We Keep Coding

How to append contents of one array onto another without intermediate copy of all the data?

Is there any clean and efficient way to add the contents of one array directly to another array without making an intermediate/temporary copy of all the data?
For example, you can use .push() to add the contents of one array directly onto another like this:
// imagine these are a both large arrays
let base = [1,2,3];
let data = [4,5,6]
base.push(...data);
But, that seemingly makes a copy of all the items in data as it makes them arguments to .push(). The same is true for .splice():
// imagine these are a both large arrays
let base = [1,2,3];
let data = [4,5,6]
base.splice(base.length, 0, ...data);
Both of these seem inefficient from a memory point of view (extra copy of all the data) and from an execution point of view (the data gets iterated twice).
Methods like .concat() don't add the contents of one array to another, but rather make a new array with the combined contents (which copies the contents of both arrays).
I've got some big arrays with lots of manipulations and I'm trying to ease the burden on the CPU/garbage collector by avoiding unnecessary intermediate copies of things and I find it curious that I haven't found such a built-in operation. So far, my best option for avoiding unnecessary copies has been this:
// imagine these are a both large arrays
let base = [1,2,3];
let data = [4,5,6];
for (let item of data) {
base.push(item);
}
which seems like it's probably not as efficient in execution as it could be if it was one operation and obviously it's multiple lines of code when you'd like it to be one line.

Per, Sebastian's helpful comments, the ideal case would be the fast-path optimization for code like this:
array.push(...data)
that V8 now deploys where it detects data as a known type of iterable and then takes optimized shortcuts to grow the target array once and then copy the data over without making the intermediate copy on the stack.
But, apparently the current V8 does not apply such an optimization to this specific case. When I tried this in node v14.3:
const targetCnt = 100_000;
const sourceCnt = 100_000_000;
// create initial arrays
let target = new Array(targetCnt);
let source = new Array(sourceCnt);
target.fill(1);
source.fill(2);
let b1 = new Bench().markBegin();
target.push(...source);
b1.markEnd();
console.log(`.push(...source): ${b1.formatNs()}`);
I got this error:
FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
And, when I reduced the sourceCnt to 1_000_000, then I got this error:
RangeError: Maximum call stack size exceeded
So, I guess that optimization only applies to other circumstances mentioned in the article, not to this one.
So, it seems that until that fast path optimization is considered ubiquitous in all possible targets and you know exactly which situations it is applicable to in your code (I wonder if it would ever be codified in a spec?) and there's no danger of passing an unknown iterable in your code that wouldn't get such preferential treatment, perhaps the best option is to just make your own mini-version of the optimization as a function:
function appendToArray(targetArray, sourceArray) {
// grow the target
let targetIndex = targetArray.length;
let sourceLen = sourceArray.length;
targetArray.length = targetIndex + sourceLen;
// copy the data
for (let sourceIndex = 0; sourceIndex < sourceLen; sourceIndex++, targetIndex++) {
targetArray[targetIndex] = sourceArray[sourceIndex];
}
return targetArray;
}

ES6 class getter, temporary return or alternative solution

I am trying to solve a problem I am seeing when rendering a list of items in my ui that is coming out of a es6 class I have created. The model is working great, however I am using animations that are listening to (in react) mount, onEnter, and onLeave of the items.
When I apply my filters and sorting via the model and spit back the new list of items via the getter, the animations do not apply to some items because the list is just being re sorted, not necessarily changed.
So my getter just grabs this.products of the class and returns it and applies a sort order to it. And if filters are applied (which are tracked by this._checkedList in the class), the this.products is reduced based on which filters are selected then sorted. So that getter looks like so :
get productList() {
if (this._checkedList.length > 0) {
const filteredProducts = _.reduce(this.filterMap, reduceFilters, []);
const deDuped = _.uniq(filteredProducts, 'id');
return this.applySort(deDuped);
}
const deDuped = _.uniq(this.products, 'id');
return this.applySort(deDuped);
}
What I am trying to figure out, is a way to to temporarily send back an empty array while the filters or sorting run. The reason being the ui would receive an empty array (even if for a split second) and react would register the new sorted/filtered list as a new list and fire the enter/leave/mount animations again.
My attempt was to set a local property of the class like -
this._tempReturn = false;
then in the functions where the sort or filter happen, I set it to true, then back to false when the function is done like this -
toggleFilter(args) {
this._tempReturn = true;
...toggle logic
this._tempReturn = false;
}
Then changed the getter to check for that property before i do anything else, and if it's true, send back an empty array -
get productList() {
if (this._tempReturn) {
return [];
}
...
}
However, this does not seem to work. Even putting a console.log in the if (this._tempReturn) { didn't show any logs.
I also tried sending back a new list with lodash's _.cloneDeep like so :
get productList() {
if (this._checkedList.length > 0) {
const filteredProducts = _.reduce(this.filterMap, reduceFilters, []);
const deDuped = _.uniq(filteredProducts, 'id');
return _.cloneDeep(this.applySort(deDuped));
}
const deDuped = _.uniq(this.products, 'id');
return _.cloneDeep(this.applySort(deDuped));
}
this did not work either. So it seems the empty array return might be a better approach.
I am wondering if there is some way to achieve this - I would like to have the array be return empty for a second perhaps while the filters and sort are applying.
Very stuck on how to achieve, perhaps I am even looking at this problem from the wrong angle and there is a much better way to solve this. Any advice would be welcomed, thanks for reading!

