rspack

The rspack package hooks into the Meteor lifecycle to run the rspack bundler independently, compiling app code while preserving Meteor packages as external. It automatically integrates the rspack dev server and HMR mechanism, and manages client and server bundles for development and production. By default, rspack is configured to support secured code for client and server, tree shaking, full ESM support with export fields in package.json, and so on. It also enables the user to provide custom configuration.

Legacy and architecture-specific entry points

Starting with Meteor 3.6, explicit meteor.mainModule entries for modern, legacy (or web.browser.legacy), web.browser, and web.cordova are compiled separately through the same Rspack configuration. Aliases, loaders, and plugins apply to those entries too. Legacy compilation defaults to ES5 for both application transforms and the Rspack runtime, including Cordova when modern.cordova is false. Meteor.arch and Meteor.isLegacy are available in the Rspack configuration callback for architecture-specific adjustments. Explicit false disables that entry, while omitted entries retain the existing client fallback.

Select the legacy source in package.json, rather than overriding Rspack's entry option:

1{
2  "meteor": {
3    "mainModule": {
4      "client": "client/main.js",
5      "legacy": "client/legacy.js",
6      "server": "server/main.js"
7    },
8    "modern": { "webArchOnly": false }
9  }
10}

The existing defineConfig(Meteor => ...) callback runs for each compilation. Use Meteor.isLegacy to select legacy-specific settings in that callback; there is no automatically loaded rspack.legacy.config.js. With only mainModule.client, the shared-client behavior is preserved, so declare legacy explicitly when it needs separate configuration. A testModule.legacy entry selects separate tests.

Each explicit architecture has its own generated modules, cache, chunks, and assets. In development, those builds use Rspack watch and Meteor reload; the default client keeps Rspack HMR. The normal architecture inclusion settings still apply, so enable legacy with modern.webArchOnly: false when testing it in dev.

Dependencies excluded from transpilation and custom compiler rules remain the application's responsibility. Use the existing compileWithRspack helper for npm dependencies that need transpilation for the legacy target.

See the legacy workflow guide for conditional configuration, development, builds, and browser testing. ES5 output still requires appropriate runtime APIs and compatible dependencies in the browsers your app supports.