I'd suggest passing the facets object to the function as a parameter, you can then easily write the input and output of the function:

        
const someFunction = (input) => {
    let final = {
      names: Object.keys(input.names).join(' | '),
      nat: Object.keys(input.nationalities).join(' | '),
    }
  return final;
}

const facets = { names: { John: true, Mary: true, }, nationalities: { US: true, CA: true, }, }
const output = someFunction(facets)
console.log('Input:', facets);
console.log('Output:', output);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Solution #1: Declare facets on module level, for accessing to it outside of someFunction:

const facets = {
  names: {
    John: true,
    Mary: true
  },
  nationalities: {
    US: true,
    CA: true
  }
};
// Your method: do the same, but simplified
const someFunction = () => ({
  names: Object.keys(facets.names).join(" | "),
  nat: Object.keys(facets.nationalities).join(" | ")
});
// Reverted method: just return initial value
const revertFuntion = () => facets

Solution #2: if you need to revert any value returned by someFunction to it's inital state, you can do something like this:

const revertChanges = obj => {
  for (const key in obj) {
    const res = {}
    obj[key].split(' | ').forEach(value => {
      res[value] = true;
    })
    obj[key] = res;
  }
  return obj;
}

let whatever = someFunction();
console.log(whatever);
// {
//   "names": "John | Mary",
//   "nat": "US | CA"
// }
let reverted = revertChanges(whatever);
console.log(reverted);
// Initial facets

Not sure what you mean, but if you need both facets and final you can return an Object.

const someFunction = (facets, unify) => {
  const result = {
    names: Object.keys(facets.names).join(" | "),
    nat: Object.keys(facets.nationalities).join(" | ")
  };
  return unify ? {...result, original: facets } : {original: facets, result};
};

const facets = {
  names: {
    John: true,
    Mary: true
  },
  nationalities: {
    US: true,
    CA: true
  }
};
const whatever = someFunction(facets);
const pre = document.querySelector(`pre`);

pre.textContent = `Object with original/result properties: ${
  JSON.stringify(whatever, null, 2)}`;

// or include facets as 'original' within the return value
pre.textContent += `\n\nSingle object: ${JSON.stringify(someFunction(facets, true), null, 2)}`;

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