Best way extract gameId from array object

I have a 2D array matches where I store the game id and another array of objects games where I store gameId with the playerId.

Here is what I have done so far:

Step 1: I get the opponent’s gameId against the gameId from “matches” array.

e.g. gameId “2” has an opponent “1”

Step 2: check opponent_game in games and return game.

    let gameId = 2
    
    let matches = [
        [1, 2],
        [3, 4]
    ]
    
    const opponent_game = matches.filter((v, i) => {
        if (v.includes(gameId)) {
            var index = v.indexOf(gameId);
            if (index > -1) {
                return v.splice(index, 1);
            }
        }
    })
    
    let games = [{
        gameId: 1,
        playerId: 222
    }, {
        gameId: 4,
        playerId: 222
    }]
    
    const game = games.filter((v, i) => {
        return v.gameId === opponent_game[0][0] 
    })
    
    console.log(game)

Please suggest best practices in javascript or an optimized and clean way to write the same solution. TIA

.includes() and checking .indexOf() against -1 do the same thing, you only need one.

Also the function should be returning a boolean value. It currently works because the array returned by .splice() is considered true and not returning anything (or undefined) is considered false. It would be better to explicitly return a boolean:

 const opponent_game = matches.filter((v, i) => {
    var index = v.indexOf(gameId);
    if (index > -1) {
        v.splice(index, 1);
        return true;
    }
    return false;
})

However modifying the content of an array in a filter is bad practice, because it is unexpected. Since you only looking for a single value .reduce() would probably be the better choice:

 const opponent_game = matches.reduce((acc, v) => {
    if (acc !== null) {
      return acc;
    }
    var index = v.indexOf(gameId);
    if (index > -1) {
        return v[1 - index];
    }
    return null;
}, null)

In this case opponent_game is a number and no longer an array containing an array containing a number so that simplifies the second part too. Also since you are looking for a single item .find() would be more appropriate:

const game = games.find(v => {
    return v.gameId === opponent_game
})