vnc: 0.137 socket: 0.130 network: 0.125 other: 0.103 device: 0.102 semantic: 0.063 debug: 0.062 PID: 0.053 performance: 0.052 files: 0.049 permissions: 0.045 graphic: 0.038 boot: 0.021 KVM: 0.020 debug: 0.563 vnc: 0.067 files: 0.064 network: 0.047 socket: 0.044 other: 0.040 PID: 0.039 semantic: 0.035 boot: 0.027 performance: 0.023 KVM: 0.014 device: 0.014 graphic: 0.012 permissions: 0.011 tcg bug master / 4.0.0 v8 operation >>> and |= vm guest is linux, executed with tcg running this Node.js snippet leads to $ node > a = undefined undefined > a >>> 0 4294967295 host node $ node > a = undefined undefined > a >>> 0 0 same with |= node Welcome to Node.js v12.4.0. Type ".help" for more information. > let buffer undefined > buffer |= 0 0 vm with tcg: $ ./out/Release/node --version v12.4.0 ./out/Release/node -e "let buffer; buffer |= 0; console.log(buffer);" -1 vm guest is debian x86_64 latest release vm guest is started with ./x86_64-softmmu/qemu-system-x86_64 -vnc :0 -cdrom debian-9.9.0-amd64-netinst.iso -m 4G -smp cores=6,threads=1,sockets=1 -nic user,hostfwd=tcp:ipv4addr:2233-:22 -cpu qemu64 debian.img git tag v4.0.0 and master, commit a578cdfbdd8f9beff5ced52b7826ddb1669abbbf, for building qemu-system-x86_64 was used. Node.js as compiled on the vm guest (v12.4.0 / master) see also https://github.com/nodejs/node/issues/19348#issuecomment-500465502 I need further assistance to track down the cause of the bug. Kind regards Manuel This might be the same underlying problem as LP:1815423 which also mentions some issues with Javascript calculations involving arithmetic operations on a js "undefined" value. That bug has a C-only reproduce case so is probably a good place to start for anybody interesting in investigating and fixing it. https://