[javascript] Looping through array and removing items, without breaking for loop

I have the following for loop, and when I use splice() to remove an item, I then get that 'seconds' is undefined. I could check if it's undefined, but I feel there's probably a more elegant way to do this. The desire is to simply delete an item and keep on going.

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

Two examples that work:

(Example ONE)
// Remove from Listing the Items Checked in Checkbox for Delete
let temp_products_images = store.state.c_products.products_images
if (temp_products_images != null) {
    for (var l = temp_products_images.length; l--;) {
        // 'mark' is the checkbox field
        if (temp_products_images[l].mark == true) {
            store.state.c_products.products_images.splice(l,1);         // THIS WORKS
            // this.$delete(store.state.c_products.products_images,l);  // THIS ALSO WORKS
        }
    }
}

(Example TWO)
// Remove from Listing the Items Checked in Checkbox for Delete
let temp_products_images = store.state.c_products.products_images
if (temp_products_images != null) {
    let l = temp_products_images.length
    while (l--)
    {
        // 'mark' is the checkbox field
        if (temp_products_images[l].mark == true) {
            store.state.c_products.products_images.splice(l,1);         // THIS WORKS
            // this.$delete(store.state.c_products.products_images,l);  // THIS ALSO WORKS
        }
    }
}

You can just look through and use shift()

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

Try to relay an array into newArray when looping:

var auctions = Auction.auctions;
var auctionIndex;
var auction;
var newAuctions = [];

for (
  auctionIndex = 0; 
  auctionIndex < Auction.auctions.length;
  auctionIndex++) {

  auction = auctions[auctionIndex];

  if (auction.seconds >= 0) { 
    newAuctions.push(
      auction);
  }    
}

Auction.auctions = newAuctions;

Here is another example for the proper use of splice. This example is about to remove 'attribute' from 'array'.

for (var i = array.length; i--;) {
    if (array[i] === 'attribute') {
        array.splice(i, 1);
    }
}

Another simple solution to digest an array elements once:

while(Auction.auctions.length){
    // From first to last...
    var auction = Auction.auctions.shift();
    // From last to first...
    var auction = Auction.auctions.pop();

    // Do stuff with auction
}

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.

There are lot of wonderful answers on this thread already. However I wanted to share my experience when I tried to solve "remove nth element from array" in ES5 context.

JavaScript arrays have different methods to add/remove elements from start or end. These are:

arr.push(ele) - To add element(s) at the end of the array 
arr.unshift(ele) - To add element(s) at the beginning of the array
arr.pop() - To remove last element from the array 
arr.shift() - To remove first element from the array 

Essentially none of the above methods can be used directly to remove nth element from the array.

A fact worth noting is that this is in contrast with java iterator's using which it is possible to remove nth element for a collection while iterating.

This basically leaves us with only one array method Array.splice to perform removal of nth element (there are other things you could do with these methods as well, but in the context of this question I am focusing on removal of elements):

Array.splice(index,1) - removes the element at the index 

Here is the code copied from original answer (with comments):

var arr = ["one", "two", "three", "four"];_x000D_
var i = arr.length; //initialize counter to array length _x000D_
_x000D_
while (i--) //decrement counter else it would run into IndexOutBounds exception_x000D_
{_x000D_
  if (arr[i] === "four" || arr[i] === "two") {_x000D_
    //splice modifies the original array_x000D_
    arr.splice(i, 1); //never runs into IndexOutBounds exception _x000D_
    console.log("Element removed. arr: ");_x000D_
_x000D_
  } else {_x000D_
    console.log("Element not removed. arr: ");_x000D_
  }_x000D_
  console.log(arr);_x000D_
}

Another noteworthy method is Array.slice. However the return type of this method is the removed elements. Also this doesn't modify original array. Modified code snippet as follows:

var arr = ["one", "two", "three", "four"];_x000D_
var i = arr.length; //initialize counter to array length _x000D_
_x000D_
while (i--) //decrement counter _x000D_
{_x000D_
  if (arr[i] === "four" || arr[i] === "two") {_x000D_
    console.log("Element removed. arr: ");_x000D_
    console.log(arr.slice(i, i + 1));_x000D_
    console.log("Original array: ");_x000D_
    console.log(arr);_x000D_
  }_x000D_
}

Having said that, we can still use Array.slice to remove nth element as shown below. However it is lot more code (hence inefficient)

var arr = ["one", "two", "three", "four"];_x000D_
var i = arr.length; //initialize counter to array length _x000D_
_x000D_
while (i--) //decrement counter _x000D_
{_x000D_
  if (arr[i] === "four" || arr[i] === "two") {_x000D_
    console.log("Array after removal of ith element: ");_x000D_
    arr = arr.slice(0, i).concat(arr.slice(i + 1));_x000D_
    console.log(arr);_x000D_
  }_x000D_
_x000D_
}

The Array.slice method is extremely important to achieve immutability in functional programming à la redux

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.

Give this a try

RemoveItems.forEach((i, j) => {
    OriginalItems.splice((i - j), 1);
});

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);

Auction.auctions = Auction.auctions.filter(function(el) {
  return --el["seconds"] > 0;
});

Here is a simple linear time solution to this simple linear time problem.

When I run this snippet, with n = 1 million, each call to filterInPlace() takes .013 to .016 seconds. A quadratic solution (e.g. the accepted answer) would take a million times that, or so.

// Remove from array every item such that !condition(item).
function filterInPlace(array, condition) {
   var iOut = 0;
   for (var i = 0; i < array.length; i++)
     if (condition(array[i]))
       array[iOut++] = array[i];
   array.length = iOut;
}

// Try it out.  A quadratic solution would take a very long time.
var n = 1*1000*1000;
console.log("constructing array...");
var Auction = {auctions: []};
for (var i = 0; i < n; ++i) {
  Auction.auctions.push({seconds:1});
  Auction.auctions.push({seconds:2});
  Auction.auctions.push({seconds:0});
}
console.log("array length should be "+(3*n)+": ", Auction.auctions.length)
filterInPlace(Auction.auctions, function(auction) {return --auction.seconds >= 0; })
console.log("array length should be "+(2*n)+": ", Auction.auctions.length)
filterInPlace(Auction.auctions, function(auction) {return --auction.seconds >= 0; })
console.log("array length should be "+n+": ", Auction.auctions.length)
filterInPlace(Auction.auctions, function(auction) {return --auction.seconds >= 0; })
console.log("array length should be 0: ", Auction.auctions.length)

Note that this modifies the original array in place rather than creating a new array; doing it in place like this can be advantageous, e.g. in the case that the array is the program's single memory bottleneck; in that case, you don't want to create another array of the same size, even temporarily.

If you are e using ES6+ - why not just use Array.filter method?

Auction.auctions = Auction.auctions.filter((auction) => {
  auction['seconds'] --;
  return (auction.seconds > 0)
})  

Note that modifying the array element during filter iteration only works for objects and will not work for array of primitive values.

Deleting Parameters

        oldJson=[{firstName:'s1',lastName:'v1'},
                 {firstName:'s2',lastName:'v2'},
                 {firstName:'s3',lastName:'v3'}]
        
        newJson = oldJson.map(({...ele}) => {
          delete ele.firstName;
          return ele;
          })

it deletes and and create new array and as we are using spread operator on each objects so the original array objects are also remains unharmed