migration:generate / seed:generate always emit .js, which breaks in "type": "module" projects — allow configuring the file extension (e.g. .cjs) #1575

What you are doing?

In a project whose package.json has "type": "module" (common with TypeScript ESM setups), generating a migration and running it fails, because the generated file is CommonJS but has a .js extension, which Node treats as ESM in that scope.

// package.json
{ "type": "module" }

npx sequelize-cli migration:generate --name add-users-table
# -> migrations/20261001000000-add-users-table.js   (content: module.exports = { up, down })

npx sequelize-cli db:migrate

What do you expect to happen?

A way to tell the CLI which extension to use for generated migrations and seeders, for example:

npx sequelize-cli migration:generate --name add-users-table --extension cjs

or once in .sequelizerc (whose keys are already passed to yargs as config, see src/core/yargs.js#L6-L17):

// .sequelizerc
module.exports = {
  'migrations-path': 'migrations',
  extension: 'cjs',
};

so the generated file is 20261001000000-add-users-table.cjs, which db:migrate already loads correctly.

What is actually happening?

The extension is hardcoded to js:

• src/helpers/path-helper.js#L47-L49: getFileExtension() always returns 'js', even though addFileExtension(basename, options) already passes an options argument to it.
• src/commands/migration_generate.js#L21 and src/commands/seed_generate.js#L19 build the path through it.
• The templates are CommonJS: src/assets/migrations/skeleton.js#L4, src/assets/seeders/skeleton.js#L4.

Migrations are then loaded with a plain require() by Umzug v2 (umzug/src/migration.js#L68). Under "type": "module" a .js file is ESM, so loading it fails:

# Node 22
module is not defined in ES module scope

# Node < 20.19 / < 22.12
ERR_REQUIRE_ESM

The loader side already supports .cjs: the migrator pattern is /^(?!.*\.d\.ts$).*\.(cjs|js|cts|ts)$/ (src/core/migrator.js#L49), added in #905. Only generation is missing.

Current workaround

We wrap the generate command in a small script that renames the output from .js to .cjs:

#!/usr/bin/env bash
# bin/generate-migration.sh — usage: bin/generate-migration.sh --name <name>
set -euo pipefail

MIGRATIONS_DIR="migrations"
before=$(ls "$MIGRATIONS_DIR")

npx sequelize-cli migration:generate "$@"

for f in "$MIGRATIONS_DIR"/*.js; do
  name=$(basename "$f")
  if ! grep -qxF "$name" <<< "$before"; then
    mv "$f" "${f%.js}.cjs"
    echo "Renamed $name -> ${name%.js}.cjs"
  fi
done

This works, but every project using "type": "module" has to reinvent it, and it's easy to forget when someone runs sequelize-cli migration:generate directly. A built-in option would remove the need for it.

Proposed change

• Add an --extension option to migration:generate and seed:generate, settable from .sequelizerc like other options.
• Accept only extensions the migrator actually loads (js, cjs, ts, cts) via yargs choices, default js, so behaviour is unchanged for existing users.
• Reject mjs, since the migrator pattern skips .mjs files silently and Umzug v2 loads with require().
• Pass the extension explicitly through getMigrationPath / getSeederPath → getFileName → getFileExtension(extension) (returning extension || 'js'), rather than reading global args inside getFileExtension(). The same helper is also used by getModelPath() (path-helper.js#L76) and init (init-helper.js#L58), and the generated models/index.js only loads .js files (models/index.js#L25), so a global setting would silently break model loading.
• Template content stays CommonJS, since every accepted extension is loaded through require().

Related

• support a migration with a cjs extension #905: added support for running .cjs migrations (this is the generation counterpart)
• When on typescript project, sequelize-cli still generates javascript files. #1436: same root cause (hardcoded extension in getFileExtension), asking for .ts generation. The proposed option would cover that request too.
• models/index.js doesn't work with ES6 modules #960: generated files not working in ESM projects; option to choose CJS vs ESM
• feat: add support for ESM config files #987 / v6: Improve support for ESM & CJS config files #990: ESM/CJS support for config and .sequelizerc

Dialect: any
Database version: N/A
Sequelize CLI version: 6.6.5
Sequelize version: 6.37.7
Node version: 22.x

Would you resolve this issue by submitting a Pull Request?

• Yes, I have the time and I know how to start.