System calls are the bridge between your application and your operating system. They are used whenever you access something outside of your application's memory, for example when you write to the console, when you read or write files or when you access the network. In Go, system calls are mostly used by the os package, hence the name. When using GopherJS you need to consider if system calls are available or not.
If system calls are not available in your environment (see below), then a special redirection of os.Stdout and os.Stderr is applied. It buffers a line until it is terminated by a line break and then prints it via JavaScript's console.log to your browser's JavaScript console or your system console. That way, fmt.Println etc. work as expected, even if system calls are not available.
The JavaScript environment of a web browser is completely isolated from your operating system to protect your machine. You don't want any web page to read or write files on your disk without your consent. That is why system calls are not and will never be available when running your code in a web browser.
GopherJS has support for system calls on Linux and macOS. Before running your code with Node.js, you need to install the system calls module. The module is compatible with Node.js versions from 10.0.0 to 11.15.0 (Node.js version 12 is not yet supported). If you want to use an older version you can opt to not install the module, but then system calls are not available.
Compile and install the module with:
# Install node-gyp globally
npm install --global node-gyp
# Change directory into gopherjs/node-syscall
cd $GOPATH/src/github.com/gopherjs/gopherjs/node-syscall/
# Rebuild using a specific node version
# Use can install a specific node version using https://github.com/tj/n
# To install Node.js version 11.15.0 use:
# n v11.15.0
node-gyp --target=v11.15.0 rebuild
# Create a Node Libraries folder and copy the artifact over
mkdir -p ~/.node_libraries/
cp build/Release/syscall.node ~/.node_libraries/syscall.node
When running your code with Node.js on Windows, it is theoretically possible to use system calls. To do so, you would need a special Node.js module that provides direct access to system calls. However, since the interface is quite different from the one used on Linux and macOS, the system calls module included in GopherJS currently does not support Windows. Sorry. Get in contact if you feel like you want to change this situation.
Note that even with syscalls enabled in Node.js, some programs may not behave as expected due to the fact that the current implementation blocks other goroutines during a syscall, which can lead to a deadlock in some situations. This is not considered a bug, as it is considered sufficient for most test cases (which is all Node.js should be used for). Get in contact if you feel like you want to change this situation.