SpiderMonkey is Mozilla's JavaScript engine written in C and C++. The Debugger server uses its API to do everything from setting breakpoints to receiving pause events.

It's fairly easy to install js shell once you have the appropriate dependencies. The MDN docs have a detailed installation guide.

Quick Version:

cd <gecko>/js/src
autoconf-2.13

# This name should end with "_OPT.OBJ" to make the version control system ignore it.
mkdir build_OPT.OBJ
cd build_OPT.OBJ
../configure --enable-debug --disable-optimize
# Use "mozmake" on Windows
make


# Run the shell
./js/src/js

One way to speed up the builds is to add mk_add_options MOZ_MAKE_FLAGS="-j6 -s" to your mozconfig file.

NOTE: First you have to build the JS Shell

diff --git a/js/src/jit-test/tests/debug/wasm-11.js b/js/src/jit-test/tests/debug/wasm-11.js
index 6c84c1c..1da17f2 100644
--- a/js/src/jit-test/tests/debug/wasm-11.js
+++ b/js/src/jit-test/tests/debug/wasm-11.js
@@ -11,6 +11,8 @@ g.eval(`
     var dbg = new Debugger(parent);
 `);

+console.log("YO")
+
 var i = new WebAssembly.Instance(new WebAssembly.Module(wasmTextToBinary(`
     (module
         (func (export "f2")

cd <gecko>/js/src/jit-test
./jit_test.py --no-progress -o ../build_DBG.OBJ/dist/bin/js wasm-11

Exit code: 0
YO
Exit code: 0
PASSED ALL