day8.js

const signalToNumberMap = (inputByLength) => {
  const signalMap = { };

  const countIncludedChars = (input, number) => (
    number.split('').reduce((count, numberChar) => {
      if (input.includes(numberChar)) count++;
      return count;
    }, 0)
  );

  const number1 = inputByLength[2][0];
  const number7 = inputByLength[3][0];
  const number4 = inputByLength[4][0];
  const number8 = inputByLength[7][0];

  const number3 = inputByLength[5].find(input => (
    countIncludedChars(input, number7) == number7.length
  ));

  const number9 = inputByLength[6].find(input => (
    countIncludedChars(input, number3) == number3.length
  ));

  const number0 = inputByLength[6].find(input => (
    countIncludedChars(input, number1) == number1.length && input != number9
  ));

  const number6 = inputByLength[6].find(input => (
    input != number9 && input != number0
  ));

  const number5 = inputByLength[5].find(input => (
    countIncludedChars(input, number6) == number6.length - 1
  ));

  const number2 = inputByLength[5].find(input => (
    input != number3 && input != number5
  ));

  signalMap[number0] = 0;
  signalMap[number1] = 1;
  signalMap[number2] = 2;
  signalMap[number3] = 3;
  signalMap[number4] = 4;
  signalMap[number5] = 5;
  signalMap[number6] = 6;
  signalMap[number7] = 7;
  signalMap[number8] = 8;
  signalMap[number9] = 9;

  return signalMap;
};