package from the same entry points when using Node. This has several benefits, the foremost being: less code shipped/smaller package size, avoiding dual package hazard entirely, distributables are not packed/bundled/uglified, a drastically less complex build process, and CJS consumers aren't shafted.

Each entry point (i.e. ENTRY) in package.json's exports[ENTRY] object includes one or more export conditions. These entries may or may not include: an exports[ENTRY].types condition pointing to a type declaration file for TypeScript and IDEs, a exports[ENTRY].module condition pointing to (usually ESM) source for Webpack/Rollup, a exports[ENTRY].node and/or exports[ENTRY].default condition pointing to (usually CJS2) source for Node.js require/import and for browsers and other environments, and other conditions not enumerated here. Check the package.json file to see which export conditions are supported.

Note that, regardless of the { "type": "..." } specified in package.json, any JavaScript files written in ESM syntax (including distributables) will always have the .mjs extension. Note also that package.json may include the sideEffects key, which is almost always false for optimal tree shaking where appropriate.

License

See LICENSE.

Contributing and Support

New issues and pull requests are always welcome and greatly appreciated! 🤩 Just as well, you can star 🌟 this project to let me know you found it useful! ✊🏿 Or buy me a beer, I'd appreciate it. Thank you!

See CONTRIBUTING.md and SUPPORT.md for more information.

Contributors

Thanks goes to these wonderful people (emoji key):

Bernard 🚇 💻 📖 🚧 ⚠️ 👀

Add your contributions

This project follows the all-contributors specification. Contributions of any kind welcome!