do not understand how to pass arguments to node's child_process.spawn

ls-on-file-watch.js

"use strict";
const fs = require('fs'),
      spawn = require('child_process').spawn,
      fileName = process.argv[2],
      spawnCmd = process.argv[3],
      spawnArgs = process.argv.slice(4, process.argv.length);

console.log([typeof spawnArgs, spawnArgs]);

if (!fileName) {
    throw Error("need to specify file to watch!");
}

// TODO: does not handle deletes
fs.watch(fileName, function() {
    console.log("file '"+ fileName + "' just changed");

    // works
    // const ls = spawn('ls', ['-lh', fileName]);

    // but this does not work. here
    // works on the bash terminal
    // nor does it error out. frustrating
    // const ls = spawn(spawnCmd, ['-l -h', fileName]);

    // this works.
    // so it is a special array then :-)
    // const ls = spawn(spawnCmd, ['-l', '-h', fileName]);

    // why ? bug ?
    // or this is just POSIX
    // and bash has added a feature to make it work ?

    const ls = spawn(spawnCmd, spawnArgs.concat([fileName]));

    ls.stdout.pipe(process.stdout);

    let readStream = fs.createReadStream(fileName)

    readStream.pipe(process.stdout);
    // will not throw error if file is deleted
    readStream.on('error', function(err) {
        console.log(err);
    });
});

console.log("now watching '"+ fileName + "' for changes");