Transform an array of objects structure

This seems like a use case for a for loop, that creates or pushes each "result" into the appropriate Country index, then uses similar logic to organize the matches for each country by League. The code below is pretty verbose-- if you use a utility library like Lodash, you can achieve some of this much more succinctly via something like groupBy (although, that would produce an object instead of an array).

const results = [/* ...your original data... */];
const resultsByCountry = [];

// The first loop just indexes the matches by Country:
for (let r = 0; r < results.length; r++) {
  let result = results[r];
  let existingCountryIndex = resultsByCountry.findIndex((country) => country.country_name == result.country_name);

  // Temporarily, we're just stashing the matches flat, here. We'll take another pass to group them into leagues, once we know they're all sorted into the right countries:
  if (existingCountryIndex > -1) {
    resultsByCountry[existingCountryIndex].entries.push(result);
  } else {
    resultsByCountry.push({
      country_name: result.country_name,
      entries: [result]
    });
  }
}


// The second loop organizes the matches into leagues, by a very similar means:
for (let c = 0; c < resultsByCountry.length; c++) {
  const country = resultsByCountry[c]
  let matches = country.entries;
  let matchesByLeague = [];

  for (let m = 0; m < matches.length; m++) {
    let match = matches[m];
    let existingLeagueIndex = matchesByLeague.findIndex((league) => league.league_name == match.league_name);

    if (existingLeagueIndex > -1) {
      matchesByLeague[existingLeagueIndex].events.push(match);
    } else {
      matchesByLeague.push({
        league_name: match.league_name,
        events: [match]
      });
    }
  }

  // Overwrite the old `entries` key with the grouped array:
  country.entries = matchesByLeague;
}

console.log(resultsByCountry);

Again, this is pretty crude. There is probably a way to implement it with map and reduce, but because the data structure is unusual, and the question specifically asked about transforming from an array to an array (not an object), I've provided this as a simple + explicit answer.

You could store all information for building nested group/value pairs and take an array for all keys for the final objects and reduce the array by reducing the group array.

var data = [{ match_id: "255232", country_id: "41", country_name: "England", league_id: "152", league_name: "National League", match_date: "2020-01-01", match_status: "", match_time: "16:00" }, { match_id: "255232", country_id: "41", country_name: "Italy", league_id: "152", league_name: "Serie a", match_date: "2020-01-01", match_status: "", match_time: "16:00" }, { match_id: "255232", country_id: "41", country_name: "Italy", league_id: "153", league_name: "Serie b", match_date: "2020-01-01", match_status: "", match_time: "16:00" }],
    groups = [['country_name', 'entries'], ['league_name', 'events']],
    values = ['match_date', 'match_status', 'match_time'],
    result = data.reduce((r, o) => {
        groups
            .reduce((group, [key, values]) => {
                var temp = group.find(q => q[key] === o[key]);
                if (!temp) group.push(temp = { [key]: o[key], [values]: [] });
                return temp[values];
            }, r)
            .push(Object.assign(...values.map(k => ({ [k]: o[k] }))));
        return r;
    }, []);

console.log(result);

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const map1 = {};
const list = matches.map(match => {
  if (!map1[match.country_name]) {
    map1[match.country_name] = {};
  }
  map1[match.country_name]["entries"] =
    map1[match.country_name]["entries"] &&
    map1[match.country_name]["entries"].length > 0
      ? map1[match.country_name]["entries"].concat(match)
      : [match];
});

const result = [];
Object.keys(map1).forEach(country => {
  const m = {};
  m["country_name"] = country;
  m["entries"] = [];
  const entries = map1[country]["entries"];

  entries.forEach(entry => {
    m["entries"].push({
      league_name: entry.league_name,
      events: entry
    });
  });

  result.push(m);
});
// result has the required info.