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Missing ECMAScript module utils for Node.js

Missing ECMAScript module utils for Node.js

While ESM Modules are evolving in Node.js ecosystem, there are still many required features that are still experimental or missing or needed to support ESM. This package tries to fill in the gap.


Install npm package:

# using yarn
yarn add mlly

# using npm
npm install mlly

Note: Node.js 14+ is recommended.

Import utils:

// ESM
import {} from "mlly";

// CommonJS
const {} = require("mlly");

Resolving ESM modules

Several utilities to make ESM resolution easier:

  • Respecting ECMAScript Resolver algorithm
  • Exposed from Node.js implementation
  • Windows paths normalized
  • Supporting custom extensions and /index resolution
  • Supporting custom conditions
  • Support resolving from multiple paths or urls

resolve / resolveSync

Resolve a module by respecting ECMAScript Resolver algorithm (using wooorm/import-meta-resolve).

Additionally supports resolving without extension and /index similar to CommonJS.

import { resolve, resolveSync } from "mlly";

// file:///home/user/project/module.mjs
console.log(await resolve("./module.mjs", { url: import.meta.url }));

Resolve options:

  • url: URL or string to resolve from (default is pwd())
  • conditions: Array of conditions used for resolution algorithm (default is ['node', 'import'])
  • extensions: Array of additional extensions to check if import failed (default is ['.mjs', '.cjs', '.js', '.json'])

resolvePath / resolvePathSync

Similar to resolve but returns a path instead of URL using fileURLToPath.

import { resolvePath, resolveSync } from "mlly";

// /home/user/project/module.mjs
console.log(await resolvePath("./module.mjs", { url: import.meta.url }));


Create a resolve function with defaults.

import { createResolve } from "mlly";

const _resolve = createResolve({ url: import.meta.url });

// file:///home/user/project/module.mjs
console.log(await _resolve("./module.mjs"));

Example: Ponyfill import.meta.resolve:

import { createResolve } from "mlly";

import.meta.resolve = createResolve({ url: import.meta.url });


Resolve all static and dynamic imports with relative paths to full resolved path.

import { resolveImports } from "mlly";

// import foo from 'file:///home/user/project/bar.mjs'
  await resolveImports(`import foo from './bar.mjs'`, { url: import.meta.url }),

Syntax Analyzes


Using various syntax detection and heuristics, this method can determine if import is a valid import or not to be imported using dynamic import() before hitting an error!

When result is false, we usually need a to create a CommonJS require context or add specific rules to the bundler to transform dependency.

import { isValidNodeImport } from "mlly";

// If returns true, we are safe to use `import('some-lib')`
await isValidNodeImport("some-lib", {});


  • Check import protocol - If is data: return true (✅ valid) - If is not node:, file: or data:, return false ( ❌ invalid)
  • Resolve full path of import using Node.js Resolution algorithm
  • Check full path extension
    • If is .mjs, .cjs, .node or .wasm, return true (✅ valid)
    • If is not .js, return false (❌ invalid)
    • If is matching known mixed syntax (.esm.js, .es.js, etc) return false ( ❌ invalid)
  • Read closest package.json file to resolve path
  • If type: 'module' field is set, return true (✅ valid)
  • Read source code of resolved path
  • Try to detect CommonJS syntax usage
    • If yes, return true (✅ valid)
  • Try to detect ESM syntax usage
    • if yes, return false ( ❌ invalid)


  • There might be still edge cases algorithm cannot cover. It is designed with best-efforts.
  • This method also allows using dynamic import of CommonJS libraries considering Node.js has Interoperability with CommonJS.


Detect if code, has usage of ESM syntax (Static import, ESM export and import.meta usage)

import { hasESMSyntax } from "mlly";

hasESMSyntax("export default foo = 123"); // true


Detect if code, has usage of CommonJS syntax (exports, module.exports, require and global usage)

import { hasCJSSyntax } from "mlly";

hasCJSSyntax("export default foo = 123"); // false


Tests code against both CJS and ESM.

isMixed indicates if both are detected! This is a common case with legacy packages exporting semi-compatible ESM syntax meant to be used by bundlers.

import { detectSyntax } from "mlly";

// { hasESM: true, hasCJS: true, isMixed: true }
detectSyntax('export default require("lodash")');

CommonJS Context


This utility creates a compatible CommonJS context that is missing in ECMAScript modules.

import { createCommonJS } from "mlly";

const { __dirname, __filename, require } = createCommonJS(import.meta.url);

Note: require and require.resolve implementation are lazy functions. createRequire will be called on first usage.

