tap operator for side effects in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the tap operator from RxJS.

Angular

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

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

Atap

BswitchMap

Cfilter

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Importing map instead of tap

Forgetting to import tap

Importing from wrong RxJS path

The tap operator is imported from rxjs/operators to perform side effects without altering the stream.

2fill in blank

medium

Complete the code to add a tap operator that logs the emitted value.

Angular

observable.pipe([1](value => console.log(value)))

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

Amap

BcatchError

Cfilter

Dtap

Attempts:

3 left

💡 Hint

Common Mistakes

Using map which changes the data

Using filter which filters emissions

Using catchError which handles errors

The tap operator is used here to log each emitted value for side effects.

3fill in blank

hard

Fix the error in the tap usage to correctly perform a side effect.

Angular

observable.pipe(tap([1]))

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

Aconsole.log(value)

Bvalue => console.log(value)

C() => console.log

Dconsole.log()

Attempts:

3 left

💡 Hint

Common Mistakes

Passing console.log() which calls the function immediately

Passing console.log without wrapping in a function

Passing a non-function value

The tap operator expects a function. Passing value => console.log(value) correctly logs each value.

4fill in blank

hard

Fill both blanks to create a tap operator that logs 'Start' before and 'End' after the observable emits.

Angular

observable.pipe(tap([1], [2]))

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

A() => console.log('Start')

B() => console.log('End')

C() => alert('End')

D() => console.error('Error')

Attempts:

3 left

💡 Hint

Common Mistakes

Using alert instead of console.log

Swapping the order of functions

Using console.error instead of console.log

The first function runs on each emission (logging 'Start'), the second runs on completion (logging 'End').

5fill in blank

hard

Fill all three blanks to create a tap operator that logs the value, errors, and completion messages.

Angular

observable.pipe(tap([1], [2], [3]))

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

Avalue => console.log('Value:', value)

Berror => console.error('Error:', error)

C() => console.log('Completed')

D() => alert('Done')

Attempts:

3 left

💡 Hint

Common Mistakes

Using alert instead of console methods

Missing the error handler function

Passing non-functions as arguments

The tap operator can take three functions: for next values, errors, and completion respectively.

Practice

1. What is the main purpose of the tap operator in Angular's RxJS?

easy

A. To transform the data emitted by an observable

B. To perform side effects without modifying the data stream

C. To filter out unwanted values from the observable

D. To combine multiple observables into one

Solution

1. Step 1: Understand the role of tap

   The tap operator allows you to execute side effects like logging or debugging without changing the data passing through the observable.

2. Step 2: Compare with other operators

   Operators like map transform data, filter removes values, and combineLatest merges streams. tap only peeks at data.

3. Final Answer:

   To perform side effects without modifying the data stream -> Option B

4. Quick Check:

   tap = side effects only [OK]

Hint: Remember: tap is for side effects, not data changes [OK]

Common Mistakes:

• Thinking tap changes the data
• Confusing tap with map or filter
• Using tap outside pipe

2. Which of the following is the correct syntax to use tap inside an observable pipe in Angular?

easy

A. observable.pipe(map(value => tap(console.log(value))))

B. observable.tap(value => console.log(value))

C. tap(observable, value => console.log(value))

D. observable.pipe(tap(value => console.log(value)))

Solution

1. Step 1: Recall the RxJS pipe syntax

   Operators like tap are used inside the pipe() method of an observable.

2. Step 2: Check each option

   observable.pipe(tap(value => console.log(value))) correctly uses pipe(tap(...)). observable.tap(value => console.log(value)) tries to call tap directly on observable, which is invalid. observable.pipe(map(value => tap(console.log(value)))) misuses map and tap. tap(observable, value => console.log(value)) is not valid RxJS syntax.

3. Final Answer:

   observable.pipe(tap(value => console.log(value))) -> Option D

4. Quick Check:

   Use pipe(tap(...)) syntax [OK]

Hint: Always put tap inside pipe() for observables [OK]

Common Mistakes:

• Calling tap directly on observable
• Mixing tap with map incorrectly
• Using tap outside pipe

3. Given the following code, what will be logged to the console?

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

of(1, 2, 3).pipe(
  tap(x => console.log('Before map:', x)),
  map(x => x * 10),
  tap(x => console.log('After map:', x))
).subscribe();

medium

A. Before map: 1
After map: 10
Before map: 2
After map: 20
Before map: 3
After map: 30

B. After map: 10
After map: 20
After map: 30

C. Before map: 10
Before map: 20
Before map: 30

D. Before map: 1
Before map: 2
Before map: 3

Solution

1. Step 1: Understand the order of operators

   The observable emits 1, 2, 3. The first tap logs 'Before map:' with original values. Then map multiplies each by 10. The second tap logs 'After map:' with transformed values.

2. Step 2: Trace the console logs

   For each value: log 'Before map: x', then after map, log 'After map: x*10'. So logs appear in pairs for each value.

3. Final Answer:

   Before map: 1 After map: 10 Before map: 2 After map: 20 Before map: 3 After map: 30 -> Option A

4. Quick Check:

   tap logs before and after map [OK]

Hint: tap logs values without changing them [OK]

Common Mistakes:

• Assuming tap changes values
• Ignoring order of operators
• Expecting only one set of logs

4. Identify the error in this code snippet using tap and fix it:

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

const source = of(5, 10, 15);
source.tap(value => console.log(value)).subscribe();

medium

A. Replace source.tap(...) with source.pipe(tap(...))

B. Import tap from 'rxjs' instead of 'rxjs/operators'

C. Add subscribe() before tap()

D. Change of to from

Solution

1. Step 1: Identify incorrect usage of tap

   The code calls tap directly on the observable, which is invalid. Operators must be used inside pipe().

2. Step 2: Correct the syntax

   Change source.tap(...) to source.pipe(tap(...)) to properly apply the operator.

3. Final Answer:

   Replace source.tap(...) with source.pipe(tap(...)) -> Option A

4. Quick Check:

   Use pipe(tap(...)) not tap() directly [OK]

Hint: Operators go inside pipe(), never called directly [OK]

Common Mistakes:

• Calling tap directly on observable
• Wrong import path for tap
• Missing pipe() usage

5. You want to log each value emitted by an HTTP request observable without changing the data. Which is the best way to do this using tap? Consider this code:

this.http.get('/api/data').pipe(
  // What goes here?
).subscribe(data => this.process(data));

hard

A. subscribe(data => console.log('Received:', data))

B. map(data => { console.log('Received:', data); return data; })

C. tap(data => console.log('Received:', data))

D. filter(data => { console.log('Received:', data); return true; })

Solution

1. Step 1: Understand the goal

   You want to log data without changing it before passing it to subscribe.

2. Step 2: Choose the operator that logs without modifying

   tap is designed for side effects like logging without altering data. map transforms data, filter filters it, and logging inside subscribe happens after pipe.

3. Final Answer:

   tap(data => console.log('Received:', data)) -> Option C

4. Quick Check:

   tap logs side effects without data change [OK]

Hint: Use tap inside pipe for logging without data change [OK]

Common Mistakes:

• Using map for logging (changes data)
• Logging inside subscribe instead of pipe
• Using filter which may drop data