Track By

Angular: The Full Gamut Edition

Charlie Greenman
April 26, 2021
3 min read

In Angular, there are performance enhancements that are valuable here and there. It can be easy to miss them. So they are not integral towards the core documentation.

In particular when it comes to for loops for Angular, the way that change detection works is that if any part of the array is changed Angular will aggressively change the entire DOM. This is of course can lead to performance issues, if all we need is a particular piece of data to be changed. The Angular framework has an internal trackBy function to combat these performance leaks.

Using Track By in Angular

Using *ngFors in Angular is actually quite common. Suffice to say,an ngFor that parses through alot of data, as well as optimizing it for performance reasons, is very important. Even it's an application that has pagination through the backend, and we only allow a maximum of 50 rows at a time.

So What is trackBy?

What trackBy does is check the set as a whole for order changes (sorting, insertion, deletion). When the order changes, instead of removing all elements from the DOM and creating new ones, the trackBy function is used to identify which elements do not need to be removed from the DOM. This reduces the number of DOM calls, and also reduces the number of angular digest cycles.

Under the hood trackBy tracks an order change. It does this by taking in index of each item, and a unique identifier of that item. If item does not exists, from prior id's and indexes, then it knows to insert that one particular item into the DOM.

Track By Within Data Tables

Specifically, within our architecture, we call for using Material. By default, if a trackBy function is not given, Material Table will deeply compare the elements in the set. So, a trackBy function is really used to reduce the amount of checks necessary to compare elements in a set. Instead of a deep copy, you can check for a single unique property.

Track By in Practice

//.. this code is inside of our component class  trackByBuyerId(index, item) {return; // unique id corresponding to the item}

By doing the above, track by will in a material setting make sure that objects in the data table are not deeply compared, comparing objects directly to each other.

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