How can I obtain an object with the values as keys and vice versa - and with duplicate values stored in an array?

Question

1. My task

In the object, how can I get an array in which the corresponding keys are stored if there are duplicate values in the input object?

const object5 = { a: 1, b: 2, c: 3, d: 1 };

const object6 = { a: 1, b: 1, c: 1, d: 1 };

/**          
1. @param {object} ???
2. @returns {object} An object with the given object's values 
as keys, and keys as values. If there are duplicate values in 
the input object, the corresponding keys should be stored in 
an array.  
*/

2. Tests

console.log(swapPairs3(object5))// , { 1: ["a", "d"], 2: "b", 3: "c" };　　　　　　　　　　　　 console.log(swapPairs3(object6))// , { 1: ["a", "b", "c", "d"] };

Answer 1

You can reduce the entries of the object using Array.prototype.reduce.

const groupByValue = (obj) =>
  Object.entries(obj).reduce((r, [k, v]) => ((r[v] ??= []).push(k), r), {});

console.log(groupByValue({ a: 0, b: 0, c: 3, d: 1 }));
console.log(groupByValue({ a: 1, b: 1, c: 1, d: 1 }));

A more comprehensive version of the above solution:

function groupByValue(obj) {
  return Object.entries(obj).reduce((r, [k, v]) => {
    r[v] = r[v] ?? [];
    r[v].push(k);
    return r;
  }, {});
}

console.log(groupByValue({ a: 1, b: 2, c: 3, d: 1 }));
console.log(groupByValue({ a: 1, b: 1, c: 1, d: 1 }));

Documentation: Nullish Coalescing Operator (??)