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1.0.73 • Public • Published


Open-source implementations of the programming language Source. Source is a series of small subsets of JavaScript, designed for teaching university-level programming courses for computer science majors, following Structure and Interpretation of Computer Programs, JavaScript Adaptation (https://sourceacademy.org/sicpjs/).

Table of Contents



To build,

$ git clone --recurse-submodules https://github.com/source-academy/js-slang.git
$ cd js-slang
$ yarn
$ yarn build

This repository uses git submodules. To update existing repositories with a submodule,

# Init is only required on the very first time.
$ git submodule update --init --recursive
# Required subsequently every time you want to update the submodules.
$ git submodule update --recursive --remote

To add "js-slang" to your PATH, build it as per the above instructions, then run

$ cd dist
$ npm link

If you do not wish to add "js-slang" to your PATH, replace "js-slang" with "node dist/repl/repl.js" in the following examples.

To try out Source in a REPL, run

$ js-slang -c [chapter] # default: 1

You can set additional options:

Usage: js-slang [PROGRAM_STRING] [OPTION]

  -c, --chapter=CHAPTER set the Source chapter number (i.e., 1-4)                                                              (default: 1)
  -v, --variant=VARIANT set the Source variant (i.e., default, interpreter, substituter, typed, lazy, non-det, concurrent, wasm, gpu) (default: default)
  -h, --help            display this help
  -e, --eval            don't show REPL, only display output of evaluation

Currently, valid CHAPTER/VARIANT combinations are:

  • --chapter=1 --variant=default
  • --chapter=1 --variant=wasm
  • --chapter=1 --variant=lazy
  • --chapter=1 --variant=substituter
  • --chapter=1 --variant=interpreter
  • --chapter=1 --variant=typed
  • --chapter=2 --variant=default
  • --chapter=2 --variant=lazy
  • --chapter=2 --variant=substituter
  • --chapter=2 --variant=interpreter
  • --chapter=2 --variant=typed
  • --chapter=3 --variant=default
  • --chapter=3 --variant=concurrent
  • --chapter=3 --variant=non-det
  • --chapter=3 --variant=interpreter
  • --chapter=3 --variant=typed
  • --chapter=4 --variant=default
  • --chapter=4 --variant=gpu
  • --chapter=4 --variant=interpreter

Hint: In bash you can take the PROGRAM_STRING out of a file as follows:

$ js-slang -n --chapter=1 -e "$(< my_source_program.js)"


Source is documented here: https://docs.sourceacademy.org/


  • bash: known working version: GNU bash, version 5.0.16
  • latexmk: Version 4.52c
  • pdflatex: known working versions
    • pdfTeX 3.14159265-2.6-1.40.18 (TeX Live 2017)

To build the documentation, run

$ git clone https://github.com/source-academy/js-slang.git
$ cd js-slang
$ yarn
$ yarn install
$ yarn jsdoc  # to make the web pages in js-slang/docs/source
$ cd docs/specs
$ make        # to make the PDF documents using LaTeX

Note: The documentation may not build on Windows, depending on your bash setup, see above.

Documentation on the Source libraries are generated from inline documentation in the library sources, a copy of which are kept in docs/lib/*.js. The command yarn jsdoc generates the documentation and places it in the folder docs/source. You can test the documentation using a local server:

$ cd docs/source;  python -m http.server 8000

Documentation of libraries is displayed in autocomplete in the frontend. This documentation is generated by ./scripts/updateAutocompleteDocs.py and placed in src/editors/ace/docTooltip/*.json files. This script is run by yarn buildprior totsc. To add a Source variant to the frontend autocomplete, edit src/editors/ace/docTooltip/index.ts and./scripts/updateAutocompleteDocs.py.


js-slang comes with an extensive test suite. To run the tests after you made your modifications, run yarn test. Regression tests are run automatically when you want to push changes to this repository. The regression tests are generated using jest and stored as snapshots in src/\_\_tests\_\_. After modifying js-slang, carefully inspect any failing regression tests reported in red in the command line. If you are convinced that the regression tests and not your changes are at fault, you can update the regression tests as follows:

$ yarn test -- --updateSnapshot

Error messages

To enable verbose messages, have the statement "enable verbose"; as the first line of your program. This also causes the program to be run by the interpreter.

There are two main kinds of error messages: those that occur at runtime and those that occur at parse time. The first can be found in interpreter-errors.ts, while the second can be found in rules/.

Each error subclass will have explain() and elaborate(). Displaying the error will always cause the first to be called; the second is only called when verbose mode is enabled. As such, explain() should be made to return a string containing the most basic information about what the error entails. Any additional details about the error message, including specifics and correction guides, should be left to elaborate().

Please remember to write test cases to reflect your added functionalities. The god of this repository is self-professed to be very particular about test cases.

Using your js-slang in Source Academy

js-slang is used by the Source Academy, the immersive online experiential environment for learning programming. For this, js-slang is deployed as an NPM package. The frontend of the Source Academy then includes the js-slang package in its deployment bundle.

Using your js-slang in your local Source Academy

A common issue when developing modifications to js-slang is how to test it using your own local frontend. Assume that you have built your own frontend locally, here is how you can make it use your own js-slang, instead of the one that the Source Academy team has deployed to npm.

First, build and link your local js-slang:

$ cd js-slang
$ yarn build
$ yarn link

Then, from your local copy of frontend:

$ cd frontend
$ yarn link "js-slang"

Then start the frontend and the new js-slang will be used.

Building and publishing SICP package

To build SICP package

$ cd js-slang
$ yarn
$ yarn build_sicp_package

To publish SICP package, update version number in sicp_publish/package.json

$ cd js-slang/sicp_publish
$ npm publish

Talks and Presentations

  • How js-slang works under the hood (17th Jan 2023 – The Gathering) (slides)



All sources in this repository are licensed under the Apache License Version 2.

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