new.js

// define dispatch table OR import it from somewhere else
// const commands = require('./commands/live/somewhere');
const commands = {
  first: (foo, bar) => { /* ... */ },
  second: (baz) => { /* ... */ },
  help: (commandName) => { /* ... */ }
};

function usingDispatch(commandName, ...args) {
  const command = commands[commandName] || commands.help;
  command(args);
  doTheRest();
}

old.js

function doFirst(foo, bar) { /* ... */ }

function doSecond(baz) { /* ... */ }

function doHelp(commandName) { /* ... */ }

function usingSwitch(commandName, ...args) {
  switch (commandName) {
    case 'first':
      doFirst(args);
      break;
    case 'second':
      doSecond(args);
      break;
    default:
      doHelp(args[0]);
      break;
  }
  doTheRest(); // ...
}