Kiesel

⚡ A JavaScript engine written in Zig

Introduction

Kiesel (/ˈkiːzəl/) is a JavaScript engine written from scratch for the purpose of me learning the Zig programming language, with the eventual goal of full support for the latest ECMAScript standard. It features a custom bytecode VM and runtime, and aims to implement as much as possible in pure Zig - currently with the following exceptions:

• bdwgc for garbage collected allocations
• libregexp for regular expressions
• ICU4X for Intl-related functionality

A simple CLI (REPL/file interpreter) is available but being easy to embed in other projects is also a goal.

The implementation style is similar to LibJS - the JS engine used in SerenityOS and the Ladybird browser which I worked on before - in various aspects, most notably the desire to stay very close to spec. Development is still at an early stage and the engine's usefulness thus limited. While there are various things that already work well, there's an equal amount of things that don't :^)

Please refrain from filing issues for missing features - many are missing! Similarly, please get in touch before starting to work on something larger than a bug fix.

Further reading:

• Kiesel Devlog #1: Now passing 25% of test262!
• Kiesel Devlog #2: Iterators, more math, and a bug in the Zig stdlib
• Kiesel Devlog #3: Accessors, Date/Map/Set, test262 history graph
• Kiesel Devlog #4: The biggest update yet!
• Kiesel Devlog #5: Progress powered by the Shadow web engine
• Kiesel Devlog #6: Catching Up :^)
• Kiesel Devlog #7: Happy Birthday!

Build

Dependencies managed through build.zig.zon will be installed automatically when building for the first time.

To build and run the Kiesel CLI:

zig build run

NOTE: Kiesel targets the master branch of Zig. This means that the build breaks semi-regularly, which I will usually fix as soon as I notice.

Build Options

These can be set by passing -D<name>=<value> to zig build.

Name	Default	Description
enable-annex-b	true	Enables support for ECMA-262 Annex B language features.
enable-intl	true	Enables support for ECMA-402 (Intl), which depends on cargo being available for the ICU4X library.
enable-legacy	true	Enables support for legacy language features

Usage

Usage: kiesel [options] [file]

Options:
  -c, --command                            Run the given code instead of reading from a file
      --disable-gc                         Disable garbage collection
  -m, --module                             Run code as a module instead of a script
      --print-ast                          Print the parsed AST
      --print-bytecode                     Print the generated bytecode
      --print-gc-warnings                  Print GC warnings, e.g. OOM
      --print-promise-rejection-warnings   Print promise rejection warnings
  -p, --print-result                       Print the evaluated result
  -v, --version                            Print version information and exit
  -h, --help                               Print help text and exit

Links

• Website
• Wasm-based online playground
• Builds for the main branch
• test262 results