editor-code-challenge.js

const COMMAND_PRINT = '3';
const COMMAND_UNDO = '4';
const COMMAND_TO_FUNCTION = {
  '1': appendToString,
  '2': deleteLastNLetters,
  [COMMAND_PRINT]: printNthLetter,
  [COMMAND_UNDO]: undo
};
const WHITESPACE = ' ';

function processData(input = '') {
  const commands = input
    .split('\n')
    .slice(1)
    .map((rawCommand) => parseCommand(rawCommand));

  const initialEditorStateHistory = [''];
  const editorStateHistory = commands.reduce(
    (stateHistory, [command, argument]) => reduceToNextEditorState(stateHistory, command, argument),
    initialEditorStateHistory
  );
}

function parseCommand(rawCommand) {
  return rawCommand
    .split(WHITESPACE);
}

function reduceToNextEditorState(stateHistory, command, argument) {
  const newEditorString = COMMAND_TO_FUNCTION[command](stateHistory, argument);
  // For performance optimizations, we mutate the state instead of being "pure" and
  // creating a new array for the state each time.
  if (command !== COMMAND_PRINT && command !== COMMAND_UNDO) {
    stateHistory.push(newEditorString);
  }

  return stateHistory;
}

function appendToString(stateHistory, suffix) {
  return stateHistory[stateHistory.length - 1] + suffix;
}

function deleteLastNLetters(stateHistory, numOfLettersToDrop) {
  return stateHistory[stateHistory.length - 1].slice(0, -numOfLettersToDrop);
}

function printNthLetter(stateHistory, n) {
  const index = parseInt(n, 10) - 1;
  const editorString = stateHistory[stateHistory.length - 1];
  console.log(editorString[index]);
  return editorString;
}

function undo(stateHistory) {
  return stateHistory.pop();
}


process.stdin.resume();
process.stdin.setEncoding('ascii');
_input = '';
process.stdin.on('data', function (input) {
  _input += input;
});

process.stdin.on('end', function () {
  processData(_input);
});