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javascript create a mapping of string to array of objects from an array of objects

May 23, 2022

Is there a way to create a map (mapping string to array) from an array in the example below using map function? Not sure how to append an object to a key that already exists instead of overwriting what’s inside already

WANT
{"Germany": [{score: 1}], "Austria": [{score: 1}], "Switzerland": [{score: 2},{score: 3}]}

GETTING
{"Germany": [{score: 1}], "Austria": [{score: 1}], "Switzerland": [{score: 3}]}

let data = [
  {country: 'Germany', score: 1},
  {country: 'Austria', score: 1},
  {country: 'Switzerland', score: 2},
  {country: 'Switzerland', score: 3}
];

let dictionary = Object.assign({}, ...data.map((x) => ({[x.country]: [{score: x.score}]})));

>Solution :

Fairly straightforward to do with reduce(), some destructuring and spread syntax:

const data = [
  {country: 'Germany', score: 1},
  {country: 'Austria', score: 1},
  {country: 'Switzerland', score: 2},
  {country: 'Switzerland', score: 3}
];

const result = data.reduce((a, {country, score}) => ({
  ...a,
  [country]: [...a[country] || [], {score}]
}), {});

console.log(result);