smallworld.js is a translation of Timothy Budd's SmallWorld into JavaScript. SmallWorld is a Java implementation of a very basic Smalltalk virtual machine. Thus, smallworld.js is a Smalltalk virtual machine written in JavaScript.

This was a hand translation of Java to JavaScript, and accepts the same bytecode format as the one I used in my SmallWorld fork. Similar to the Java version, every Smalltalk object corresponds to a JavaScript object. Thus, the JavaScript runtime does all the memory management and garbage collection. This isn't a particularly fast or elegant Smalltalk VM, but it is very simple, and relies on the fact that modern systems are very fast, and JavaScript runtimes are quite good, so actually using the system has reasonable performance.

The current implementation is extremely minimalist. The GUI is rough and incomplete. However, what is here is capable of running a fairly large amount of Smalltalk code correctly, including basic arbitrary precision arithmetic, string manipulation, and adding new methods to existing classes. See the tests in interpreter.js (and related) for more details.

The main interpreter can be used "headless" - you can run basic Smalltalk code without a GUI. You can optionally add a UI Handler (which adds the UI primitives) but the base system doesn't depend on it. The UI Handler itself doesn't do much either, it depends on a UI implementation (called the UI factory) which presents the Smalltalk UI concepts in a browser using the HTML DOM APIs. These are all included here, but since the interpreter doesn't depend directly on any UI, it could be used for embedding in other systems.

After cloning, use npm install to install the dependencies (only needed for running unit tests). Running npm test will run the test suite in test.js.

There is a simple REPL in the scripts directory that you can run from the command line with node repl.js or ./repl.js. Lines on the command line will be compiled and run with doIt.

You can run a very rudimentary version of the interpeter by loading index.html in a Web browser. You can evaluate expressions and the result will be printed below.

I have always been fascinated by Smalltalk, especially the programming paradigm of using a virtual image (snapshot of all of the objects in a system). This kind of Smalltalk provides a level of system introspection and manipulation that is rare in other programming systems. Budd's Little Smalltalk and SmallWorld are especially interesting Smalltalk variants, because a very few Smalltalk classes and a relatively small virtual machine can accomplish a great deal. The base image is made from 41 classes. The whole environment (full source code, bytecode, programming tools, and so on) is made from 4924 objects, taking up less than 170K of disk space (and considerably less with some deduplication).

I've written many interpreters and compilers over the years, and I even took Tim Budd's original SmallWorld and cleaned up the code a bit. However, I never really understood his Java code, or deeply how a Smalltalk virtual machine works and how the illusion of "everything is an object" can be maintained.

Porting from Java to JavaScript has therefore been a personal journey of discovery to understand the inner workings of the virtual machine. My rough notes along the way are in NOTES.md.

Here is the HTML UI showing off SmallWorld's integration with the HTML Canvas API: