How to add a new Key value pair in Node.js/Javascript

Question

Here is the JSON that I have:

[
    "Spain",
    "2010",
    "Brazil",
    "1994",
    "Brazil",
    "1970",
    "Brazil",
    "1962",
    "Brazil",
    "2002",
    "Brazil",
    "1958",
    "Germany",
    "2014",
    "Germany",
    "1990",
    "Germany",
    "1974",
    "Germany",
    "1954",
    "Italy",
    "2006",
    "Italy",
    "1982",
    "Italy",
    "1938",
    "Italy",
    "1934",
    "France",
    "2018",
    "France",
    "1998",
    "Argentina",
    "1986",
    "Argentina",
    "1978",
    "Uruguay",
    "1930",
    "Uruguay",
    "1950",
    "England",
    "1966"
]

I'm trying to convert it into multiple array objects of key-value pair like below.

{
        "name": "Spain",
        "year": 2010
    },
    {
        "name": "Brazil",
        "year": 1994, 1970, 1962, 2002, 1958
    },
    ...
]

Is there any way to achieve the above structure? I have tried couple of methods using array map function but it doesn't give in the desired output.

const dataMap = Object.keys(s).map((filename) => {
      return {
        name: filename,
        year: s[filename]
    }
    })

How can it be done in Node.js??

Shouldn't year property be an array?

DecPK
– DecPK

2021-09-05 11:49:42 +00:00
Commented Sep 5, 2021 at 11:49 — DecPK
– DecPK, Commented Sep 5, 2021 at 11:49

DecPK · Accepted Answer · 2021-09-05 11:56:45Z

You can easily achieve the result using Map

const arr=["Spain","2010","Brazil","1994","Brazil","1970","Brazil","1962","Brazil","2002","Brazil","1958","Germany","2014","Germany","1990","Germany","1974","Germany","1954","Italy","2006","Italy","1982","Italy","1938","Italy","1934","France","2018","France","1998","Argentina","1986","Argentina","1978","Uruguay","1930","Uruguay","1950","England","1966"];

const map = new Map();
for (let i = 0; i < arr.length; i += 2) {
  const country = arr[i],
    year = arr[i + 1];

  if (map.has(country)) map.get(country).push(year);
  else map.set(country, [year]);
}

const result = [];
for (let [name, year] of map.entries()) {
  result.push({ name, year });
}
console.log(result);

/* This is not a part of answer. It is just to give the output fill height. So IGNORE IT */
.as-console-wrapper { max-height: 100% !important; top: 0; }

Nick Parsons · Accepted Answer · 2021-09-05 11:57:46Z

You can create a Map by looping over your array 2 indexes at a time. For each iteration, you can obtain the place + the year, and create a key-value pair using the place as the key and an array of years as the value. If you come across a place that you've already seen, you can add it to the array at your key in the Map. Lastly, you can use Array.from() with a mapping function to build your array of objects from the Map:

const arr = ["Spain","2010","Brazil","1994","Brazil","1970","Brazil","1962","Brazil","2002","Brazil","1958","Germany","2014","Germany","1990","Germany","1974","Germany","1954","Italy","2006","Italy","1982","Italy","1938","Italy","1934","France","2018","France","1998","Argentina","1986","Argentina","1978","Uruguay","1930","Uruguay","1950","England","1966"];

const map = new Map;
for(let i = 0; i < arr.length; i+=2) {
  const name = arr[i];
  const year = arr[i+1];
  map.set(name, (map.get(name) || []).concat(year));
}
const res = Array.from(map, ([name, year]) => ({name, year}));
console.log(res);

.as-console-wrapper { max-height: 100% !important;} /* ignore */

Eldar · Accepted Answer · 2021-09-05 12:08:26Z

You can achieve what you want with a few steps. Create a temporary map (using a plain object or a Map class). Fill that map using even and odd indexes (even indexes become key and the odd indexes become part of the array) and then convert the map into an array.

const sourceArray = [
    "Spain",
    "2010",
    "Brazil",
    "1994",
    "Brazil",
    "1970",
    "Brazil",
    "1962",
    "Brazil",
    "2002",
    "Brazil",
    "1958",
    "Germany",
    "2014",
    "Germany",
    "1990",
    "Germany",
    "1974",
    "Germany",
    "1954",
    "Italy",
    "2006",
    "Italy",
    "1982",
    "Italy",
    "1938",
    "Italy",
    "1934",
    "France",
    "2018",
    "France",
    "1998",
    "Argentina",
    "1986",
    "Argentina",
    "1978",
    "Uruguay",
    "1930",
    "Uruguay",
    "1950",
    "England",
    "1966"
];

const map = {};

for(let i =0; i < sourceArray.length;i+=2){
  const country = sourceArray[i];
  if(map[country]){
      map[country].push(+sourceArray[i+1])
  }else{
     map[country] = [+sourceArray[i+1]]
  }
}

const finalArray = Object.entries(map).map(([k,v])=> ({ name:k, years :v}))
console.log(finalArray)

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