Snowpack implements HMR via the esm-hmr spec, an attempted standard for ESM-based Hot Module Replacement (HMR).

// HMR Code Snippet Example
if (undefined /* [snowpack] import.meta.hot */ ) {
  undefined /* [snowpack] import.meta.hot */ .accept(({module}) => {
    // Accept the module, apply it into your application.
  });
}

Full API Reference: snowpack/esm-hmr on GitHub

Learn more about HMR, Fast Refresh, and how it’s meant to work in Snowpack.