*ngIf for conditional rendering in Angular - Performance & Optimization

Performance: *ngIf for conditional rendering

MEDIUM IMPACT

Controls whether elements are added or removed from the DOM, impacting initial load and interaction speed.

Conditionally showing or hiding UI elements

Angular

<div *ngIf="isVisible">Content</div>

Element is added or removed from the DOM, reducing DOM size and improving load time.

📈 Performance GainReduces initial DOM nodes and memory, improving LCP and interaction speed.

Conditionally showing or hiding UI elements

Angular

<div [hidden]="!isVisible">Content</div>

Element stays in the DOM and still consumes resources even when hidden.

📉 Performance CostTriggers no reflows on toggle but increases initial DOM size and memory usage.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
[hidden] binding	No DOM changes	0 on toggle	Low but persistent	[!] OK
*ngIf directive	Adds/removes nodes	1 per toggle	Moderate on toggle	[OK] Good

Rendering Pipeline

*ngIf controls DOM insertion/removal, affecting layout and paint stages by changing node count dynamically.

→Layout

→Paint

→Composite

⚠️ BottleneckLayout stage due to recalculating positions when nodes are added or removed.

Core Web Vital Affected

LCP

Controls whether elements are added or removed from the DOM, impacting initial load and interaction speed.

Optimization Tips

1Use *ngIf to remove elements from the DOM instead of just hiding them.

2Avoid toggling *ngIf too often to reduce layout thrashing.

3Prefer *ngIf for large or complex elements to improve initial load speed.

Performance Quiz - 3 Questions

What is the main performance benefit of using *ngIf over [hidden] for conditional rendering?

A It only hides elements visually without changing the DOM.

B It removes elements from the DOM, reducing initial load and memory usage.

C It delays rendering until user interaction.

D It caches elements for faster toggling.

DevTools: Elements and Performance

How to check: Use Elements panel to see if elements exist in DOM when toggled; use Performance panel to record and analyze layout and paint events during toggle.

What to look for: Check DOM node count changes and layout recalculations; fewer nodes and minimal layout thrashing indicate good use of *ngIf.