Why operators transform data streams in Angular - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the map operator from RxJS.

Angular

import { [1] } from 'rxjs/operators';

Drag options to blanks, or click blank then click option'

AmergeMap

Bfilter

Ctap

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Importing the wrong operator like filter or tap.

Forgetting to import from 'rxjs/operators'.

The map operator is used to transform data streams in Angular with RxJS.

2fill in blank

medium

Complete the code to transform the emitted values by doubling them.

Angular

source$.pipe([1](value => value * 2))

Drag options to blanks, or click blank then click option'

Amap

Bfilter

Ctap

Dscan

Attempts:

3 left

💡 Hint

Common Mistakes

Using filter which only filters values.

Using tap which is for side effects, not transformation.

The map operator applies a function to each emitted value, transforming the stream.

3fill in blank

hard

Fix the error in the code by choosing the correct operator to transform the stream.

Angular

const doubled$ = source$.pipe([1](value => value * 2));

Drag options to blanks, or click blank then click option'

Amap

Bfilter

Csubscribe

Dpipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using subscribe instead of map.

Trying to call pipe as an operator.

The map operator transforms each value. Using subscribe or pipe incorrectly here causes errors.

4fill in blank

hard

Fill both blanks to create a stream that filters even numbers and then doubles them.

Angular

source$.pipe([1](value => value % 2 === 0), [2](value => value * 2))

Drag options to blanks, or click blank then click option'

Afilter

Bmap

Ctap

Dscan

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of operators.

Using tap instead of map for transformation.

First, filter selects even numbers. Then, map doubles them.

5fill in blank

hard

Fill all three blanks to create a stream that filters odd numbers, doubles them, and logs each value.

Angular

source$.pipe([1](value => value % 2 !== 0), [2](value => value * 2), [3](value => console.log(value)))

Drag options to blanks, or click blank then click option'

Afilter

Bmap

Ctap

Dscan

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of tap for logging.

Filtering even numbers instead of odd.

filter selects odd numbers, map doubles them, and tap logs each value without changing the stream.

Practice

1. What is the main purpose of operators in Angular data streams?

easy

A. To stop the stream permanently

B. To create new streams from scratch

C. To transform or change the data inside the stream

D. To convert streams into plain objects

Solution

1. Step 1: Understand what operators do in streams

   Operators are functions that take data from a stream and change it in some way.

2. Step 2: Identify the main role of operators

   They transform the data to fit what the app needs, like filtering or mapping values.

3. Final Answer:

   To transform or change the data inside the stream -> Option C

4. Quick Check:

   Operators transform data = A [OK]

Hint: Operators change data inside streams, not create or stop them [OK]

Common Mistakes:

• Thinking operators create streams instead of transforming data
• Confusing operators with stream termination
• Believing operators convert streams to objects

2. Which is the correct way to combine operators in Angular?

easy

A. Using the merge() method

B. Using the combine() method

C. Using the concat() method

D. Using the pipe() method

Solution

1. Step 1: Recall how Angular chains operators

   Angular uses the pipe() method to chain multiple operators together on a stream.

2. Step 2: Confirm the correct method name

   The method pipe() is the standard way to combine operators like map and filter.

3. Final Answer:

   Using the pipe() method -> Option D

4. Quick Check:

   Combine operators with pipe() = C [OK]

Hint: Remember: pipe() chains operators in Angular streams [OK]

Common Mistakes:

• Confusing pipe() with combine() or merge()
• Using merge() or concat() which combine streams, not operators
• Trying to chain operators without pipe()

3. Given the code:

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

of(1, 2, 3, 4, 5).pipe(
  filter(x => x % 2 === 0),
  map(x => x * 10)
).subscribe(console.log);

What will be printed to the console?

medium

A. 20, 40

B. 10, 20, 30, 40, 50

C. 2, 4

D. 1, 3, 5

Solution

1. Step 1: Apply the filter operator

   The filter keeps only even numbers: 2 and 4.

2. Step 2: Apply the map operator

   Each filtered number is multiplied by 10: 2*10=20, 4*10=40.

3. Final Answer:

   20, 40 -> Option A

4. Quick Check:

   Filter evens then multiply by 10 = B [OK]

Hint: Filter first, then map changes values; order matters [OK]

Common Mistakes:

• Ignoring filter and mapping all numbers
• Mapping before filtering (wrong order)
• Printing original numbers without transformation

4. What is wrong with this code snippet?

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

of(1, 2, 3).pipe(
  map(x => x * 2),
  filter(x => x > 3)
).subscribe(console.log);

medium

A. The operators are in the wrong order; filter should come before map

B. There is no error; the code works as expected

C. The subscribe() method is not called correctly

D. The pipe() method is missing

Solution

1. Step 1: Check the order of operators

   Mapping first doubles the numbers: 2, 4, 6. Then filtering keeps values greater than 3: 4 and 6.

2. Step 2: Verify syntax and method calls

   pipe() and subscribe() are used correctly with proper syntax.

3. Final Answer:

   There is no error; the code works as expected -> Option B

4. Quick Check:

   Correct order and syntax = D [OK]

Hint: Mapping before filtering is valid; no syntax errors here [OK]

Common Mistakes:

• Assuming filter must always come before map
• Thinking pipe() is missing
• Believing subscribe() is incorrect

5. You want to create a stream that emits only the first 3 even numbers doubled from the list [1, 2, 3, 4, 5, 6, 7, 8]. Which operator sequence correctly achieves this?

hard

A. of(...list).pipe(filter(x => x % 2 === 0), take(3), map(x => x * 2))

B. of(...list).pipe(take(3), filter(x => x % 2 === 0), map(x => x * 2))

C. of(...list).pipe(map(x => x * 2), filter(x => x % 2 === 0), take(3))

D. of(...list).pipe(map(x => x * 2), take(3), filter(x => x % 2 === 0))

Solution

1. Step 1: Filter even numbers first

   Filtering first keeps only even numbers: 2, 4, 6, 8.

2. Step 2: Take the first 3 even numbers

   Using take(3) after filtering selects 2, 4, 6.

3. Step 3: Double the selected numbers

   Mapping last doubles each: 4, 8, 12.

4. Final Answer:

   of(...list).pipe(filter(x => x % 2 === 0), take(3), map(x => x * 2)) -> Option A

5. Quick Check:

   Filter -> take -> map order = A [OK]

Hint: Filter first, then take, then map to transform last [OK]

Common Mistakes:

• Taking first 3 items before filtering
• Mapping before filtering, changing filter logic
• Taking after mapping, causing wrong values