Pure vs impure pipes in Angular - Performance Comparison

Performance: Pure vs impure pipes

MEDIUM IMPACT

This concept affects how often Angular recalculates and updates the displayed data, impacting rendering speed and responsiveness.

Transforming data in templates efficiently

Angular

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

@Pipe({
  name: 'pureExample',
  pure: true
})
export class PureExamplePipe implements PipeTransform {
  transform(value: any): any {
    // same calculation
    return value.map(item => item * 2);
  }
}

Pure pipes run only when input references change, reducing unnecessary recalculations and improving rendering speed.

📈 Performance Gainsingle recalculation per input change, reducing reflows and improving interaction responsiveness

Transforming data in templates efficiently

Angular

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

@Pipe({
  name: 'impureExample',
  pure: false
})
export class ImpureExamplePipe implements PipeTransform {
  transform(value: any): any {
    // expensive calculation
    return value.map(item => item * 2);
  }
}

Impure pipes run on every change detection cycle, causing repeated expensive calculations even if input data hasn't changed.

📉 Performance Costtriggers change detection recalculation every cycle, causing multiple reflows and blocking rendering for tens of milliseconds on complex data

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Pure Pipe	Minimal - only on input change	Few - only when data changes	Low - fewer repaints	[OK] Good
Impure Pipe	Repeated on every change detection	Many - triggers frequent reflows	High - frequent repaints	[X] Bad

Rendering Pipeline

Angular runs change detection cycles to update the DOM. Pure pipes execute only when their input changes, minimizing recalculations. Impure pipes run every cycle, increasing CPU and rendering work.

→Change Detection

→Layout

→Paint

⚠️ BottleneckChange Detection recalculations triggered by impure pipes

Core Web Vital Affected

INP

This concept affects how often Angular recalculates and updates the displayed data, impacting rendering speed and responsiveness.

Optimization Tips

1Use pure pipes to limit recalculations to input changes only.

2Avoid impure pipes for expensive operations to prevent frequent reflows.

3Immutable data helps pure pipes detect changes efficiently.

Performance Quiz - 3 Questions

What is the main performance difference between pure and impure pipes in Angular?

A Impure pipes run only when input changes; pure pipes run every change detection cycle.

B Both run equally often but pure pipes use less memory.

C Pure pipes run only when input changes; impure pipes run every change detection cycle.

D Impure pipes cache results; pure pipes do not.

DevTools: Performance

How to check: Record a performance profile while interacting with the app. Look for frequent change detection cycles and pipe recalculations.

What to look for: High CPU usage spikes and long scripting times indicate impure pipes causing repeated recalculations.

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