Pure vs impure pipes in Angular

Introduction

Pipes help change data in templates. Pure and impure pipes decide when to update the displayed data.

When you want to format data that rarely changes, like dates or currency.

When you need to update the display every time any change happens, like filtering a list as you type.

When you want to improve app speed by avoiding unnecessary updates.

When you want to always show the latest data even if the data object reference stays the same.

Syntax

Angular

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

@Pipe({
  name: 'examplePipe',
  pure: true // or false
})
export class ExamplePipe implements PipeTransform {
  transform(value: any, ...args: any[]): any {
    // transform logic here
    return value;
  }
}

The pure property controls if the pipe is pure (true) or impure (false).

Pure pipes run only when input references change. Impure pipes run on every change detection cycle.

Examples

This is a pure pipe. It updates only when the input string changes.

Angular

@Pipe({ name: 'purePipe', pure: true })
export class PurePipe implements PipeTransform {
  transform(value: string): string {
    return value.toUpperCase();
  }
}

This is an impure pipe. It updates every time Angular checks for changes.

Angular

@Pipe({ name: 'impurePipe', pure: false })
export class ImpurePipe implements PipeTransform {
  transform(value: string): string {
    return value + ' updated';
  }
}

Sample Program

This example shows two pipes: one pure and one impure. Clicking the button sets the text to the same string. The pure pipe runs only if the input reference changes, so it may not run. The impure pipe runs every time Angular checks for changes, so it always runs.

Angular

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

@Pipe({
  name: 'purePipe',
  pure: true
})
export class PurePipe implements PipeTransform {
  transform(value: string): string {
    console.log('PurePipe transform called');
    return value.toUpperCase();
  }
}

@Pipe({
  name: 'impurePipe',
  pure: false
})
export class ImpurePipe implements PipeTransform {
  transform(value: string): string {
    console.log('ImpurePipe transform called');
    return value + ' updated';
  }
}

@Component({
  selector: 'app-root',
  template: `
    <h3>Pure Pipe Output: {{ text | purePipe }}</h3>
    <h3>Impure Pipe Output: {{ text | impurePipe }}</h3>
    <button (click)="changeText()">Change Text</button>
  `
})
export class AppComponent {
  text = 'hello';

  changeText() {
    this.text = 'hello'; // same string value, new reference
  }
}

Important Notes

Pure pipes improve performance by running less often.

Impure pipes can slow down your app if overused because they run very often.

Use impure pipes only when you need to detect changes inside objects or arrays without changing their reference.

Summary

Pure pipes run only when input references change.

Impure pipes run on every change detection cycle.

Choose pure pipes for better performance and impure pipes when you need frequent updates.

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