Pure vs impure pipes in Angular - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a pure pipe in Angular.

Angular

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
  name: 'example',
  pure: [1]
})
export class ExamplePipe implements PipeTransform {
  transform(value: string): string {
    return value.toUpperCase();
  }
}

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

Afalse

Btrue

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Setting pure to false when you want a pure pipe.

Leaving pure undefined and expecting impure behavior.

Pure pipes have pure: true by default, meaning they only run when input changes.

2fill in blank

medium

Complete the code to declare an impure pipe in Angular.

Angular

@Pipe({
  name: 'impureExample',
  pure: [1]
})
export class ImpureExamplePipe implements PipeTransform {
  transform(value: any): any {
    return value;
  }
}

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

Afalse

Btrue

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using pure: true for impure pipes.

Not setting pure property at all and expecting impure behavior.

Impure pipes have pure: false, so they run on every change detection cycle.

3fill in blank

hard

Fix the error in the pipe declaration to make it impure.

Angular

@Pipe({
  name: 'fixPipe',
  pure: [1]
})
export class FixPipe implements PipeTransform {
  transform(value: any): any {
    return value;
  }
}

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

Atrue

Bundefined

Cfalse

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving pure as true for impure pipes.

Using undefined or null instead of false.

To make a pipe impure, set pure: false.

4fill in blank

hard

Fill both blanks to create a pure pipe that transforms input to lowercase.

Angular

@Pipe({
  name: 'lowercase',
  pure: [1]
})
export class LowercasePipe implements [2] {
  transform(value: string): string {
    return value.toLowerCase();
  }
}

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

Atrue

Bfalse

CPipeTransform

DOnInit

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnInit instead of PipeTransform.

Setting pure to false for a pure pipe.

Pure pipes have pure: true and implement PipeTransform.

5fill in blank

hard

Fill all three blanks to create an impure pipe that reverses a string.

Angular

@Pipe({
  name: 'reverse',
  pure: [1]
})
export class ReversePipe implements [2] {
  transform(value: string): string {
    return value.split('').[3]().join('');
  }
}

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

Atrue

Bfalse

Creverse

DPipeTransform

Attempts:

3 left

💡 Hint

Common Mistakes

Setting pure to true for impure pipes.

Not implementing PipeTransform.

Using a wrong method instead of reverse().

Impure pipes have pure: false, implement PipeTransform, and use reverse() to reverse arrays.

Practice

1. What is the main difference between a pure pipe and an impure pipe in Angular?

easy

A. Pure pipes can modify input data, impure pipes cannot.

B. Pure pipes run every change detection cycle, impure pipes run only when input references change.

C. Pure pipes run only when input references change, impure pipes run every change detection cycle.

D. Pure pipes are slower than impure pipes.

Solution

1. Step 1: Understand pure pipe behavior

   Pure pipes execute only when the input reference changes, meaning they do not run on every change detection cycle.

2. Step 2: Understand impure pipe behavior

   Impure pipes run on every change detection cycle regardless of input changes, which can impact performance.

3. Final Answer:

   Pure pipes run only when input references change, impure pipes run every change detection cycle. -> Option C

4. Quick Check:

   Pure vs impure pipe execution = A [OK]

Hint: Pure pipes run less often; impure pipes run every cycle [OK]

Common Mistakes:

• Confusing when pure and impure pipes run
• Thinking pure pipes run every cycle
• Assuming impure pipes only run on input change

2. Which of the following is the correct way to declare an impure pipe in Angular?

easy

A. @Pipe({ name: 'examplePipe', pure: false })

B. @Pipe({ name: 'examplePipe', impure: true })

C. @Pipe({ name: 'examplePipe' })

D. @Pipe({ name: 'examplePipe', pure: true })

Solution

1. Step 1: Recall pipe metadata syntax

   Angular pipes use the @Pipe decorator with a 'pure' property to indicate purity.

2. Step 2: Identify impure pipe declaration

   Setting 'pure: false' marks the pipe as impure, causing it to run every change detection cycle.

3. Final Answer:

   @Pipe({ name: 'examplePipe', pure: false }) -> Option A

4. Quick Check:

   Impure pipe uses pure: false [OK]

Hint: Impure pipes set pure to false in @Pipe decorator [OK]

Common Mistakes:

• Using pure: true for impure pipes
• Omitting pure property defaults to pure
• Using non-existent 'impure' property

3. Given this Angular pipe usage:

items = [1, 2, 3];
// Template: {{ items | customPipe }}

If customPipe is pure and the array items is mutated by items.push(4) without changing the reference, what will happen?

medium

A. The pipe will run and update the output to include 4.

B. Angular throws an error due to mutation.

C. The pipe runs only if change detection is manually triggered.

D. The pipe will not run; output remains unchanged.

Solution

1. Step 1: Understand pure pipe input change detection

   Pure pipes run only when the input reference changes, not when the content inside the object changes.

2. Step 2: Analyze mutation effect on pipe execution

   Mutating the array with push() does not change the reference, so the pure pipe does not run again and output stays the same.

3. Final Answer:

   The pipe will not run; output remains unchanged. -> Option D

4. Quick Check:

   Pure pipe ignores internal mutations = C [OK]

Hint: Pure pipes need new object references to update [OK]

Common Mistakes:

• Assuming mutation triggers pure pipe
• Thinking Angular throws error on mutation
• Believing manual change detection is required here

4. You have an Angular pipe declared as @Pipe({ name: 'filterPipe', pure: true }). It filters an array based on user input. The filtered results do not update when the user types. What is the likely cause?

medium

A. The pipe is impure and runs too often causing errors.

B. The pipe is pure and input array reference does not change on filtering.

C. The pipe is missing the transform method.

D. The pipe is declared without a name.

Solution

1. Step 1: Identify pipe purity and input behavior

   A pure pipe runs only when the input reference changes, not when the content changes.

2. Step 2: Understand why filtered results don't update

   If the array is filtered in place without creating a new array reference, the pure pipe does not detect change and does not update output.

3. Final Answer:

   The pipe is pure and input array reference does not change on filtering. -> Option B

4. Quick Check:

   Pure pipe needs new input reference to update [OK]

Hint: Pure pipes need new input references to update output [OK]

Common Mistakes:

• Assuming impure pipe causes this issue
• Forgetting to return new array reference
• Ignoring pipe naming or method errors

5. You want to create a pipe that formats the current time every second in Angular. Which pipe type should you use and why?

hard

A. Impure pipe, because it runs every change detection cycle and updates time frequently.

B. Pure pipe, because it runs only when input changes, improving performance.

C. Pure pipe, because it automatically triggers updates every second.

D. Impure pipe, because pure pipes cannot be used for formatting.

Solution

1. Step 1: Understand the need for frequent updates

   Formatting current time every second requires the pipe to run frequently to update the display.

2. Step 2: Choose pipe type based on update frequency

   Impure pipes run every change detection cycle, making them suitable for frequent updates like a clock.

3. Final Answer:

   Impure pipe, because it runs every change detection cycle and updates time frequently. -> Option A

4. Quick Check:

   Frequent updates need impure pipe [OK]

Hint: Use impure pipes for frequent, automatic updates [OK]

Common Mistakes:

• Choosing pure pipe for frequent updates
• Thinking pure pipes auto-update every second
• Confusing pipe purpose with formatting capability