Changing array indices in JavaScript by splicing elements

I'm experiencing a curious issue that seems to involve overwriting array indices after splicing, or at least that's what I suspect. This problem arises in a small game project built using phaser 2 - essentially, it's a multiplayer jumping game aimed at gaining experience with client/server architecture. We're utilizing socket.io and express for this project. The trouble lies on the server side: when a client disconnects, their removal from the player list causes the remaining player(s) to overwrite the disconnected player's index. To debug this, I have been heavily relying on console logs, using a for loop to iterate through the list of players and display their respective socket IDs.

Initially, I considered an aliasing issue within the onNewPlayer(data) function because of duplication in variable names (var currentInfo). I subsequently changed the second object to var info. So, could this be an aliasing problem, or should I investigate elsewhere for the root cause? If necessary, I can provide additional code; thus far, all callbacks related to player creation and movement appear to be functioning correctly. Thank you.

Below is the relevant server-side code:

var players[];
//When a new player is created, save them
function onNewPlayer(data) {
  var newPlayer = new Player(data.x, data.y, this.id);

  var currentInfo = {
    x: newPlayer.x,
    y: newPlayer.y,
    id: newPlayer.id,
  };

  for(i = 0; i < players.length; i++) {
    //broadcast the new player's information to all other players in the list  
    this.broadcast.emit("newEnemy", currentInfo);
  }

  //check if existing players are present, then send their info to the new player
  if(players.length > 0) {
    for(i = 0; i < players.length; i++) {
        var info = {
            x: players[i].x,
            y: players[i].y,
            id: players[i].id,
        };
        this.emit("newEnemy", info);
    }
  }

  players.push(newPlayer);
  for(i = 0; i < players.length; i++) {
    console.log(players[i].id);
  }
}

function onDisconnect(){
    console.log("User " + this.id + " disconnected");
    //find the user in the list of players and remove them, then inform the client
    for(i = 0; i < players.length; i++) {
        if(players[i].id === this.id) {
            console.log("removing this player " + this.id);
            //TODO trying a different broadcast
            this.broadcast.emit("playerDisconnect", this.id);
            console.log(players[i].id);
            players.splice(i, 1);
        }
    }
}

Below is the relevant client-side code:

//We've lost connection with the server!
function onSocketDisconnect() {
    console.log("Lost connection with server!");
};

//When the server notifies the client that an enemy has disconnected,
//search for it in the enemy list and stop rendering it
function onEnemyDisconnect(data) {
    //TODO
    for(i = 0; i < enemies.length; i++) {
        if(enemies[i].id == data) {
            //TODO
            console.log("destroying");
            enemies[i].destroy();
            enemies.splice(i, 1);
        }
    }
}

Answer №1

When iterating through an array in the forward direction using a for loop and removing elements with .splice(), issues may arise. This method does not work as expected because when an element is removed from the array, the indexes of the subsequent elements shift down by one. Consequently, if the index in the loop points to the next element after removal, certain items will be skipped during iteration.

To address this issue, there are several potential solutions:

  1. Iterate through the array backwards instead of forwards. This way, unprocessed elements remain intact while utilizing .splice().

  2. Avoid modifying the array within the loop. Collect the indexes of elements to be deleted and remove them after the loop completes, preferable from back to front.

  3. Utilize .filter() to create a new array based on specific criteria. Upon completion, assign this filtered array to the original variable for continued use.

  4. If retaining the current iteration order is necessary, adjust the loop index post-.splice() operation by decreasing it by one.

An illustration of reverse array iteration is demonstrated here:

// When informed about an enemy disconnecting, search and remove from enemies list
function onEnemyDisconnect(data) {
    // Reverse iteration prevents skipping due to .splice()
    for (let i = enemies.length - 1; i >= 0; i--)
        if(enemies[i].id == data) {
            console.log("Destroying enemy");
            enemies[i].destroy();
            enemies.splice(i, 1);
        }
    }
}

An example showcasing the use of .filter(), assuming the new array permanently replaces the existing one, is provided below:

// Respond to enemy disconnect notification by filtering out enemies
function onEnemyDisconnect(data) {
    enemies = enemies.filter(item => {
        if (item.id === data) {
            console.log("Destroying enemy");
            item.destroy();
            return false;    // Exclude this item
        }
        return true;
    });
}

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