Looping through array and removing items, without breaking for loop

Solution 1:

The array is being re-indexed when you do a .splice(), which means you'll skip over an index when one is removed, and your cached .length is obsolete.

To fix it, you'd either need to decrement i after a .splice(), or simply iterate in reverse...

var i = Auction.auctions.length
while (i--) {
    ...
    if (...) { 
        Auction.auctions.splice(i, 1);
    } 
}

This way the re-indexing doesn't affect the next item in the iteration, since the indexing affects only the items from the current point to the end of the Array, and the next item in the iteration is lower than the current point.

Solution 2:

This is a pretty common issue. The solution is to loop backwards:

for (var i = Auction.auctions.length - 1; i >= 0; i--) {
    Auction.auctions[i].seconds--;
    if (Auction.auctions[i].seconds < 0) { 
        Auction.auctions.splice(i, 1);
    }
}

It doesn't matter if you're popping them off of the end because the indices will be preserved as you go backwards.

Solution 3:

Recalculate the length each time through the loop instead of just at the outset, e.g.:

for (i = 0; i < Auction.auctions.length; i++) {
      auction = Auction.auctions[i];
      Auction.auctions[i]['seconds'] --;
      if (auction.seconds < 0) { 
          Auction.auctions.splice(i, 1);
          i--; //decrement
      }
}

That way you won't exceed the bounds.

EDIT: added a decrement in the if statement.

Solution 4:

Although your question is about deleting elements from the array being iterated upon and not about removing elements (in addition to some other processing) efficiently, I think one should reconsider it if in similar situation.

The algorithmic complexity of this approach is O(n^2) as splice function and the for loop both iterate over the array (splice function shifts all elements of array in the worst case). Instead you can just push the required elements to the new array and then just assign that array to the desired variable (which was just iterated upon).

var newArray = [];
for (var i = 0, len = Auction.auctions.length; i < len; i++) {
    auction = Auction.auctions[i];
    auction.seconds--;
    if (!auction.seconds < 0) { 
        newArray.push(auction);
    }
}
Auction.auctions = newArray;

Since ES2015 we can use Array.prototype.filter to fit it all in one line:

Auction.auctions = Auction.auctions.filter(auction => --auction.seconds >= 0);