filter operator for selection in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter the list to only include items with status 'active'.

Angular

const activeItems = items.[1](item => item.status === 'active');

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

Afilter

Bmap

Creduce

DforEach

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of filter, which transforms items but doesn't select.

Using forEach, which doesn't return a new filtered array.

The filter operator selects items that meet a condition. Here, it keeps only items with status 'active'.

2fill in blank

medium

Complete the Angular template code to display only users whose age is greater than 18 using the filter pipe.

Angular

<div *ngFor="let user of users | [1]: user => user.age > 18">{{user.name}}</div>

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

Afilter

Basync

Cmap

Dslice

Attempts:

3 left

💡 Hint

Common Mistakes

Using map pipe which transforms but does not filter.

Using async pipe which handles observables, not filtering.

The filter pipe selects array elements that satisfy the condition. Here, it shows users older than 18.

3fill in blank

hard

Fix the error in the RxJS observable pipeline to filter values greater than 10.

Angular

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

of(5, 15, 8, 20).pipe(
  [1](value => value > 10)
).subscribe(console.log);

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

Amap

Btap

Cfilter

Dreduce

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of filter, which changes values instead of filtering.

Using tap which does not filter values.

The filter operator in RxJS selects emitted values that pass the test. Here, it keeps values greater than 10.

4fill in blank

hard

Fill both blanks to create a filtered list of products with price less than 100 and map to their names.

Angular

const cheapProductNames = products.[1](p => p.price < 100).[2](p => p.name);

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

Afilter

Bmap

Creduce

DforEach

Attempts:

3 left

💡 Hint

Common Mistakes

Using reduce instead of map for extracting names.

Using forEach which does not return a new array.

First, filter selects products cheaper than 100. Then, map extracts their names.

5fill in blank

hard

Fill all three blanks to create objects mapping user names to ages for users older than 21.

Angular

const userAges = users
  .[1](user => user.age > 21)
  .[2](user => ({[[3]]: user.age}));

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

Afilter

Bmap

Cuser.name

Dreduce

Attempts:

3 left

💡 Hint

Common Mistakes

Using reduce instead of map for transformation.

Using age instead of name as the key.

First, filter selects users older than 21. Then, map creates objects with user names as keys and ages as values.

Practice

1. What does the filter operator do in Angular's RxJS streams?

easy

A. It delays the stream values by a set time.

B. It selects only the values that meet a condition.

C. It combines multiple streams into one.

D. It transforms each value into a new value.

Solution

1. Step 1: Understand the purpose of filter

   The filter operator is used to pick values from a stream that satisfy a condition.

2. Step 2: Compare with other operators

   Unlike map (which transforms) or merge (which combines), filter only selects values.

3. Final Answer:

   It selects only the values that meet a condition. -> Option B

4. Quick Check:

   filter = select values by condition [OK]

Hint: Filter picks values that return true in its function [OK]

Common Mistakes:

• Confusing filter with map operator
• Thinking filter changes values instead of selecting
• Assuming filter combines streams

2. Which is the correct syntax to use the filter operator inside an RxJS pipe() in Angular?

easy

A. source$.pipe(filter(value => value > 10))

B. source$.filter(value => value > 10)

C. source$.pipe(map(value => value > 10))

D. source$.pipe(filter(value, value > 10))

Solution

1. Step 1: Recall RxJS pipe syntax

   Operators like filter are used inside pipe as functions: pipe(filter(...)).

2. Step 2: Check the filter function argument

   The filter function takes a callback returning true/false, e.g. value => value > 10.

3. Final Answer:

   source$.pipe(filter(value => value > 10)) -> Option A

4. Quick Check:

   filter inside pipe with callback arrow function [OK]

Hint: Use filter inside pipe with a function returning true/false [OK]

Common Mistakes:

• Calling filter directly on observable without pipe
• Using map instead of filter
• Passing multiple arguments incorrectly to filter

3. Given this code snippet, what will be the output values logged?

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

of(1, 5, 10, 15).pipe(
  filter(x => x > 5)
).subscribe(console.log);

medium

A. 5, 10, 15

B. 1, 5

C. 10, 15

D. 1, 5, 10, 15

Solution

1. Step 1: Identify the filter condition

   The filter condition is x > 5, so only values greater than 5 pass.

2. Step 2: Check which values pass

   From the values 1, 5, 10, 15, only 10 and 15 are greater than 5.

3. Final Answer:

   10, 15 -> Option C

4. Quick Check:

   Values > 5 = 10, 15 [OK]

Hint: Filter keeps values where condition is true [OK]

Common Mistakes:

• Including values equal to 5 (should be > 5)
• Confusing filter with map output
• Assuming all values pass filter

4. What is wrong with this Angular RxJS code using filter?

source$.pipe(
  filter(x => {
    if (x % 2 === 0) return true;
  })
).subscribe(console.log);

medium

A. The filter callback does not return false for odd numbers.

B. The filter operator is used outside pipe.

C. The subscribe method is missing.

D. The filter callback should return a number, not boolean.

Solution

1. Step 1: Analyze the filter callback function

   The callback returns true for even numbers but returns undefined (no return) for odd numbers.

2. Step 2: Understand filter expects boolean return

   Filter needs true or false; undefined is treated as false but is unclear and error-prone.

3. Final Answer:

   The filter callback does not return false for odd numbers. -> Option A

4. Quick Check:

   Filter callback must return true or false explicitly [OK]

Hint: Always return true or false in filter callback [OK]

Common Mistakes:

• Missing explicit false return in filter
• Using filter outside pipe
• Confusing filter callback return types

5. You want to filter a stream of user objects to only those who are active and have age over 18. Which filter function correctly applies both conditions inside pipe()?

users$.pipe(
  filter( /* your code here */ )
).subscribe(activeAdults => console.log(activeAdults));

hard

A. user => user.isActive && user.age >= 18

B. user => user.isActive || user.age > 18

C. user => user.isActive && user.age > 18 && user.name

D. user => user.isActive && user.age > 18

Solution

1. Step 1: Understand the conditions

   We want users who are active AND older than 18 (strictly greater).

2. Step 2: Check each option's logic

   user => user.isActive && user.age > 18 uses && with user.isActive and user.age > 18, matching requirements exactly.

3. Final Answer:

   user => user.isActive && user.age > 18 -> Option D

4. Quick Check:

   Both conditions true with AND operator [OK]

Hint: Use && to combine multiple filter conditions [OK]

Common Mistakes:

• Using || instead of && to combine conditions
• Using >= instead of > if strict condition needed
• Adding unrelated conditions like user.name