JavaScript Convert Nested Map To Object

I want to convert an es6 nested Map to an object. I tried this code: mapToObjectRec(m) { let lo = {} for(let[k,v] of m) { if(v instanceof Map) {

Solution 1:

If the Map's keys are numbers, strings, or symbols, the following technique will work -

const m =
  new Map
    ( [ [ 'a', 1 ]
      , [ 'b', 2 ]
      , [ 'c'
        , new Map
            ( [ [ 'd', 3 ]
              , [ 'e', 4 ]
              , [ 'f'
                , new Map ([[ 'g', 6 ]])
                ]
              ]
            )
        ]
      ]
    )
    
const toObject = (map = new Map) =>
  Object.fromEntries
    ( Array.from
        ( map.entries()
        , ([ k, v ]) =>
            v instanceof Map
              ? [ k, toObject (v) ]
              : [ k, v ]
        )
    )
    
console .log (toObject (m))
// { a: 1
// , b: 2
// , c:
//     { d: 3
//     , e: 4
//     , f: { g: 6 }
//     }
// }

Otherwise, if the Map keys are complex objects and cannot be reliably coerced into a string, you'll have to come up with another mapping that preserves the key's actual value, such as -

const m =
  new Map
    ( [ [ 'a', 1 ]
      , [ 'b', 2 ]
      , [ 'c'
        , new Map
            ( [ [ 'd', 3 ]
              , [ 'e', 4 ]
              , [ 'f'
                , new Map ([[ 'g', 6 ]])
                ]
              ]
            )
        ]
      ]
    )
    
const toObject = (map = new Map) =>
  Array.from
    ( map.entries()
    , ([ k, v ]) =>
        v instanceof Map
          ? { key: k, value: toObject (v) }
          : { key: k, value: v }
    )
    
console .log (toObject (m))
// [ { key: "a", value: 1 }
// , { key: "b", value: 2 }
// , { key: "c"
//   , value:
//       [ { key: "d", value: 3 }
//       , { key: "e", value: 4 }
//       , { key: "f", value: [ { key: "g", value: 6 } ] }
//       ]
//   }
// ]

Solution 2:

If a value in the map can be an array of maps you need:

const toObject = (map = new Map) => {
  if (!(map instanceof Map)) return map
  return Object.fromEntries(Array.from(map.entries(), ([k, v]) => {
    if (v instanceof Array) {
      return [k, v.map(toObject)]
    } else if (v instanceof Map) {
      return [k, toObject(v)]
    } else {
      return [k, v]
    }
  }))
}