In this article, I am going to discuss the Angular ngFor trackBy with some examples. Please read our previous article before proceeding to this article where we discussed Angular ngFor Directive in detail. The ngFor trackBy is used to improve the performance of an angular application. At the end of this article, you will understand what exactly is Angular ngFor trackBy and when and how to use this in Angular Application.
The use of trackBy is to improve the performance of the angular application. It is usually not needed by default but needed only when your application running into performance issues.
The Angular ngFor directive may perform poorly with the large collections. Suppose, we have some data coming from some API and we are storing these data into some kind of collection like an array and then we need to update these data over the webpage using ngFor directive. By default, what angular framework will do is, it will remove all the DOM elements that are associated with the data and will create them again in the DOM tree even if the same data is coming.
Let us understand the need for angular ngFor trackBy with an example step by step. Open app. We are going to use some styles in our HTML page. So, open app. Open your root module i. To confirm this launch the browser developer tools by pressing the F12 key. This is because Angular Framework by default keeps track of the objects using the object references. Image what could happen if the data size is huge. In order to solve the above performance issue problem, the angular framework provides one function called trackBy which will help us to track the items which have been added or removed.
The trackBy function will take two arguments, first is the index and the second one is the current item and it will return one unique identifier as return a value using which we can track that item. In our example, we are going to track by Student ID as the student id is unique for each student. Here we are using the trackBy along with ngFor directive.
At this point, run the application and then launch the browser developer tools by pressing the F12 key. In the next article, I am going to discuss Angular ngIf Directive with then and else block in detail with some examples. Here, in this article, I try to explain the Angular ngFor trackBy with examples.
I saw a few articles hereherehereand here that say using trackBy will improve performance and has better memory management. But I was wondering if trackBy is such an improvement, then why isn't it default behavior?
Is it default behavior and everything I am looking at is out of date? I also want to avoid adding a service and importing that 90 times. Returning index alone has a use case but it's a fairly uncommon one. It's basically telling angular to never rerender the existing items in this list since the index of a given item will never change.
This usually is a very unexpected behavior for developers since the initialization lifecycle hooks of sub components won't reexecute. This is generally safe to do though in ngFor's that don't have sub components but these kinds of lists are generally more performant anyway and you won't see much benefit unless the lists are very long or change frequently.
The idea of trackBy is to allow you to reinitialize items in lists that need it and not reinitialize ones that don't.
It isn't a silver bullet for blindly increasing performance like some people treat it, it's purpose and functionality should be fully understood. Keep in mind that just because an item has a unique ID doesn't mean it is appropriate to use in a trackBy function. If the ID stays the same but the contents can change, depending on how you've built a certain component, that component might need to be reinitialized anyway. Learn more. Angular efficiently using trackBy with ngFor Ask Question. Asked 1 year, 11 months ago.
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Angular ngFor - Learn all Features including trackBy, why is it not only for Arrays?
There is no global way to set trackBy as far as I know, nor would it make much sense since it would need to be overridden in some cases. The closest you could get is some kind of base "ListComponent" that implements a default trackByFn of your own design, but you would still need to explicitly declare its usage in your templates.
Thank you for the response, and lets assume all of my objects have id's because they all come from a db. We have a component where we add common functions, but we would need to add it to the import, providers, and constructor to be able to reference it in the html, which is the same number of lines as a return item.
This may be the same number of lines as importing and constructing, but it will be infinitely easier to change if it ever needs to be changed. If you get good enough with angular cli, you can actually set it up so that it can scaffold this component for you. Sign up or log in Sign up using Google.
Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog.We will show you how to use ngFor using an example code.
We further look at the local variables like IndexFirstLastodd and evenwhich is exported by the ngFor directive. Finally, we look at the Track By trackBy clausewhich enhances the performances of the ngFor. The let keyword creates a local variable named item. You can then use this variable to reference the individual item in the items collection. You can use this variable anywhere inside your template. Then use the ngFor directive to tell angular to repeat that block of HTML elements for each item in the list.
First, you need to create an angular Application. You can also download the code for this tutorial from the gitHub.
The starting point for the project is found in Start folder and the completed project is in the folder ngFor. Open the app. We will build a template to display these movies in a tabular form.
Angular 9/8/7 ngFor with Index and trackBy Example
The next step is to create HTML template. Our purpose is to display the list of movies. Hence we build a tr element of a table, which can display a single movie. We want the tr element to be repeated for each item in the movies collection.
Hence we apply ngFor directive to the tr element of the table. You can use the variable to create the template. Run the project using npm start command and you should be able to see the list of movies displayed in your browser. We can assign the values of these exported values to the local variable and use it in our template.
Using additional let operator, we can create an additional local variable and assign the value of the index to it. The syntax is shown below. Then use this to change the class name to either odd or even. The example code is shown below.
