filter operator for selection in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Angular component using filter operator?

Consider this Angular component using RxJS filter operator to select values:

import { Component, signal } from '@angular/core';
import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

@Component({
  selector: 'app-filter-demo',
  standalone: true,
  template: `<div>Filtered Values: {{ filteredValues() }}</div>`
})
export class FilterDemoComponent {
  filteredValues = signal([]);

  constructor() {
    const source$ = from(['apple', 'banana', 'cherry', 'date']);
    source$.pipe(
      filter(fruit => fruit.length > 5)
    ).subscribe(value => {
      this.filteredValues.update(arr => [...arr, value]);
    });
  }
}

What will be displayed inside the <div> after component initialization?

Angular

import { Component, signal } from '@angular/core';
import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

@Component({
  selector: 'app-filter-demo',
  standalone: true,
  template: `<div>Filtered Values: {{ filteredValues() }}</div>`
})
export class FilterDemoComponent {
  filteredValues = signal<string[]>([]);

  constructor() {
    const source$ = from(['apple', 'banana', 'cherry', 'date']);
    source$.pipe(
      filter(fruit => fruit.length > 5)
    ).subscribe(value => {
      this.filteredValues.update(arr => [...arr, value]);
    });
  }
}

A Filtered Values: banana

B Filtered Values: apple,banana,cherry,date

C Filtered Values: apple,date

D Filtered Values: banana,cherry

💡 Hint

Think about which fruits have names longer than 5 characters.

The filter operator selects only fruits with length greater than 5. 'banana' and 'cherry' have 6 letters, so they pass the filter. 'apple' and 'date' do not.

📝 Syntax

intermediate

Which option correctly uses the filter operator in Angular with RxJS?

Choose the correct syntax to filter emitted values from an observable numbers$ to only even numbers.

Angular

import { filter } from 'rxjs/operators';

numbers$.pipe(
  // filter condition here
).subscribe(console.log);

A filter(n => n % 2 === 0)

B filter(n => n / 2 === 0)

C filter(n => n % 2 = 0)

D filter(n => n & 2 === 0)

💡 Hint

Remember the modulo operator % returns the remainder.

Option A correctly uses n % 2 === 0 to check if a number is even. Option A uses division incorrectly, C uses assignment instead of comparison, and D uses bitwise AND incorrectly.

🔧 Debug

advanced

Why does this Angular RxJS filter code not emit any values?

Look at this code snippet:

import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

const source$ = from([1, 2, 3, 4]);
source$.pipe(
  filter(x => x > 5)
).subscribe(console.log);

Why does it not print anything?

Angular

import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

const source$ = from([1, 2, 3, 4]);
source$.pipe(
  filter(x => x > 5)
).subscribe(console.log);

A Because the subscribe callback is missing.

B Because the observable source$ is empty.

C Because no values in the array are greater than 5, so filter excludes all.

D Because the filter operator is not imported correctly.

💡 Hint

Check the condition inside the filter and the source array values.

The filter condition x > 5 excludes all numbers since none in [1,2,3,4] are greater than 5. So no values are emitted.

❓ state_output

advanced

What is the final state of the signal after filtering and updating?

Given this Angular component snippet:

import { Component, signal } from '@angular/core';
import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

@Component({
  selector: 'app-signal-filter',
  standalone: true,
  template: `<div>Count: {{ count() }}</div>`
})
export class SignalFilterComponent {
  count = signal(0);

  constructor() {
    const source$ = from([10, 15, 20, 25]);
    source$.pipe(
      filter(x => x % 10 === 0)
    ).subscribe(() => {
      this.count.update(c => c + 1);
    });
  }
}

What will be the value of count() after subscription completes?

Angular

import { Component, signal } from '@angular/core';
import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

@Component({
  selector: 'app-signal-filter',
  standalone: true,
  template: `<div>Count: {{ count() }}</div>`
})
export class SignalFilterComponent {
  count = signal(0);

  constructor() {
    const source$ = from([10, 15, 20, 25]);
    source$.pipe(
      filter(x => x % 10 === 0)
    ).subscribe(() => {
      this.count.update(c => c + 1);
    });
  }
}

A Count: 4

B Count: 2

C Count: 3

D Count: 0

💡 Hint

Count how many numbers in the array are divisible by 10.

Only 10 and 20 satisfy x % 10 === 0. So the count increments twice, resulting in 2.

🧠 Conceptual

expert

Which statement best describes the behavior of the filter operator in Angular RxJS streams?

Choose the most accurate description of how the filter operator works in Angular RxJS streams.

A It selectively emits only those values from the source observable that satisfy the given condition.

B It transforms each emitted value into a new value based on a condition.

C It blocks all values until a condition is true, then emits all values after.

D It accumulates values over time and emits the accumulated result when the condition is met.

💡 Hint

Think about whether filter changes values or just decides which to pass through.

The filter operator does not change values; it only lets through values that meet the condition, ignoring others.