In order to force a re-render of items in a list when updating them you just need to make sure that each items has a unique key property.
Instead of rendering the list, then rendering it as empty, then re-rendering a changed list make sure each child has a unique key. Changing the key property on a child in an array will always cause it to re-render.

Create multiple arrays from one set

I have an array of items, each have an item_type property that could be one of four different types (this is a cart of items). When I attempt to "pay" for the items in the cart, I need to separate all the items remaining into arrays of each item type to send back to the server so the proper tables can be updated. I was just going to use a map to do this but that would require multiple passes (at least the way I was thinking I needed to do it) but is there a quicker way to take one array and split it into multiple arrays based on a property?
Specifically I need to know which array is which (which one has raffle tickets, donations, etc.) so I can post them to the server with the correct property name so the server knows what array is what and how to proceed with each.
Currently in ember this is how I am working through this. Not a lot of code but still I wonder if there is some refactoring that can be done here
// ARRAYS OF CLASSES
itemsArray: Ember.computed.filterBy('model.items','item_type','bid'),
donationsArray: Ember.computed.filterBy('model.items','item_type','donation'),
ticketsArray: Ember.computed.filterBy('model.items','item_type','ticket'),
rafflesArray: Ember.computed.filterBy('model.items','item_type','raffle'),
// ARRAYS OF JUST CLASS IDS
itemIds: Ember.computed.mapBy('itemsArray','id'),
donationIds: Ember.computed.mapBy('donationsArray','id'),
ticketIds: Ember.computed.mapBy('ticketsArray','id'),
raffleIds: Ember.computed.mapBy('rafflesArray','id'),

Ok, so first of all your code is absolutely fine.
I was interested in comparing performance of using observer instead of creating 4 computed properties, so I created this twiddle. I haven't found good way to be absolutely sure 1 way is absolutely faster than another, but you here's another way you could use in your code.
createArrays: Ember.on('init', Ember.observer('model.items.#each.item_type', function() {
console.log('observer fired');
const items = this.get('model.items'),
itemsArray = [],
donationsArray = [],
rafflesArray = [],
ticketsArray = [];
items.forEach(item => {
switch (Ember.get(item, 'item_type')) {
case 'bid':
itemsArray.pushObject(item);
break;
case 'donation':
donationsArray.pushObject(item);
break;
case 'ticket':
ticketsArray.pushObject(item);
break;
default:
rafflesArray.pushObject(item);
break;
}
});
this.setProperties({
itemsArray,
donationsArray,
ticketsArray,
rafflesArray
});
})),
But I don't think it's worth changing to.
Another thing you could do is programatically create those 8 computed properties, but it would be more code than it is now probably. If you would have 10 instead of 4 item_type's it would be worth to do.

Is there a way to map a value in an object to the index of an array in javascript?

