Janet 1.16.1-87f8fe1 Documentation
(Other Versions: 1.15.0 1.13.1 1.12.2 1.11.1 1.10.1 1.9.1 1.8.1 1.7.0 1.6.0 1.5.1 1.5.0 1.4.0 1.3.1 )


Most default installs of Janet come with a build tool, jpm, that makes managing dependencies, building Janet code, and distributing Janet applications easy. While jpm is not required by Janet, it is a very handy build tool that removes much of the boilerplate for creating new Janet projects, and is updated in step with Janet. jpm also works on Windows, Linux, and macOS so you don't need to write multiple build scripts and worry about platform quirks to make your scripts, binaries, and applications cross-platform.

jpm's main functions are installing dependencies and building native Janet modules, but it is meant to be used for much of the life-cycle for Janet projects. Since Janet code doesn't usually need to be compiled, you don't always need jpm, especially for scripts, but jpm comes with some functionality that is difficult to duplicate, like compiling Janet source code and all imported modules into a statically linked executable for distribution.


A self contained unit of Janet source code as recognized by jpm is called a project. A project is a directory containing a project.janet file, which contains build recipes. Often, a project will correspond to a single git repository, and contain a single library. However, a project's project.janet file can contain build recipes for as many libraries, native extensions, and executables as it wants. Each of these recipes builds an artifact. Artifacts are the output files that will either be distributed or installed on the end-user or developer's machine.

Building projects with jpm

Once you have the project on your machine, building the various artifacts should be pretty simple.

Global install

sudo jpm deps
jpm build
jpm test
sudo jpm install

(On Windows, sudo is not required. Use of sudo on POSIX systems depends on whether you installed janet to a directory owned by the root user.)

Local install

export JANET_PATH=/home/me/janet
jpm deps
jpm build
jpm test
jpm install


jpm deps is a command that installs Janet libraries that the project depends on recursively. It will automatically fetch, build, and install all required dependencies for you. As of August 2019, this only works with git, which you need to have installed on your machine to install dependencies. If you don't have git you are free to manually obtain the requisite dependencies and install them manually with sudo jpm install.


Next, we use the jpm build command to build artifacts. All built artifacts will be created in the build subdirectory of the current project. Therefore, it is probably a good idea to exclude the build directory from source control. For building executables and native modules, you will need to have a C compiler on your PATH where you run jpm build. For POSIX systems, the compiler is cc.

If you make changes to the source code after building once, jpm will try to only rebuild what is needed on a rebuild. If this fails for any reason, you can delete the entire build directory with jpm clean to reset things.


For Windows, the C compiler used by jpm is cl.exe, which is part of MSVC. You can get it with Visual Studio, or standalone with the C and C++ Build Tools from Microsoft. You will then need to run jpm build in a Developer Command Prompt, or source vcvars64.bat in your shell to add cl.exe to the PATH.


Once we have built our software, it is a good idea to test it to verify that it works on the current machine. jpm test will run all Janet scripts in the test directory of the project and return a non-zero exit code if any fail.


Finally, once we have built our software and tested that it works, we can install it on our system. For an executable, this means copying it to the bin directory, and for libraries it means copying them to the global syspath. You can optionally install into any directory if you don't want to pollute your system or you don't have permission to write to the directory where janet itself was installed. You can specify the path to install modules to via the --modpath option, and the path to install binaries to with the --binpath option. These need to be given before the subcommand install.

The project.janet file

To create your own software in Janet, it is a good idea to understand what the project.janet file is and how it defines rules for building, testing, and installing software. The code in project.janet is normal Janet source code that is run in a special environment.

A project.janet file is loaded by jpm and evaluated to create various recipes, or rules. For example, declare-project creates several rules, including "install", "build", "clean", and "test". These are a few of the rules that jpm expects project.janet to create when executed.

Declaring a project

Use the declare-project as the first declare- macro towards the beginning of your project.janet file. You can also pass in any metadata about your project that you want, and add dependencies on other Janet projects here.

  :name "mylib" # required
  :description "a library that does things" # some example metadata.

  # Optional urls to git repositories that contain required artifacts.
  :dependencies ["https://github.com/janet-lang/json.git"])

Creating a module

A 100% Janet library is the easiest kind of software to distribute in Janet. Since it does not need to be built and since installing it means simply moving the files to a system directory, we only need to specify the files that comprise the library in project.janet.

  # :source is an array or tuple that can contain
  # source files and directories that will be installed.
  # Often will just be a single file or single directory.
  :source ["mylib.janet"])

For information on writing modules, see the modules docpage.

Creating a native module

Once you have written your C code that defines your native module (see the embedding page on how to do this), you must declare it in project.janet in order for jpm to build the native modules for you.

 :name "mynative"
 :source ["mynative.c" "mysupport.c"]
 :embedded ["extra-functions.janet"])

This makes jpm create a native module called mynative when jpm build is run, the arguments for which should be pretty straightforward. The :embedded argument is Janet source code that will be embedded as an array of bytes directly into the C source code. It is not recommended to use the :embedded argument, as one can simply create multiple artifacts, one for a pure C native module and one for Janet source code.

Creating an executable

The declaration for an executable file is pretty simple.

 :name "myexec"
 :entry "main.janet")

jpm is smart enough to figure out from the one entry file what libraries and other code your executable depends on, and bundles them into the final application for you. The final executable will be located at build/myexec, or build\myexec.exe on Windows.

Also note that the entry of an executable file should look different than a normal Janet script. It should define a main function that can receive a variable number of parameters, the command-line arguments. It will be called as the entry point to your executable.

(import mylib1)
(import mylib2)

# This will be printed when you run `jpm build`
(print "build time!")

(defn main [& args]
    # You can also get command-line arguments through (dyn :args)
    (print "args: " ;(interpose ", " args))

It's important to remember that code at the top level will run when you invoke jpm build, not at executable runtime. This is because in order to create the executable, we marshal the main function of the app and write it to an image. In order to create the main function, we need to actually compile and run everything that it references, in the above case mylib1 and mylib2.

This has a number of benefits, but the largest is that we only include bytecode for the functions that our application uses. If we only use one function from a library of 1000 functions, our final executable will not include the bytecode for the other 999 functions (unless our one function references some of those other functions, of course). This feature, called tree-shaking, only works for Janet code. Native modules will be linked to the final executable statically in full if they are used at all. A native module is considered "used" if is imported at any time during jpm build. This may change, but it is currently the most reliable way to check if a native modules needs to be linked into the final executable.

There are some limitations to this approach. Any dynamically required modules will not always be linked into the final executable. If require or import is not called during jpm build, then the code will not be linked into the executable. The module can still be required if it is available at runtime, though.

For an example Janet executable built with jpm, see https://github.com/bakpakin/littleserver.