Installation | MCMC.js

MCMC.js ships as the mcmcjs npm package, which installs the mcmc binary. Inference runs in a backend runtime (Julia, or CmdStan for Stan), which the CLI can install for you.

1. Install the CLI

You need Node.js 22 or newer.

npm install -g mcmcjs

This puts the mcmc binary on your PATH. Check it:

mcmc --version

2. Provision Julia

Inference needs Julia and a managed set of packages (Turing.jl, JuliaBUGS, and friends). mcmc setup installs the toolchain through juliaup and precompiles the managed project, so you do not configure Julia by hand.

mcmc setup

Pass --dry-run to see what would be installed without making changes, or --verbose to see the full install and precompile output instead of a collapsed spinner.

The first setup downloads and precompiles a Julia project, which takes a few minutes. After that, fits start fast.

3. Verify

mcmc doctor reports every engine’s toolchain and tells you whether you are ready to fit.

mcmc doctor

Julia
juliaup  1.20.7  /home/you/.juliaup/bin/juliaup
julia    1.12.6  /home/you/.juliaup/bin/julia
ready for inference

Stan (CmdStan)
cmdstan  2.39.0  /home/you/.cmdstan/cmdstan-2.39.0
stanc    2.39.0  /home/you/.cmdstan/cmdstan-2.39.0/bin/stanc
make     4.4.1  make
c++      16.1.1  g++
ready for inference

The exit code is 0 as long as at least one engine is ready. Add --json for a machine-readable report, or --engine <id> to check one engine in the original flat format.

Stan (optional)

Stan models run through a local CmdStan, which mcmc setup can download and build for you.

mcmc setup --engine stan

Building CmdStan uses make and a C++ compiler (g++ or clang++) from your system; Linux and macOS are supported today. Pass --stan-version <v> to install a specific CmdStan release instead of the pinned default. Check readiness the same way as for Julia:

mcmc doctor --engine stan

If you already have CmdStan, the CLI also finds it through the MCMCJS_CMDSTAN or CMDSTAN environment variable, or at ~/.cmdstan, shared with other Stan interfaces.

Libraries

The CLI is the unscoped mcmcjs package. The internal libraries it is built from publish under the @mcmcjs/* scope (for example @mcmcjs/core and @mcmcjs/diagnostics). You do not need to install those directly to use the CLI; see Packages if you want to build on them.

Next steps

Fit and diagnose your first model in the Quickstart.