https://www.codewars.com/kata/double-rainbow/train/javascript

willThereEverBeARainbow.js

function willThereEverBeARainbow(string) {
  const rainbow = ['Red', 'Orange', 'Yellow', 'Green', 'Blue', 'Indigo', 'Violet'].join('').toUpperCase();
  const characterFrequencyMap = (str) => str.split('').reduce((acc, letter) => {
    return Object.assign({}, acc, {
      [letter]: acc[letter] ? acc[letter] + 1 : 1
    });
  }, {});
  const expectedLettersMap = characterFrequencyMap(rainbow); // => { R: 3, E: 7, D:2, ... }
  const inputLettersMap = characterFrequencyMap(string);
  
  const tooShort = string.length < rainbow.length;
  if (tooShort) return 'Too short for a rainbow!';
  
  const noRainbow = Object.keys(expectedLettersMap).some((key) => !inputLettersMap[key] || inputLettersMap[key] < expectedLettersMap[key]);
  if (noRainbow) return 'No rainbow to be found!';
  
  const double = Object.keys(expectedLettersMap).every((key) => inputLettersMap[key] >= expectedLettersMap[key] * 2);
  if (double) return 'Double rainbow all the way!';

  const normal = Object.keys(expectedLettersMap).every((key) => inputLettersMap[key] >= expectedLettersMap[key]);
  if (normal) return 'Look! A rainbow!';
}

// test cases

Test.assertEquals(willThereEverBeARainbow('REDORANGEYELLOWGREENBLUEINDIGOVIOLET'), 'Look! A rainbow!');
Test.assertEquals(willThereEverBeARainbow('EDORANGEYELLOWGREENBLUEINDIGOVIOLET'), 'Too short for a rainbow!');
Test.assertEquals(willThereEverBeARainbow('EORECDIDSEPYKWNTQVAZIESJKEOOBCCQNHQEJOXJRGLULRPGEGSN'), 'No rainbow to be found!');
Test.assertEquals(willThereEverBeARainbow('REDORANGEYELLOWGREENBLUEINDIGOVIOLETREDORANGEYELLOWGREENBLUEINDIGOVIOLET'), 'Double rainbow all the way!');
Test.assertEquals(willThereEverBeARainbow('JDOIDNRETNMLLEFLORAUOHIIVLTRZRAYBEENGESCGUDEPKOENGIVYWLGBQXOE'), 'Look! A rainbow!');
Test.assertEquals(willThereEverBeARainbow('LQUEXEEARELNEPOIEBWVTYNIVEGZOLELDOIEOEAADLDTGSDREOORYNGNRNEGRENELROLYNVUODBOKCWILEUFHIGIBIJGMGTLR'), 'Double rainbow all the way!');