HMR + Fast Refresh

Hot Module Replacement (HMR) is the ability to push file updates to the browser without triggering a full page refresh. Imagine changing some CSS, hitting save, and then instantly seeing your change reflected on the page without a refresh. That’s HMR.

HMR is not unique to Snowpack. However, Snowpack’s ability to leverage ESM for unbundled development introduces near-instant single file builds that only take 10-25ms to load and update in the browser.

Snowpack ships with ready, out-of-the-box HMR support for the following file types:

  • CSS
  • CSS Modules
  • JSON

JavaScript HMR is also supported out-of-the-box, but often requires a few additional lines of code to properly integrate with your frontend framework’s “render” function. See “Enabling HMR + Fast Refresh” below.

Fast Refresh

In addition to normal HMR, Snowpack also supports Fast Refresh for most popular frameworks like React, Preact and Svelte. Fast Refresh is a framework-specific enhancement to HMR, which applies single file updates in a way that preserves component state across updates. Changes to a <Timer /> component, for example, would be applied without resetting the component’s internal state.

Fast Refresh makes development even faster, especially when working on popups and other secondary view states that normally would require a click to re-open or re-visit after every change.

Enabling HMR + Fast Refresh

Snowpack supports HMR for all popular frontend frameworks. Create Snowpack App (CSA) ships with HMR enabled by default. You can setup HMR yourself with just a few lines of code, and Fast Refresh can be enabled automatically via plugin:

For more advanced HMR integrations, Snowpack created the esm-hmr spec, a standard HMR API for any ESM-based dev environment:

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

Check out the full ESM-HMR API reference on GitHub.