merge.js

#!/usr/bin/env node
'use strict';

const fs = require('fs');

let input_A = process.argv[2] || null;
let input_B = process.argv[3] || null;
let output = process.argv[4] || null;

if (input_A, input_B) {
  let output_json = JSON.parse(fs.readFileSync(input_A).toString());
  let file_B = JSON.parse(fs.readFileSync(input_B).toString());

  for (let key in file_B) {
    console.log('has %s? %s', key, typeof output_json[key] !== 'undefined');
    if (typeof output_json[key] !== 'undefined') {
      console.log('concatenating arrays');
      output_json[key] = output_json[key].concat(file_B[key]);
      output_json[key] = Array.from(new Set(output_json[key]));
    } else {
      console.log('setting solitary array');
      output_json[key] = file_B[key];
    }
  }

  if (output_json) {
    if (output) {
      output_json = JSON.stringify(output_json).replace(new RegExp('","', 'g'), '",\n    "').replace(new RegExp('],"', 'g'),'],\n"');

      fs.writeFile(output,
        output_json,
        function (err) {
          if (err) {return console.log('everything sucks because: ', err);}
          console.log('wrote grammar to %s', output);
          process.exit(0);
        });
    } else {
      console.log(output_json);
      process.exit(0);
    }
  } else {
    console.log("something went wrong merging files");
    process.exit(1);
  }

} else {
  console.error('please specify 2 input files');
  process.exit(1);
}