Ngfor not updating when array changes angular 6

I update the children array when changes are detected add, update, remove. When an item is added or updated inside the array, the ngFor is triggered and the DOM is updated. However, when an item is removed, the array is changed, but the DOM does not remove the entry for that item. It does when adding or updating. The same thing happens, adding and removing works fine, removing not. I generate the slides using ngFor, based on an array.

When I update a slide, no problem. But when I try to remove a slide, nothing happens. I recommend you read about ngZone. But it was zonejs who decides when to update the DOM. PS I never used it. If you missed something by the update, it might result in such a weird behavior. Check out the following post for more details:. DOM not updating when removing item from ngFor array ionic ionic-v3. Any help would be greatly appreciated. I also tried replacing the entire array, but no luck.

Even if I do: this. You debugged it? Are you sure that your remove method is called? Yes I am sure. The array has 0 items, however the DOM still contains one or more items. For me it works. I use splice too… However I use object. In you example, what is the type of bars? Does it contain primitive types or complex objects.

How does NgZone work?Stay connected to Telerik Blogs for. I am getting an array of "product"s from a resolver getting data from a json endpoint. Parameters are ComparatorFunction is a function which specifies the order of the elements, If not passed, Default sorting is applied. An example of a filter is the 'uppercase' filter which takes on a string output and formats the string and displays all the characters in the string as uppercase.

In this tutorial, we are going to discuss how to use a simple Array in AngularJs application. The orderBy filter facilitates you to sort an array. Following is the syntax of implementing sorting functionality for columns in angularjs ui grid.

This time Angular detects that the array reference has changed. Sort Array with the array as a paramater. Please try again later. This filter reduces arrays into sub arrays based on conditions. This tells ng-options directive to loop through the array, and generate option element containing the current value.

Definition and Usage. We will build the searching and sorting of the list of data that loaded to an array. Angular newer released decided not to support these filters out-of-box. Following is the content of the modified module descriptor app. This is the simplest and commonly used expression.

Angular 6 Features. Syntax: angular. It was written to demonstrate to a friend, the simplicity and power of Angular to address programming challenges - in recruitment processes - quickly and effectively.

This question already has answers here: Sort array of objects by string property value 46 answers Closed 2 years ago.A tip in the spirit of performance optimization: optimize your coding by building, sharing and consuming reusable Angular components with Bit. This directive is used for rendering an array of iterable objects or simply arrays. It is the same as ng-repeat in AngularJS. The output will look like this:.

We have a User with name, address, and phone properties. We initialize the users array property in the NgForEx class.

We can extend the app to enable us to add more users to the users array. When we fill the form and click the Add User button, the values of the inputs are aggregated and added to the users array. The component re-renders to update the view of the new entry in the users array.

The rendering of the array in ngFor causes excessive DOM manipulations. Our above example is too simple to show any slowdowns in the app. Here, we populate the users array with users. No worries lemme explain with visuals. Now, when we add a new user to the users array like. The whole DOM. What problem we will face here? This will increase the time it takes for the browser to render the changes to the DOM.

The whole destruction and creation of DOM nodes especially on a large scale will seriously hamper the speed and performance of our app. When it sees a change, it runs a re-render or CD on the directive. Note : Components are Directives, but they are Directives with a view. What is are reference changes? A reference in a complex data structure in JS is the memory address where the object is stored in the Heap.In this post we are going to go over the ngFor core directive, namely we are going to go over the following:.

So let's get started diving into ngFor! Below you can also find a video version of this post if you prefer, and the running code of this post is available here. The core directive ngFor allows us to build data presentation lists and tables in our HTML templates.

Let's take for example the following data:. With ngFor we can print this data to the screen under the form of a data table, by generating HTML similar to this:. This template will generate the HTML table that we showed just above. We can see in this example the most common syntax for using ngFor :.

ngfor not updating when array changes angular 6

Note that the loop variable hero is only visible inside the loop, you would not be able to access it outside the ngFor section. If you have an AngularJs background, you will see this error a few times before you get used to the new Angular syntax:. This is because you have accidentally either used item in items instead of item of itemsor forgot to add the let keyword at the beginning of the expression:.