Track By clause allows you to specify your own key to identify objects. Angular 2 generates unique IDs. Although the retrieved items are similar to those available in the DOM, the Angular have no means to identify them.Structural directives are used to make changes in HTML layout by adding, removing, repeating or manipulating elements in DOM structure. NgFor directive is added to element as an attribute property which takes a collection which can be an Array of Items or an Object.
In some situations, we may need to get Index of the current item in the collection. Suppose we want to show S. There are available keywords which returns boolean values true or false. Which can be used as follows:. Skip to content. Can't bind to 'ngForOf' since it isn't a known property of 'div'. Subscribe to Latest Tutorials. Connect with. I allow to create an account.Angular 6 Tutorial 10: Looping with *ngFor
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Once your account is created, you'll be logged-in to this account. Disagree Agree. This comment form is under antispam protection. Notify of.In this quick how-to tutorial we'll be learning about the ngFor directive in Angular and how to use it to display arrays of data in your Angular templates. For example, let's consider the following array of objects representing a set of products in a component:.
Let's see how ngFor works by building a practical example. We'll only see how to create a component with an associated HTML template so you should already have an Angular development environment ready with Angular CLI v9 installed and a project generated.
Both these two things can be done with the following two commands:. Go ahead and navigate inside inside your project's folder and generate a component with the following commands:. Let's understand the basic syntax if ngFor :. The variable used to represent the data availabe for each iteration inside an ngFor directive is only only visible inside the loop.
You can get the index of the current element in the ngFor loop by using the index variable:. Check out this tutorial for a complete Angular 8 example that demonstrates how we can use ngFor to display an array of fetched data in our HTML template.
We'll be using Angular 9 for the example. We'll learn about these things: What ngFor does in Angular and how it's used? How to find the index position of an element inside an ngFor loop?
Getting the first and the last element inside an ngFor loop Let's get started with our tutorial to master ngFor in Angular!
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Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. So, I am trying to get the index of the ngFor in order to do a comparison to array of data before triggering another function.
For example, the test function will be triggered when I click a button, then it will check the current index, and do something with it. You can follow this answer. Learn more. Angular: Get current index in ngFor Ask Question. Asked 2 years, 1 month ago. Active 1 year, 6 months ago. Viewed 27k times. How would I go about so that I can get the current index? Steve Kim. Steve Kim Steve Kim 3, 7 7 gold badges 29 29 silver badges 61 61 bronze badges.
Where are you going to get index? What is the current index outside ngFor scope in your opinion? For example, the test function will be triggered when I click a button, then it will check the current indexand do something with it.
Outside of that div, your "loop" is complete, you just have the data array. I see. Thank you. I understand. Active Oldest Votes. As per OP's comment on question : So, I am trying to get the index of the ngFor in order to do a comparison to array of data before triggering another function. Vivek Doshi Vivek Doshi Yeah, I understand about your last comment. I will be careful.
Just a quick question. What would be the best way to call the test function only when the ngFor is done iterating? Thus, I can see the last index number. If you want to get last index then you can use array.
The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. As pointed out in Eric comment, and after lots of reading and playing around, here is how to use trackBy in angular2. As you already recognized, using a function is the only way to use trackBy in Angular 2. Starting with Angular 2. To avoid this expensive operation, you can customize the default tracking algorithm.
If trackBy is given, Angular tracks changes by the return value of the function. It's especially usefull when using Subscription which often looks like what I schematized with array. So it would looks like:. So, if you create all new objects in the list and then use ngForit will render whole list. Let's consider a scenario where despite of all ngFor optimizations, the rendering is still taking time. In that case we use trackBy. So that, we can provide another parameter to track objects than the object identity which is a default tracking criteria.
Learn more. Asked 4 years ago. Active 7 months ago. Viewed 87k times. Sujatha Girijala 7 7 silver badges 20 20 bronze badges. Zalaboza Zalaboza 7, 15 15 gold badges 52 52 silver badges bronze badges. Can you please update me what is the use of trackBy in ngFor? Eric's answer seems to be working fine. Any other link? Still I'm not clear with clean usage of trackBy. Active Oldest Votes. As pointed out in Eric comment, and after lots of reading and playing around, here is how to use trackBy in angular2 the first thing you need to know its not same syntax as angular1, now you need to separate it from the for loop with a.
I do not think that "trackBy:post?. In my case, the state of the child element is lost. A separate trackBy function works though but is definitely not convenient. By the way, the elvis operator itself indicates that something is wrong. If it worked, then you would not have to use it here unless the collection items can be null.
I confirm, trackBy only works with functions, it doesn't error with "post?. In addition to Olivier comment. Right Zalaboza, only function is allowed, please update your post. Discussed here: github.