map operator for transformation in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will be the output of this Angular component using map operator?

Consider this Angular component that uses RxJS map operator to transform data from an observable. What will be displayed in the template?

Angular

import { Component } from '@angular/core';
import { of } from 'rxjs';
import { map } from 'rxjs/operators';

@Component({
  selector: 'app-transform',
  template: `<div>{{ transformedValue | async }}</div>`
})
export class TransformComponent {
  source$ = of(5);
  transformedValue = this.source$.pipe(
    map(x => x * 3)
  );
}

A Observable { }

B 5

C 15

D undefined

💡 Hint

Remember that map transforms the emitted value before it reaches the template.

The map operator multiplies the emitted value 5 by 3, resulting in 15. The async pipe unwraps the observable and displays 15.

📝 Syntax

intermediate

Which option correctly uses the map operator to transform an observable of strings to their lengths?

Given an observable emitting strings, which code snippet correctly uses map to transform each string to its length?

Angular

import { of } from 'rxjs';
import { map } from 'rxjs/operators';

const source$ = of('apple', 'banana', 'cherry');
const lengths$ = source$.pipe(
  // transformation here
);

A map(str => str.length)

B map(str => str.toUpperCase())

C map(str => str.length())

D map(str => str.length + 1)

💡 Hint

Check how to get the length of a string in JavaScript.

Option A correctly accesses the length property of the string. Option A tries to call length() as a function, which is invalid. Option A transforms to uppercase, not length. Option A adds 1 to length, which is incorrect.

🔧 Debug

advanced

Why does this Angular observable transformation cause a runtime error?

Examine the following code snippet. Why does it cause a runtime error when subscribed?

Angular

import { of } from 'rxjs';
import { map } from 'rxjs/operators';

const source$ = of(null);
const transformed$ = source$.pipe(
  map(x => x.toString())
);

transformed$.subscribe(console.log);

A Because <code>subscribe</code> is missing a callback function

B Because <code>map</code> operator is not imported correctly

C Because <code>of(null)</code> does not emit any value

D Because <code>x</code> is null and calling <code>toString()</code> on null causes a TypeError

💡 Hint

Think about what happens when you call a method on null.

The observable emits null. Calling toString() on null causes a runtime TypeError because null has no methods.

❓ state_output

advanced

What is the final emitted value after applying multiple map operators?

Given this observable pipeline, what is the final value emitted?

Angular

import { of } from 'rxjs';
import { map } from 'rxjs/operators';

const source$ = of(2);
const result$ = source$.pipe(
  map(x => x + 3),
  map(x => x * 2),
  map(x => `Value: ${x}`)
);

result$.subscribe(console.log);

A Value: 7

B Value: 10

C Value: 5

D Value: 4

💡 Hint

Apply each map transformation step by step.

Starting with 2, add 3 → 5, multiply by 2 → 10, then format as string → 'Value: 10'.

🧠 Conceptual

expert

Which statement best describes the behavior of the map operator in Angular's RxJS?

Choose the most accurate description of how the map operator works in Angular's RxJS library.

A It transforms each emitted value from the source observable by applying a given function, emitting the transformed value.

B It combines multiple observables into one by merging their emissions.

C It filters out values from the source observable that do not satisfy a condition.

D It delays the emission of values from the source observable by a specified time.

💡 Hint

Recall the purpose of map in functional programming and RxJS.

The map operator applies a function to each value emitted by the source observable and emits the result. Other options describe different operators like filter, merge, and delay.