A very common requirement is to add to a list the numeric index position of its element. We can get the index of the current element by using the index variable:. Note that you need the let keyword to get the value of the index, otherwise you will get an error similar to this one:.

Another very common functionality needed when building tables is to be able to stripe a table by adding a different css class to the even or odd rows. Let's say that to the above table we want to add a CSS class even if the row is even and the CSS class odd if the row is odd. In order to so, we have a couple of variables available for that: even and oddthat can be used in the following way together with ngClass :. Just like the even and odd functionality, there are also two other variables that can be used to identify the first and the last elements of the list:.

This will add a CSS class named first to the first element of the list, and a CSS class named last to the last element of the list:. As the input list gets modified, ngFor will try to avoid to constantly create and destroy the DOM elements of the list, as this is an expensive operation. Also, when we pass to ngFor a new list, this does not mean that the whole list will be re-built, meaning all the DOM re-created.

Many of the existing DOM elements will be reused and only some values inside them will be overwritten, and the decision is taken for each element in the list separately.

In order to take that decision Angular needs to identify each list element in a unique way, because for example if we pass in a new list with a different order, Angular will try to identify the elements and re-order the DOM elements of the list without deleting them and recreating them. This means that if you build a list of new objects from scratch with the exact same values as the previous list and pass this newly built list to ngForAngular will not be able to tell that a given list item is already present or not.

From a point of view of object identity, the new list contains a whole new set of items, completely different from the previous set. This is the case if for example we query the data again from the backend.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Here's a link to plnkr. To fully understand the problem and possible solutions, we need to discuss Angular change detection -- for pipes and components. They don't remember anything, so they don't have any properties — just a transform method. For this question, " student of students sortByName:queryElem. One way to fix the array issue is to change the array reference each time a student is added — i.

We could do this with concat :. Although this works, our addNewStudent method shouldn't have to be implemented a certain way just because we're using a pipe. We want to use push to add to our array. Stateful pipes have state -- they normally have properties, not just a transform method. They may need to be evaluated even if their inputs haven't changed. This sounds like what we want, even though it is less efficient, since we want the pipe to execute even if the students reference hasn't changed.

If we simply make the pipe stateful, we get an error:. According to drewmoore's answer"this error only happens in dev mode which is enabled by default as of beta If you call enableProdMode when bootstrapping the app, the error won't get thrown. In development mode, tick also performs a second change detection cycle to ensure that no further changes are detected. If additional changes are picked up during this second cycle, bindings in the app have side-effects that cannot be resolved in a single change detection pass.

In this case, Angular throws an error, since an Angular application can only have one change detection pass during which all change detection must complete. We have a stateful pipe, and the output can change each time it is called — it can have side-effects and that's okay.

NgFor is evaluated after the pipe, so it should work fine. However, we can't really develop with this error being thrown, so one workaround is to add an array property i. See pixelbits's answer for this solution.

However, we can be more efficient, and as we'll see, we won't need the array property in the pipe implementation, and we won't need a workaround for the double change detection. By default, on every browser event, Angular change detection goes through every component to see if it changed — inputs and templates and maybe other stuff? If we know that a component only depends on its input properties and template eventsand that the input properties are immutable, we can use the much more efficient onPush change detection strategy.

Angular ngFor - Learn all Features including trackBy, why is it not only for Arrays?

With this strategy, instead of checking on every browser event, a component is checked only when the inputs change and when template events trigger. And, apparently, we don't get that Expression This is because an onPush component is not checked again until it is "marked" ChangeDetectorRef. So we still need a stateful pipe here. This is the solution EricMartinez suggested: stateful pipe with onPush change detection. See caffinatedmonkey's answer for this solution.

Note that with this solution the transform method doesn't need to return the same array each time. I find that a bit odd though: a stateful pipe with no state. Thinking about it some more Otherwise it could only be used with onPush components in dev mode. So after all that, I think I like a combination of Eric's and pixelbits's answers: stateful pipe that returns the same array reference, with onPush change detection if the component allows it.The Angular change detection mechanism is much more transparent and easier to reason about than its equivalent in Angular 1.

