[SOLVED] Angular efficiently using trackBy with ngFor

Issue

In Angular, is the trackBy function necessary for *ngFor? I saw a few articles here, here, here, and 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 Is it default behavior and everything I am looking at is out of date?

If it isn’t default behavior, my project has about 90 *ngFor in 90 components and I was wondering if there was a way to use the trackBy where I am not including the following function 90 times. I also want to avoid adding a service and importing that 90 times.

HTML

<mat-option *ngFor="let l of list; trackBy: trackByFn" [value]="l.id">
   {{l.name}}
</mat-option>

TS

trackByFn(index, item) {
    return index
}

Solution

notice that none of your examples use the index (besides one unreliable medium article), they all use a unique identifier of the object that angular can’t possibly know unless you tell it.

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. trackBy is meant to tell angular when an item needs to be re-rendered, ie when I need those life cycle hooks to re run. 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.

Answered By – bryan60

Answer Checked By – Pedro (BugsFixing Volunteer)

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