npm package.json OS specific script

I would like to create a package.json build script that executes slightly different set of commands when run from Windows, Linux, Mac.

The problem is that I cannot find a way to put it in package.json file that will run without problems at every system.

Here is an example that I would like to have:

"scripts" : {
    "build.windows" : "echo do windows specific stuff",
    "build.linux" : "echo do linux specific stuff",
    "build.mac" : "echo do mac specific stuff",
    "build" : "??????????????" <- what to put here to execute script designed for OS
                                  on which npm is running
}

from npm package.json OS specific script