Import/Export Analyzes

Tools to quickly analyze ESM syntax and extract static import/export

  • Super fast Regex based implementation
  • Handle most edge cases
  • Find all static ESM imports
  • Find all dynamic ESM imports
  • Parse static import statement
  • Find all named, declared and default exports


Find all static ESM imports.


import { findStaticImports } from "mlly";

// Empty line
import foo, { bar /* foo */ } from 'baz'


    type: "static",
    imports: "foo, { bar /* foo */ } ",
    specifier: "baz",
    code: "import foo, { bar /* foo */ } from 'baz'",
    start: 15,
    end: 55,


Parse a dynamic ESM import statement previously matched by findStaticImports.


import { findStaticImports, parseStaticImport } from "mlly";

const [match0] = findStaticImports(`import baz, { x, y as z } from 'baz'`);


  type: 'static',
  imports: 'baz, { x, y as z } ',
  specifier: 'baz',
  code: "import baz, { x, y as z } from 'baz'",
  start: 0,
  end: 36,
  defaultImport: 'baz',
  namespacedImport: undefined,
  namedImports: { x: 'x', y: 'z' }


Find all dynamic ESM imports.


import { findDynamicImports } from "mlly";

const foo = await import('bar')


import { findExports } from "mlly";

export const foo = 'bar'
export { bar, baz }
export default something


    type: "declaration",
    declaration: "const",
    name: "foo",
    code: "export const foo",
    start: 1,
    end: 17,
    type: "named",
    exports: " bar, baz ",
    code: "export { bar, baz }",
    start: 26,
    end: 45,
    names: ["bar", "baz"],
  { type: "default", code: "export default ", start: 46, end: 61 },


Same as findExports but returns array of export names.

import { findExportNames } from "mlly";

// [ "foo", "bar", "baz", "default" ]
export const foo = 'bar'
export { bar, baz }
export default something


Resolves module and reads its contents to extract possible export names using static analyzes.

import { resolveModuleExportNames } from "mlly";

// ["basename", "dirname", ... ]
console.log(await resolveModuleExportNames("mlly"));

Evaluating Modules

Set of utilities to evaluate ESM modules using data: imports

  • Automatic import rewrite to resolved path using static analyzes
  • Allow bypass ESM Cache
  • Stack-trace support
  • .json loader


Transform and evaluates module code using dynamic imports.

import { evalModule } from "mlly";

await evalModule(`console.log("Hello World!")`);

await evalModule(
  import { reverse } from './utils.mjs'
  console.log(reverse('!emosewa si sj'))
  { url: import.meta.url },


  • all resolve options
  • url: File URL


Dynamically loads a module by evaluating source code.

import { loadModule } from "mlly";

await loadModule("./hello.mjs", { url: import.meta.url });

Options are same as evalModule.


  • Resolves all relative imports will be resolved
  • All usages of import.meta.url will be replaced with url or from option
import { transformModule } from "mlly";
console.log(transformModule(`console.log(import.meta.url)`), {
  url: "test.mjs",

Options are same as evalModule.

Other Utils


Similar to url.fileURLToPath but also converts windows backslash \ to unix slash / and handles if input is already a path.

import { fileURLToPath } from "mlly";

// /foo/bar.js

// C:/path


Similar to url.pathToFileURL but also handles URL input and returns a string with file:// protocol.

import { pathToFileURL } from "mlly";

// /foo/bar.js

// C:/path


Ensures id has either of node:, data:, http:, https: or file: protocols.

import { ensureProtocol } from "mlly";

// file:///foo/bar.js


Read source contents of a URL. (currently only file protocol supported)

import { resolve, loadURL } from "mlly";

const url = await resolve("./index.mjs", { url: import.meta.url });
console.log(await loadURL(url));


Convert code to data: URL using base64 encoding.

import { toDataURL } from "mlly";

  // This is an example
  console.log('Hello world')


Return the default export of a module at the top-level, alongside any other named exports.

// Assuming the shape { default: { foo: 'bar' }, baz: 'qux' }
import myModule from "my-module";

// Returns { foo: 'bar', baz: 'qux' }


  • preferNamespace: In case that default value exists but is not extendable (when is string for example), return input as-is (default is false, meaning default's value is prefered even if cannot be extended)


Replace reserved characters from a segment of URI to make it compatible with rfc2396.

import { sanitizeURIComponent } from "mlly";

// foo_bar


Sanitize each path of a file name or path with sanitizeURIComponent for URI compatibility.

import { sanitizeFilePath } from "mlly";

// C:/te_st/_...slug_.jsx'


Parses an absolute file path in node_modules to three segments:

  • dir: Path to main directory of package
  • name: Package name
  • subpath: The optional package subpath

It returns an empty object (with partial keys) if parsing fails.

import { parseNodeModulePath } from "mlly";

// dir: "/src/a/node_modules/"
// name: "lib"
// subpath: "./dist/index.mjs"
const { dir, name, subpath } = parseNodeModulePath(


Parses an absolute file path in node_modules and tries to reverse lookup (or guess) the original package exports subpath for it.

import { lookupNodeModuleSubpath } from "mlly";

// subpath: "./utils"
const subpath = lookupNodeModuleSubpath(


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