Introduction to JS

JS (also referred as SpiderMonkey) is Mozilla's JavaScript and WebAssembly Engine, written in C++ and Rust. In BLFS, the source code of JS is taken from Firefox.

This package is known to build and work properly using an LFS-11.2 platform.

Package Information

JS91 Dependencies


ICU-71.1, rustc-1.60.0, and Which-2.21


User Notes: https://wiki.linuxfromscratch.org/blfs/wiki/js91

Installation of JS



Unlike most other packages in BLFS, the instructions below require you to untar firefox-91.12.0esr.tar.xz and change into the firefox-91.12.0 folder.

Extracting the tarball will reset the permissions of the current directory to 0755 if you have permission to do that. If you do this in a directory where the sticky bit is set, such as /tmp it will end with error messages:

tar: .: Cannot utime: Operation not permitted
tar: .: Cannot change mode to rwxr-xr-t: Operation not permitted
tar: Exiting with failure status due to previous errors

This does finish with non-zero status, but it does NOT mean there is a real problem. Do not untar as the root user in a directory where the sticky bit is set - that will unset it.

Install JS by running the following commands:



If you are compiling this package in chroot you must do two things. First, as the root user, ensure that /dev/shm is mounted. If you do not do this, the Python configuration will fail with a traceback report referencing /usr/lib/pythonN.N/multiprocessing/synchronize.py. Run:

mountpoint -q /dev/shm || mount -t tmpfs devshm /dev/shm

Second, either as the root user export the $SHELL environment variable using export SHELL=/bin/sh or else prepend SHELL=/bin/sh when running the configure command.

Compiling the C++ code respects $MAKEFLAGS and defaults to 'j1', the rust code will use all processors.

mkdir obj &&
cd    obj &&

sh ../js/src/configure.in --prefix=/usr            \
                          --with-intl-api          \
                          --with-system-zlib       \
                          --with-system-icu        \
                          --disable-jemalloc       \
                          --disable-debug-symbols  \
                          --enable-readline        &&

To run the JS test suite, issue: make -C js/src check-jstests JSTESTS_EXTRA_ARGS="--timeout 300 --wpt=disabled". It's recommended to redirect the output into a log. Because we are building with system ICU, more than one hundred tests (out of a total of more than 40,000) are known to fail.

To run the JIT test suite, issue: make -C js/src check-jit-test JITTEST_EXTRA_ARGS="--timeout 300".



An issue in the installation process causes any running program which links to JS91 shared library (for example, GNOME Shell) to crash if JS91 is upgraded or reinstalled. To work around this issue, remove the old version of the JS91 shared library before installation:

rm -fv /usr/lib/libmozjs-91.so

Now, as the root user:

make install &&
rm -v /usr/lib/libjs_static.ajs &&
sed -i '/@NSPR_CFLAGS@/d' /usr/bin/js91-config

Command Explanations

sh ../js/src/configure.in: configure.in is actually a shell script, but the executable bit is not set in its permission mode so it's needed to explicitly run it with sh.

--with-intl-api: This enables the internationalization functions required by Gjs.

--with-system-*: These parameters allow the build system to use system versions of the above libraries. These are required for stability.

--enable-readline: This switch enables Readline support in the JS shell.

--disable-jemalloc: This switch disables the internal memory allocator used in JS91. jemalloc is only intended for the Firefox browser environment. For other applications using JS91, if JS91 uses jemalloc, the application may crash as items allocated in jemalloc allocator are freed on system (glibc) allocator.

--disable-debug-symbols: Don't generate debug symbols since they are very large and most users won't need it. Remove it if you want to debug JS91.

rm -v /usr/lib/libjs_static.ajs: Remove a large static library which is not used by any BLFS package.

sed -i '/@NSPR_CFLAGS@/d' /usr/bin/js91-config: Prevent js91-config from using buggy CFLAGS.

CC=gcc CXX=g++: BLFS used to prefer to use gcc and g++ instead of upstream's defaults of the clang programs. With the release of gcc-12 the build takes longer with gcc and g++, primarily because of extra warnings, and is bigger. Pass these environment variables to the configure script if you wish to continue to use gcc, g++ (by exporting them and unset them after the installation, or simply prepending them before the sh ../js/src/configure.in command). If you are building on a 32-bit system, also see below.

CXXFLAGS="-msse2 -mfpmath=sse": Use SSE2 instead of 387 for double-precision floating-point operations. It's needed by GCC to satisfy the expectations of upstream (Mozilla) developers with floating-point arithmatics. Use it if you are building this package on a 32-bit system with GCC (if Clang is not installed or GCC is explicitly specified). Note that this will cause JS to crash on a processor without SSE2 capability. If you are running the system on such an old processor, Clang is strictly needed. This setting is not needed on 64-bit systems because all 64-bit x86 processors support SSE2 and the 64-bit compilers (both Clang and GCC) use SSE2 by default.


Installed Programs: js91 and js91-config
Installed Libraries: libmozjs-91.so
Installed Directories: /usr/include/mozjs-91

Short Descriptions


provides a command line interface to the JavaScript engine


is used to find the JS compiler and linker flags


contains the Mozilla JavaScript API functions