But there are still situations like when doing performance optimizations when we really need to know what's going on under the hood.

So let's dig deeper into change detection by going over the following topics:. Angular can detect when component data changes, and then automatically re-render the view to reflect that change. But how can it do so after such a low-level event like the click of a button, that can happen anywhere on the page? To understand how this works, we need to start by realizing that in Javascript the whole runtime is overridable by design.

We can override functions in String or Number if we so want. What happens is that Angular at startup time will patch several low-level browser APIs, such as for example addEventListenerwhich is the browser function used to register all browser events, including click handlers.

ngfor not updating when array changes angular 6

Angular will replace addEventListener with a new version that does the equivalent of this:. The new version of addEventListener adds more functionality to any event handler: not only the registered callback is called, but Angular is given a chance to run change detection and update the UI.

It's important to have an idea of what a zone is. A zone is nothing more than an execution context that survives multiple Javascript VM execution turns.

Angular 7 CRUD with Web API

It's a generic mechanism which we can use to add extra functionality to the browser. Angular uses Zones internally to trigger change detection, but another possible use would be to do application profiling, or keeping track of long stack traces that run across multiple VM turns.

In fact, many other browser APIs are patched by Zone. Have a look at the Zone. One limitation of this mechanism is that if by some reason an asynchronous browser API is not supported by Zone. This is, for example, the case of IndexedDB callbacks. Each Angular component has an associated change detector, which is created at application startup time.

For example, take the following TodoItem component:. This component will receive a Todo object as input and emit an event if the todo status is toggled. To make the example more interesting, the Todo class contains a nested object:. We can see that Todo has a property owner which is itself an object with two properties: first name and last name. We can actually see at runtime what the change detector looks like!

To see it lets just add some code in the Todo class to trigger a breakpoint when a certain property is accessed. When the breakpoint hits, we can walk through the stack trace and see change detection in action:.

Don't worry, you will never have to debug this code!

ngfor not updating when array changes angular 6

There is no magic involved either, it's just a plain Javascript method built at application startup time. But what does it do? This method might look strange at first, with all the strangely named variables.

But by digging deeper into it, we notice that it's doing something very simple: for each expression used in the template, it's comparing the current value of the property used in the expression with the previous value of that property. If the property value before and after is different, it will set isChanged to true, and that's it! We can see in the change detector code that also the properties of the owner nested object are being checked for differences. But only the firstname property is being compared, not the lastname property.

This is because lastname is not used in the component template! Also, the top-level id property of Todo is not compared by the same reason. By default, Angular Change Detection works by checking if the value of template expressions have changed.GitHub is home to over 50 million developers working together to host and review code, manage projects, and build software together. Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Already on GitHub? Sign in to your account. I'm using ngModel to bind values of an array into to number inputs. After some time making changes to the inputs, the bindings stop working properly updating the wrong inputs. But the issue only seems to happen on the inputs generated inside a ngFor. Keep one eye on the input at top right:. Notice that making changes in the second row of inputs does not reproduce the issue.

GitHub issues are for bug reports and feature requests. Either use a custom trackBy that tracks by index or use objects instead of primitive values. Thanks for the clarification zoechi. By reading the documentation, NgForTrackBy seems to be something made to improve change detection performance. I need does not display repeated data.

Because I need get input element value and compare with another value. I try to this way. How can I get this input element value. I was tried another way. Pls refer this link Angular 2 get Input element value Its not still wrk. Thanks a lot! This issue has been automatically locked due to inactivity. Please file a new issue if you are encountering a similar or related problem. Read more about our automatic conversation locking policy. Skip to content.

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ngfor not updating when array changes angular 6

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Angular NgFor: Everything you need to know

Copy link Quote reply. I'm submitting a Please tell us about your environment: Angular version: 2. Same issue I am facing. Hi I am beginner in angular 2,ionic 2 I create chat application using ionic2.