Understanding the Role of 'trackBy' Function in 'ngFor' Directive in Angular

The 'trackBy' function in the 'ngFor' directive plays a significant part in Angular applications by enhancing performance through tracking object identity. This function is particularly useful when dealing with lists or arrays of items, where Angular needs to keep track of the items as they are added, moved or removed.

Typically, whenever a list is changed, Angular re-renders the entire DOM for that list, which can be computationally expensive, especially for larger lists or when the list changes often. However, by using 'trackBy', Angular can keep track of each item's identity and only update the DOM for that specific item that has changed, not the entire list, thus improving performance by reducing unnecessary operations.

<my-element *ngFor="let item of items; trackBy: trackItem">...</my-element>
...
trackItem(index, item) {
  return item.id; //unique id for the item
}

In the above example, the 'trackBy' function returns a unique id (e.g., item.id) for each item. Angular uses this id to track each item's identity across updates or changes. When the items list changes, only the elements corresponding to items that added, removed, or moved are affected in the DOM.

This approach of using 'trackBy' function, while underutilized, is a powerful strategy in enhancing performance in Angular applications. It's particularly beneficial in applications with interactive lists, where items can frequently be added, removed or reordered.

It's also worth noting that while the 'trackBy' function can increase application performance, it's not always necessary to implement it. For smaller lists that don't change frequently, the overhead of implementing 'trackBy' might be more than the performance improvement it brings. However, for larger lists with frequent updates, the 'trackBy' function can make a significant difference in application performance.

In conclusion, by understanding the role of the 'trackBy' function and appropriately using it, developers can enhance the performance of their Angular applications significantly.