How can I use environment variable as compile-time constant in native nodejs addon?

Question

I am writing a native addon for NodeJS. How can I use an environment variable as a constant at compile time? That is, "inject" a constant in to the NodeJS C++ addon from an environment variable set during node-gyp build or npm install. I found this answer, however as far as I can see, there is no equivalent option for passing through variables to node-gyp

Answer 1

I found that the defines block and variable expansion in binding.gyp will achieve what I'm after:

{
  "targets": [
    {
      "target_name": "targetName",
      "sources": [ "source.cc" ],
      "defines": [
        'MY_DEFINE="<!(echo $MY_ENV_VAR)"'
      ]
    }
  ]
}

Then MY_DEFINE is available with value equal to whatever MY_ENV_VAR set set to at compile time.