Prepending that a solution only needs to work in the latest versions of Chrome, Firefox, and Safari as a bonus.
-
I am trying to use an associative array for a large data set with knockout. My first try made it a true associative array:
[1: {Object}, 3: {Object},...,n:{Object}]
but knockout was not happy with looping over that. So I tried a cheating way, hoping that:
[undefined, {Object}, undefined, {Object},...,{Object}]
where the location in the array is the PK ID from the database table. This array is about 3.2k items large, and would be iterated over around every 10 seconds, hence the need for speed. I tried doing this with a splice, e.g.
$.each(data, function (index, item) {
self.myArray.splice(item.PKID, 0, new Object(item));
}
but splice does not create indices, so since my first PKID is 1, it is still inserted at myArray[0] regardless. If my first PK was 500, it would start at 0 still.
My second thought is to initialize the array with var myArray = new Array(maxSize) but that seems heavy handed. I would love to be able to use some sort of map function to do this, but I'm not really sure how to make the key value translate into an index value in javascript.
My third thought was to keep two arrays, one for easy look up and the other to store the actual values. So it combines the first two solutions, almost, by finding the index of the object in the first example and doing a lookup with that in the second example. This seems to be how many people manage associative arrays in knockout, but with the array size and the fact that it's a live updating app with a growing data set seems memory intensive and not easily manageable when new information is added.
Also, maybe I'm hitting the mark wrong here? We're putting these into the DOM via knockout and managing with a library called isotope, and as I mentioned it updates about every 10 seconds. That's why I need the fast look up but knockout doesn't want to play with my hash table attempts.
--
clarity edits:
so on initial load the whole array is loaded up (which is where the new Array(maxLength) would go, then every 10 seconds anything that has changed is loaded back. That is the information I'm trying to quickly update.
--
knockout code:
<!-- ko foreach: {data: myArray(), afterRender: setInitialTileColor } -->
<div class="tile" data-bind="attr: {id: 'tileID' + $data.PKID()}">
<div class="content">
</div>
</div>
<!-- /ko -->
Then on updates the hope is:
$.each(data.Updated, function (index, item) {
var obj = myModel.myArray()[item.PKID];
//do updates here - need to check what kind of change, how long it's been since a change, etc
}

Here is a solution how to populate array items with correct indexes, so it doesn't start from the first one (0 (zero) I meant)
just use in loop
arr[obj.PKID] = obj;
and if your framework is smart (to use forEach but not for) it will start from your index (like 500 in case below)
http://jsfiddle.net/0axo9Lgp/
var data = [], new_data = [];
// Generate sample array of objects with index field
for (var i = 500; i < 3700; i++) {
data.push({
PKID: i,
value: '1'
});
}
data.forEach(function(item) {
new_data[item.PKID] = item;
});
console.log(new_data);
console.log(new_data.length); // 3700 but real length is 3200 other items are undefined

It's not an easy problem to solve. I'm assuming you've tried (or can't try) the obvious stuff like reducing the number of items per page and possibly using a different framework like React or Mithril.
There are a couple of basic optimizations I can suggest.
Don't use the framework's each. It's either slower than or same as the native Array method forEach, either way it's slower than a basic for loop.
Don't loop over the array over and over again looking for every item whose data has been updated. When you send your response of data updates, send along an array of the PKIds of the updated item. Then, do a single loop:
.
var indexes = []
var updated = JSON.parse(response).updated; // example array of updated pkids.
for(var i=0;i<allElements.length;i++){
if(updated.indexOf(allElements[i].pkid)>-1)
indexes.push(i);
}
So, basically the above assumes you have a simple array of objects, where each object has a property called pkid that stores its ID. When you get a response, you loop over this array once, storing the indexes of all items that match a pk-id in the array of updated pk-ids.
Then you only have to loop over the indexes array and use its elements as indexes on the allElements array to apply the direct updates.

If your indexes are integers in a reasonable range, you can just use an array. It does not have to be completely populated, you can use the if binding to filter out unused entries.
Applying updates is just a matter of indexing the array.
http://jsfiddle.net/0axo9Lgp/2/

You may want to consider using the publish-subscribe pattern. Have each item subscribe to its unique ID. When an item needs updating it will get the event and update itself. This library may be helpful for this. It doesn't depend upon browser events, just arrays so it should be fairly fast.

Display posts in descending posted order

I'm trying to test out Firebase to allow users to post comments using push. I want to display the data I retrieve with the following;
fbl.child('sell').limit(20).on("value", function(fbdata) {
// handle data display here
}
The problem is the data is returned in order of oldest to newest - I want it in reversed order. Can Firebase do this?

Since this answer was written, Firebase has added a feature that allows ordering by any child or by value. So there are now four ways to order data: by key, by value, by priority, or by the value of any named child. See this blog post that introduces the new ordering capabilities.
The basic approaches remain the same though:
1. Add a child property with the inverted timestamp and then order on that.
2. Read the children in ascending order and then invert them on the client.
Firebase supports retrieving child nodes of a collection in two ways:
by name
by priority
What you're getting now is by name, which happens to be chronological. That's no coincidence btw: when you push an item into a collection, the name is generated to ensure the children are ordered in this way. To quote the Firebase documentation for push:
The unique name generated by push() is prefixed with a client-generated timestamp so that the resulting list will be chronologically-sorted.
The Firebase guide on ordered data has this to say on the topic:
How Data is Ordered
By default, children at a Firebase node are sorted lexicographically by name. Using push() can generate child names that naturally sort chronologically, but many applications require their data to be sorted in other ways. Firebase lets developers specify the ordering of items in a list by specifying a custom priority for each item.
The simplest way to get the behavior you want is to also specify an always-decreasing priority when you add the item:
var ref = new Firebase('https://your.firebaseio.com/sell');
var item = ref.push();
item.setWithPriority(yourObject, 0 - Date.now());
Update
You'll also have to retrieve the children differently:
fbl.child('sell').startAt().limitToLast(20).on('child_added', function(fbdata) {
console.log(fbdata.exportVal());
})
In my test using on('child_added' ensures that the last few children added are returned in reverse chronological order. Using on('value' on the other hand, returns them in the order of their name.
Be sure to read the section "Reading ordered data", which explains the usage of the child_* events to retrieve (ordered) children.
A bin to demonstrate this: http://jsbin.com/nonawe/3/watch?js,console

