Building SpiderMonkey

Before you begin, make sure you have the right build tools for your computer:

This guide shows you how to build SpiderMonkey using mach, which is Mozilla’s multipurpose build tool. For builds using configure && make, and translations into other languages see these instructions on MDN.

These instructions assume you have a clone of mozilla-central and are interested in building the JS shell.

Developer (debug) build

For developing and debugging SpiderMonkey itself, it is best to have both a debug build (for everyday debugging) and an optimized build (for performance testing), in separate build directories. We’ll start by covering how to create a debug build.

Setting up a MOZCONFIG

First, we will create a MOZCONFIG file. This file describes the characteristics of the build you’d like mach to create. Since it is likely you will have a couple of MOZCONFIGs, a directory like $HOME/mozconfigs is a useful thing to have.

A basic MOZCONFIG file for doing a debug build, put into $HOME/mozconfigs/debug looks like this

# Build only the JS shell
ac_add_options --enable-application=js

# Disable Optimization, for the most accurate debugging experience
ac_add_options --disable-optimize
# Enable the debugging tools: Assertions, debug only code etc.
ac_add_options --enable-debug

To activate a particular MOZCONFIG, set the environment variable:

export MOZCONFIG=$HOME/mozconfigs/debug


Once you have activated a MOZCONFIG by setting the environment variable you can then ask mach, located in the top directory of your checkout, to do your build:

$ cd <path to mozilla-central>
$ ./mach build


Note: If you are on Mac and baldrdash fails to compile with something similar to

/usr/local/Cellar/llvm/7.0.1/lib/clang/7.0.1/include/inttypes.h:30:15: fatal error: 'inttypes.h' file not found

This is because, starting from Mojave, headers are no longer installed in /usr/include. Refer the release notes  under Command Line Tools -> New Features

The release notes also states that this compatibility package will no longer be provided in the near future, so the build system on macOS will have to be adapted to look for headers in the SDK

Until then, the following should help,

open /Library/Developer/CommandLineTools/Packages/

Once you have successfully built the shell, you can run it using mach run.


Once built, you can then use mach to run the jit-tests:

$ ./mach jit-test

Optimized Builds

To switch to an optimized build, one need only have an optimized build MOZCONFIG, and then activate it. An example $HOME/mozconfigs/optimized MOZCONFIG looks like this:

# Build only the JS shell
ac_add_options --enable-application=js

# Enable optimization for speed
ac_add_options --enable-optimize
# Enable the debugging tools: Assertions, debug only code etc.
# For performance testing you would probably want to change this
# to --disable-debug.
ac_add_options --enable-debug

# Use a separate objdir for optimized builds to allow easy
# switching between optimized and debug builds while developing.
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-opt-@CONFIG_GUESS@


It is possible to cross-compile a SpiderMonkey shell binary for another architecture. For example, one can develop and compile on an x86 host while building a js binary for AArch64 (ARM64).

Unlike the rest of this document, this section will use the old-style configure script.

To do this, first you must install the appropriate cross-compiler and system libraries for the desired target. This is system- and distribution-specific. Look for a package such as (using AArch64 as an example) aarch64-linux-gnu-gcc. This document will assume that you have the appropriate compiler and libraries; you can test this by compiling a C or C++ hello-world program.

You will also need the appropriate Rust compiler target support installed. For example:

$ rustup target add aarch64-unknown-linux-gnu

Once you have these prerequisites installed, you simply need to set a few environment variables and configure the build appropriately:

$ cd js/src/
$ export CC=aarch64-linux-gnu-gcc  # adjust for target as appropriate.
$ export CXX=aarch64-linux-gnu-g++
$ export AR=aarch64-linux-gnu-ar
$ export BINDGEN_CFLAGS="--sysroot /usr/aarch64-linux-gnu/sys-root"
$ ../configure --target=aarch64-unknown-linux-gnu
$ make

This will produce a binary that is appropriate for the target architecture. Note that you will not be able to run this binary natively on your host system; to do so, keep reading to set up Qemu-based user-space emulation.

Cross-Architecture Testing using Qemu

It is sometimes desirable to test a cross-compiled binary directly. Unlike the target-ISA emulators that SpiderMonkey also supports, testing a cross-compiled binary ensures that the actual binary, running as it would on the target system, works appropriately. As far as the JS shell is concerned, it is running on the target ISA.

This is possible using the Qemu emulator. Qemu supports a mode called “user-space emulation”, where an individual process executes a binary that targets a non-native ISA, and system calls are translated as appropriate to the host system. This allows transparent execution of cross-compiled binaries.

To set this up, you will need Qemu (check your system package manager) and shared libraries for the target system. You will likely have the necessary shared libraries already if you cross-compiled as described above.

Then, write a small wrapper script that invokes the JS shell under Qemu. For example:


# This is the binary compiled in the previous section.
CROSS_BIN=`dirname $0`/BUILD_AARCH64.OBJ/dist/bin/js

# Adjust the library path as needed; this is prefixed to paths such as
# `/lib64/`, and so should contain `lib` (and perhaps `lib64`)
# subdirectories.
exec qemu-aarch64 -L /usr/aarch64-linux-gnu/sys-root/ $CROSS_BIN "$@"

You can then invoke this wrapper as if it were a normal JS shell, and use it with to run tests:

$ jit-test/ ./js-cross-wrapper