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Javascript: Using Function Chaining to Rearrange Multidimensional Javascript Object

October 25, 2022

My title is slightly off, apologies, having trouble coming up with the right terms atm.

I am trying to do things functionally with .map(), .forEach(), etc.

I have a structure from a JSON like this, (all Items are unique strings, some contain spaces):

data = {
  Category1: ["Item_A", "Item_B", "Item_C"]
  Category2: ["Item_D", "Item_E", "Item_F"]
  Category3: ["Item_G", "Item_H"]
}

I would like this:

data = {
  Item_A: ["Category1"]
  Item_B: ["Category1"]
  Item_C: ["Category1"]
  Item_D: ["Category2"]
  Item_E: ["Category2"]
  Item_F: ["Category2"]
  Item_G: ["Category3"]
  Item_H: ["Category3"]
}

Thanks for any and all help.

>Solution :

You can iterate your data with Object.entries, using flatMap to replace each { key : array } pair with a flat array of [value, [key]] pairs, then use Object.fromEntries to create your desired output:

const data = {
  Category1: ["Item_A", "Item_B", "Item_C"],
  Category2: ["Item_D", "Item_E", "Item_F"],
  Category3: ["Item_G", "Item_H"],
}

const result = Object.fromEntries(
  Object.entries(data).flatMap(([k, a]) => a.map(v => [v, [k]]))
)

console.log(result)