Since firebase 2.0.x you can use limitLast() to achieve that:
fbl.child('sell').orderByValue().limitLast(20).on("value", function(fbdataSnapshot) {
// fbdataSnapshot is returned in the ascending order
// you will still need to order these 20 items in
// in a descending order
}
Here's a link to the announcement: More querying capabilities in Firebase

To augment Frank's answer, it's also possible to grab the most recent records--even if you haven't bothered to order them using priorities--by simply using endAt().limit(x) like this demo:
var fb = new Firebase(URL);
// listen for all changes and update
fb.endAt().limit(100).on('value', update);
// print the output of our array
function update(snap) {
var list = [];
snap.forEach(function(ss) {
var data = ss.val();
data['.priority'] = ss.getPriority();
data['.name'] = ss.name();
list.unshift(data);
});
// print/process the results...
}
Note that this is quite performant even up to perhaps a thousand records (assuming the payloads are small). For more robust usages, Frank's answer is authoritative and much more scalable.
This brute force can also be optimized to work with bigger data or more records by doing things like monitoring child_added/child_removed/child_moved events in lieu of value, and using a debounce to apply DOM updates in bulk instead of individually.
DOM updates, naturally, are a stinker regardless of the approach, once you get into the hundreds of elements, so the debounce approach (or a React.js solution, which is essentially an uber debounce) is a great tool to have.

There is really no way but seems we have the recyclerview we can have this
query=mCommentsReference.orderByChild("date_added");
query.keepSynced(true);
// Initialize Views
mRecyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
mManager = new LinearLayoutManager(getContext());
// mManager.setReverseLayout(false);
mManager.setReverseLayout(true);
mManager.setStackFromEnd(true);
mRecyclerView.setHasFixedSize(true);
mRecyclerView.setLayoutManager(mManager);

I have a date variable (long) and wanted to keep the newest items on top of the list. So what I did was:
Add a new long field 'dateInverse'
Add a new method called 'getDateInverse', which just returns: Long.MAX_VALUE - date;
Create my query with: .orderByChild("dateInverse")
Presto! :p

You are searching limitTolast(Int x) .This will give you the last "x" higher elements of your database (they are in ascending order) but they are the "x" higher elements
if you got in your database {10,300,150,240,2,24,220}
this method:
myFirebaseRef.orderByChild("highScore").limitToLast(4)
will retrive you : {150,220,240,300}

In Android there is a way to actually reverse the data in an Arraylist of objects through the Adapter. In my case I could not use the LayoutManager to reverse the results in descending order since I was using a horizontal Recyclerview to display the data. Setting the following parameters to the recyclerview messed up my UI experience:
llManager.setReverseLayout(true);
llManager.setStackFromEnd(true);
The only working way I found around this was through the BindViewHolder method of the RecyclerView adapter:
#Override
public void onBindViewHolder(final RecyclerView.ViewHolder holder, int position) {
final SuperPost superPost = superList.get(getItemCount() - position - 1);
}
Hope this answer will help all the devs out there who are struggling with this issue in Firebase.

Firebase: How to display a thread of items in reverse order with a limit for each request and an indicator for a "load more" button.
This will get the last 10 items of the list
FBRef.child("childName")
.limitToLast(loadMoreLimit) // loadMoreLimit = 10 for example
This will get the last 10 items. Grab the id of the last record in the list and save for the load more functionality. Next, convert the collection of objects into and an array and do a list.reverse().
LOAD MORE Functionality: The next call will do two things, it will get the next sequence of list items based on the reference id from the first request and give you an indicator if you need to display the "load more" button.
this.FBRef
.child("childName")
.endAt(null, lastThreadId) // Get this from the previous step
.limitToLast(loadMoreLimit+2)
You will need to strip the first and last item of this object collection. The first item is the reference to get this list. The last item is an indicator for the show more button.
I have a bunch of other logic that will keep everything clean. You will need to add this code only for the load more functionality.
list = snapObjectAsArray; // The list is an array from snapObject
lastItemId = key; // get the first key of the list
if (list.length < loadMoreLimit+1) {
lastItemId = false;
}
if (list.length > loadMoreLimit+1) {
list.pop();
}
if (list.length > loadMoreLimit) {
list.shift();
}
// Return the list.reverse() and lastItemId
// If lastItemId is an ID, it will be used for the next reference and a flag to show the "load more" button.
}

