The `OnPush` change detection strategy in Angular is designed to optimize performance by reducing the number of change detection cycles. It is based on the concept of immutability and relies on input properties and explicit change detection triggering.

To optimize an Angular application using the `OnPush` strategy, follow these steps:

Let's assume we have a parent component called `ParentComponent` and a child component called `ChildComponent`. The child component receives an array of items as an input property and displays them in a list. We'll optimize this scenario using the `OnPush` strategy. 1. Use the `OnPush` change detection strategy in components: 

// parent.component.ts
import { Component, ChangeDetectionStrategy } from '@angular/core';

@Component({
  selector: 'app-parent',
  template: `
    <app-child [items]="items"></app-child>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class ParentComponent {
  items: string[] = ['Item 1', 'Item 2', 'Item 3'];

  addItem() {
    this.items.push('New Item'); // Incorrect way to update the array
  }
}



// child.component.ts
import { Component, Input, ChangeDetectionStrategy } from '@angular/core';

@Component({
  selector: 'app-child',
  template: `
    <ul>
      <li *ngfor="let item of items">{{ item }}</li>
    </ul>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class ChildComponent {
  @Input() items: string[];
}
In the above code, both the `ParentComponent` and `ChildComponent` are using the `OnPush` change detection strategy. 2. Use immutable data for input properties: 

// parent.component.ts
import { Component, ChangeDetectionStrategy } from '@angular/core';

@Component({
  selector: 'app-parent',
  template: `
    <app-child [items]="items"></app-child>
    <button (click)="addItem()">Add Item</button>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class ParentComponent {
  items: string[] = ['Item 1', 'Item 2', 'Item 3'];

  addItem() {
    this.items = [...this.items, 'New Item']; // Correct way to update the array using spread operator
  }
}
In the `ParentComponent`, when adding a new item to the `items` array, we create a new array using the spread operator `[...this.items]` and then add the new item. This ensures that a new reference is created, which triggers change detection in the `ChildComponent` when the input property changes. Conclusion : By applying these optimization techniques, you can enhance the performance of your Angular application with the `OnPush` change detection strategy. Immutable data help minimize unnecessary change detection cycles, resulting in improved efficiency and responsiveness.