ts2mjs

Rename TypeScript created ESM .js files to .mjs

This tool takes output from tsc and copies/renames the files.

• file.js => file.mjs
• file.js.map => file.mjs.map
• file.d.ts => file.d.mts
• file.d.ts.map => file.d.mts.map
• file.ts => file.mts (by default, can be skipped with --skip-ts)

CommonJS Support

Use the --cjs flag to generate CommonJS extensions instead:

• file.js => file.cjs
• file.js.map => file.cjs.map
• file.d.ts => file.d.cts
• file.d.ts.map => file.d.cts.map
• file.ts => file.cts (by default, can be skipped with --skip-ts)

Key Features

🔄 Import/Export Rewriting

Automatically updates relative imports and exports in your files:

dist/code.js -> dist/code.mjs

import * as path from 'path';
import { lib } from 'package/lib/index.js'
-import { findFiles } from './findFiles.js';
+import { findFiles } from './findFiles.mjs';

dist/index.d.ts -> dist/index.d.mts

-export { PrimeNumber, Tuple, GUID, Address, Person, Annotation, } from './types.js';
+export { PrimeNumber, Tuple, GUID, Address, Person, Annotation, } from './types.mjs';
-export { lookUpPerson } from './lookup.js';
+export { lookUpPerson } from './lookup.mjs';

🗑️ Source File Removal

Use --remove-source to automatically delete original files after successful conversion:

ts2mjs dist/esm --remove-source
# Original .js files are deleted after being converted to .mjs

🚫 TypeScript File Skipping

Use --skip-ts to ignore TypeScript files instead of converting them:

ts2mjs dist/esm --skip-ts
# .ts and .d.ts files are left unchanged, only .js files are processed

Options

• --cjs - Generate .cjs extensions instead of .mjs
• --remove-source - Remove original files after successful conversion
• --skip-ts - Skip all TypeScript files (.ts and .d.ts) instead of converting them
• --dry-run - Preview changes without making them
• --verbose - Show detailed output
• -o, --output <dir> - Specify output directory (default: in-place)
• --root <dir> - Set the root directory for processing

Usage

This is an example on how to create a package that exports both CommonJS and ESM from TypeScript source.

tsconfig.esm.json

{
  "$schema": "https://json.schemastore.org/tsconfig",
  "extends": "@tsconfig/node20/tsconfig.json",
  "compilerOptions": {
    "declaration": true,
    "module": "ES2022",
    "moduleResolution": "node",
    "outDir": "dist/esm",
    "sourceMap": true
  },
  "include": ["src"]
}

tsconfig.cjs.json

{
  "$schema": "https://json.schemastore.org/tsconfig",
  "extends": "@tsconfig/node20/tsconfig.json",
  "compilerOptions": {
    "declaration": true,
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "outDir": "dist/cjs",
    "sourceMap": true
  },
  "include": ["src"]
}

package.json

{
  "type": "commonjs",
  "main": "dist/csj/index.js",
  "module": "dist/esm/index.mjs",
  "types": "dist/cjs/index.d.ts",
  "exports": {
    ".": {
      "import": "./dist/esm/index.mjs",
      "require": "./dist/cjs/index.js"
    }
  }
}

Build Commands

# Build both CommonJS and ESM
tsc -p tsconfig.cjs.json
tsc -p tsconfig.esm.json

# Convert ESM output to .mjs
ts2mjs dist/esm

# Convert CJS TypeScript files to .cts (if needed)
ts2mjs dist/cjs --cjs

# Advanced usage examples
ts2mjs dist/esm --remove-source  # Delete original files after conversion
ts2mjs dist/esm --skip-ts        # Only process .js files, skip .ts/.d.ts
ts2mjs dist/esm --dry-run --verbose  # Preview changes with detailed output

Help

Usage: ts2mjs [options] <files...>

Rename ESM .js files to .mjs (or .cjs if --cjs options is used)

Arguments:
  files                 Globs matching files to rename.

Options:
  --cjs                 Use .cjs extension instead of .mjs extension.
  -o, --output <dir>    The output directory.
  --cwd <dir>           The current working directory.
  --root <dir>          The root directory.
  -x,--exclude <glob>   Exclude a glob from the files. (default: [])
  --dry-run             Dry Run do not update files.
  --remove-source       Remove the original files after successful conversion.
  --skip-ts             Skip all TypeScript files (.ts and .d.ts) instead of
                        converting them.
  --no-must-find-files  No error if files are not found.
  --no-enforce-root     Do not fail if relative `.js` files outside of the root
                        are imported.
  --color               Force color.
  --no-color            Do not use color.
  -v, --verbose         Verbose mode
  -V, --version         output the version number
  -h, --help            display help for command

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