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About Jess

// I am a Jess file

@-use '#less'; // Use built-in modules!
@-use './sizes.ts'; // Use a custom module!
@-compose './theme.less'; // Compose a theme!
@-compose './mixins.scss' as *; // Import mixins from Sass into local mixin scope!

.container {
  width: $less.unit($sizes.width);
  color: $theme.primary-color;
  display: flex;
  $box();
}

What is Jess?​

Jess is a stylesheet language + compiler designed for teams that want modern CSS ergonomics without giving up compile-time power.

In practice, it can cover most day-to-day use cases people typically reach for:

  • Sass
  • Less (Node.js and in-browser compilation)
  • the Tailwind compiler (especially when you'd like the utility speed without turning every class attribute into a novella)
  • CSS-in-JS
  • CSS modules

Jess is both a compiler and a language. It started as a re-write of Less from the ground up, then kept evolving into its own language model. That means you can migrate .scss and .less incrementally to .jess while still using cross-language composition when needed.

Speed and build ergonomics​

Jess is built to keep build-time CSS workflows fast and predictable:

  • static CSS output by default
  • patch-friendly output paths when you need dynamic behavior
  • language-service tooling that targets real editor workflows (including Cursor/VSCode)

How is Jess different from CSS-in-JS?​

CSS-in-JS, for a long time, was thought to be the only way to produce "dynamic" styles, but it comes with trade-offs. Jess has these advantages over CSS-in-JS:

  1. You can keep stylesheet authoring in stylesheet files instead of moving all styling into JS runtime code.
  2. Many CSS-in-JS libraries don't produce static CSS at build-time (or take some effort to do so). Not only can Jess produce static CSS, but it can produce "patch-able" CSS, along with a module that can patch your CSS at any time. It's kinda magic.
  3. Many CSS-in-JS libraries give you dynamic styles at the cost of performance. Jess focuses on making CSS updates fast with minimal overhead.

Where to next​

Why am I not already using Jess?​

Honestly? Usually one of three reasons:

  1. You already have a working Sass/Less setup and migration takes planning.
  2. You need one or two ecosystem integrations that are still maturing.
  3. You have not looked at Jess in a while and assumed it was still in "just an experiment" mode.

If that sounds familiar, start small: convert one folder, wire editor support, and keep shipping.