The solution of the coder problem

coder.js

const fs = require('fs');
const readline = require('readline');

const defaultKeys = {
  1: ['a', 'b', 'c'],
  2: ['d', 'e', 'f'],
  3: ['g', 'h', 'i'],
  4: ['j', 'k', 'l'],
  5: ['m', 'n', 'o'],
  6: ['p', 'q', 'r', 's'],
  7: ['t', 'u', 'v'],
  8: ['w', 'x', 'y', 'z'],
}

function keysToMap(keys) {
  const map = new Map();
  Object.keys(keys).forEach((key) => {
    keys[key].forEach((letter, clicks) => {
      map.set(letter, { key, clicks });
    })
  })

  return map;
}

function encode(message, keys) {
  const keyMap = keysToMap(keys);
  return [...message]
    .reduce((result, letter, i) => {
      const char = letter.toLowerCase();
      const { key, clicks } = keyMap.get(char);
      const code = (clicks + i + 1) % keys[key].length;

      const encoded = (char !== letter) 
        ? keys[key][code].toUpperCase()
        : keys[key][code];
      return result.concat(encoded);
    }, '');
}

function decode(message, keys) {
  const keyMap = keysToMap(keys);
  return [...message]
    .reduce((result, letter, index) => {
      const char = letter.toLowerCase();
      const { key, clicks } = keyMap.get(char);

      const code = keys[key].findIndex((_value, i) => {
        return (i - clicks + 1 + index) % keys[key].length === 0
      });

      const decoded = (char !== letter)
        ? keys[key][code].toUpperCase()
        : keys[key][code];

      return result.concat(decoded);
    }, '');
}

async function run() {
  const args = process.argv.slice(2);

  switch (args[0]) {
    case 'file': {
      let keys = defaultKeys;
      const file = args[1];
      if (!file) {
        throw new Error('need file');
      }

      const keysFile = args[2];
      if (keysFile) {
        keys = JSON.parse(await fs.promises.readFile(keysFile));
      }

      const fileStream = fs.createReadStream(file);
      const lines = readline.createInterface({
        input: fileStream,
        crlfDelay: Infinity
      });

      lines.on('line', (line) => {
        if (line === '#')
          return;

        console.log(decode(line, keys));
      });
      break;
    }
    case 'test': {
      /*
       * I decided not to use assert, instead I made the tests more visual.
       */
      console.log('\nEncoding Tests');
      console.log(`ABCD encoded: ${encode('ABCD', defaultKeys)} - BACE`);
      console.log(`IhateSMS encoded: ${encode('IhateSMS', defaultKeys)} - GgaudQNS`);
      console.log(`ccd encoded: ${encode('ccd', defaultKeys)} - abd`);
      
      console.log('\nDecoding Tests');
      console.log(`BACE decoded: ${decode('BACE', defaultKeys)} - ABCD`);
      console.log(`GgaudQNS decoded: ${decode('GgaudQNS', defaultKeys)} - IhateSMS`);
      console.log(`abd decoded: ${decode('abd', defaultKeys)} - ccd`);
      break;
    }
    default: {
      throw new Error('You should call: "node coder.js test" or "node coder.js file <input> <keymap> keymap is optional"');
    }
  }
}

run();