I'm using ReactFire for easy Firebase integration.
Basically, it helps me storing the datas into the component state, as an array. Then, all I have to use is the reverse() function (read more)
Here is how I achieve this :
import React, { Component, PropTypes } from 'react';
import ReactMixin from 'react-mixin';
import ReactFireMixin from 'reactfire';
import Firebase from '../../../utils/firebaseUtils'; // Firebase.initializeApp(config);
#ReactMixin.decorate(ReactFireMixin)
export default class Add extends Component {
constructor(args) {
super(args);
this.state = {
articles: []
};
}
componentWillMount() {
let ref = Firebase.database().ref('articles').orderByChild('insertDate').limitToLast(10);
this.bindAsArray(ref, 'articles'); // bind retrieved data to this.state.articles
}
render() {
return (
<div>
{
this.state.articles.reverse().map(function(article) {
return <div>{article.title}</div>
})
}
</div>
);
}
}

There is a better way. You should order by negative server timestamp. How to get negative server timestamp even offline? There is an hidden field which helps. Related snippet from documentation:
var offsetRef = new Firebase("https://<YOUR-FIREBASE-APP>.firebaseio.com/.info/serverTimeOffset");
offsetRef.on("value", function(snap) {
var offset = snap.val();
var estimatedServerTimeMs = new Date().getTime() + offset;
});

To add to Dave Vávra's answer, I use a negative timestamp as my sort_key like so
Setting
const timestamp = new Date().getTime();
const data = {
name: 'John Doe',
city: 'New York',
sort_key: timestamp * -1 // Gets the negative value of the timestamp
}
Getting
const ref = firebase.database().ref('business-images').child(id);
const query = ref.orderByChild('sort_key');
return $firebaseArray(query); // AngularFire function
This fetches all objects from newest to oldest. You can also $indexOn the sortKey to make it run even faster

I had this problem too, I found a very simple solution to this that doesn't involved manipulating the data in anyway. If you are rending the result to the DOM, in a list of some sort. You can use flexbox and setup a class to reverse the elements in their container.
.reverse {
display: flex;
flex-direction: column-reverse;
}

myarray.reverse(); or this.myitems = items.map(item => item).reverse();

I did this by prepend.
query.orderByChild('sell').limitToLast(4).on("value", function(snapshot){
snapshot.forEach(function (childSnapshot) {
// PREPEND
});
});

Someone has pointed out that there are 2 ways to do this:
Manipulate the data client-side
Make a query that will order the data
The easiest way that I have found to do this is to use option 1, but through a LinkedList. I just append each of the objects to the front of the stack. It is flexible enough to still allow the list to be used in a ListView or RecyclerView. This way even though they come in order oldest to newest, you can still view, or retrieve, newest to oldest.

You can add a column named orderColumn where you save time as
Long refrenceTime = "large future time";
Long currentTime = "currentTime";
Long order = refrenceTime - currentTime;
now save Long order in column named orderColumn and when you retrieve data
as orderBy(orderColumn) you will get what you need.

just use reverse() on the array , suppose if you are storing the values to an array items[] then do a this.items.reverse()
ref.subscribe(snapshots => {
this.loading.dismiss();
this.items = [];
snapshots.forEach(snapshot => {
this.items.push(snapshot);
});
**this.items.reverse();**
},

For me it was limitToLast that worked. I also found out that limitLast is NOT a function:)
const query = messagesRef.orderBy('createdAt', 'asc').limitToLast(25);
The above is what worked for me.

PRINT in reverse order
Let's think outside the box... If your information will be printed directly into user's screen (without any content that needs to be modified in a consecutive order, like a sum or something), simply print from bottom to top.
So, instead of inserting each new block of content to the end of the print space (A += B), add that block to the beginning (A = B+A).
If you'll include the elements as a consecutive ordered list, the DOM can put the numbers for you if you insert each element as a List Item (<li>) inside an Ordered Lists (<ol>).
This way you save space from your database, avoiding unnecesary reversed data.