colorListMapped.js

const colors = 'red orange red yellow green blue yellow indigo violet red indigo orange green blue';

const colorsMapped = colors.split(' ').reduce((set, item, i) => {
  if (item in set) return set; 
  
  // if the item isn't in the set, put it in and record its position
  // don't do any mutations!
  return {
    ...set,
    [item]: i,
  }
  
}, {});

// Object.entries converts our unsortable object into an array we can sort 
// each item will be [color, order]
const solution = Object.entries(colorsMapped)
    /* destructure the args so each item will be [color, order]; sort by order */
  .sort(([ ,orderA], [ ,orderB]) => orderA - orderB)
    /* sort doesn't let us modify the array, so let's map over and grab only what we need */
  .map(([colorName]) => colorName)
    /* convert to a string */
  .join(' ');


console.log(solution);