Creating custom pipes in Angular - Quick Revision & Summary

Recall & Review

beginner

What is a custom pipe in Angular?

A custom pipe is a way to transform data in templates using your own logic. It lets you format or change data display without changing the original data.

beginner

How do you create a custom pipe class in Angular?

You create a class with the @Pipe decorator and implement the PipeTransform interface with a transform() method that changes the input value.

beginner

What does the transform() method do in a custom pipe?

It takes the input value and optional parameters, then returns the transformed output to display in the template.

beginner

How do you use a custom pipe in an Angular template?

Use the pipe symbol (|) followed by the pipe name, like {{ value | pipeName }} to apply the transformation.

intermediate

Why should pipes be pure by default in Angular?

Pure pipes run only when input changes, making apps faster by avoiding unnecessary recalculations.

What decorator do you use to define a custom pipe in Angular?

A @Pipe

B @Component

C @Injectable

D @Directive

The @Pipe decorator marks a class as a pipe and lets Angular know its name and metadata.

Which method must a custom pipe class implement?

A render()

B convert()

C execute()

D transform()

The transform() method is required to define how the pipe changes the input value.

How do you pass extra parameters to a custom pipe in a template?

A {{ value.pipeName(param1, param2) }}

B {{ pipeName(value, param1, param2) }}

C {{ value | pipeName:param1:param2 }}

D {{ pipeName | value:param1:param2 }}

Parameters are passed after the pipe name separated by colons.

What is the default purity setting of Angular pipes?

A Pure (runs only on input change)

B Impure (runs every change detection)

C Depends on the component

D No default, must be set manually

Pipes are pure by default to improve performance by running only when inputs change.

Where do you declare a custom pipe so Angular can use it?

A In the providers array

B In the declarations array of an NgModule

C In the bootstrap array

D In the imports array

Custom pipes must be declared in an NgModule's declarations to be available in templates.

Explain step-by-step how to create and use a custom pipe in Angular.

Think about the class, method, module, and template parts.

Why are pure pipes preferred in Angular and when might you need an impure pipe?

Consider performance and data update scenarios.

Practice

1. What is the main purpose of creating a custom pipe in Angular?

easy

A. To manage component lifecycle hooks

B. To handle HTTP requests

C. To define routing paths

D. To transform data in templates for display

Solution

1. Step 1: Understand the role of pipes in Angular

   Pipes are used to transform data before displaying it in templates.

2. Step 2: Identify what custom pipes do

   Custom pipes let you create your own data transformations to keep templates clean and reusable.

3. Final Answer:

   To transform data in templates for display -> Option D

4. Quick Check:

   Custom pipes = transform data [OK]

Hint: Pipes change how data looks in templates [OK]

Common Mistakes:

• Confusing pipes with services
• Thinking pipes handle HTTP or routing
• Mixing pipes with component lifecycle

2. Which of the following is the correct way to define a custom pipe class in Angular?

easy

A. export class MyPipe implements PipeTransform { transform(value: any): any { return value; } }

B. export class MyPipe { transform(value: any): any { return value; } }

C. export function MyPipe(value: any): any { return value; }

D. export class MyPipe implements OnInit { transform(value: any): any { return value; } }

Solution

1. Step 1: Recall Angular pipe class requirements

   A custom pipe class must implement the PipeTransform interface and define a transform method.

2. Step 2: Check each option

   export class MyPipe implements PipeTransform { transform(value: any): any { return value; } } correctly implements PipeTransform and has transform method. export class MyPipe { transform(value: any): any { return value; } } misses interface. export function MyPipe(value: any): any { return value; } is a function, not a class. export class MyPipe implements OnInit { transform(value: any): any { return value; } } wrongly implements OnInit instead of PipeTransform.

3. Final Answer:

   export class MyPipe implements PipeTransform { transform(value: any): any { return value; } } -> Option A

4. Quick Check:

   Pipe class implements PipeTransform [OK]

Hint: Custom pipes must implement PipeTransform interface [OK]

Common Mistakes:

• Forgetting to implement PipeTransform
• Using a function instead of a class
• Implementing wrong interfaces like OnInit

3. Given this custom pipe code, what will be the output of {{ 'hello' | exclaim }} if the pipe is defined as:

import { Pipe, PipeTransform } from '@angular/core';
@Pipe({ name: 'exclaim' })
export class ExclaimPipe implements PipeTransform {
  transform(value: string): string {
    return value + '!';
  }
}

medium

A. Error: Pipe not found

B. hello

C. hello!

D. !hello

Solution

1. Step 1: Understand the transform method behavior

   The transform method appends an exclamation mark to the input string.

2. Step 2: Apply the pipe to the input 'hello'

   Input 'hello' becomes 'hello!' after transformation.

3. Final Answer:

   hello! -> Option C

4. Quick Check:

   Input + '!' = hello! [OK]

Hint: transform method output is what pipe shows [OK]

Common Mistakes:

• Ignoring the transform method logic
• Assuming pipe adds prefix instead of suffix
• Not registering the pipe in module (causing errors)

4. What is wrong with this custom pipe code?

import { Pipe, PipeTransform } from '@angular/core';
@Pipe({ name: 'reverse' })
export class ReversePipe {
  transform(value: string): string {
    return value.split('').reverse().join('');
  }
}

medium

A. The class does not implement PipeTransform interface

B. The transform method is missing

C. The pipe name is invalid

D. The split method cannot be used on strings

Solution

1. Step 1: Check class declaration

   The class ReversePipe does not implement PipeTransform interface, which is required for custom pipes.

2. Step 2: Verify transform method presence

   The transform method exists and is correctly implemented.

3. Final Answer:

   The class does not implement PipeTransform interface -> Option A

4. Quick Check:

   Pipe class must implement PipeTransform [OK]

Hint: Always implement PipeTransform for custom pipes [OK]

Common Mistakes:

• Omitting PipeTransform interface
• Misnaming the transform method
• Incorrect pipe decorator usage

5. You want to create a custom pipe that formats a number to show 'Even' if the number is even, 'Odd' if odd, and 'Zero' if zero. Which transform method implementation is correct?

hard

A. transform(value: number): string { return value % 2 === 0 ? 'Even' : 'Odd'; }

B. transform(value: number): string { if (value === 0) return 'Zero'; return value % 2 === 0 ? 'Even' : 'Odd'; }

C. transform(value: number): string { if (value === 0) return 'Odd'; else return 'Even'; }

D. transform(value: number): string { if (value) return 'Even'; else return 'Zero'; }

Solution

1. Step 1: Handle zero case explicitly

   Zero is a special case and must return 'Zero'.

2. Step 2: Check even or odd for other numbers

   Use modulo operator to check if number is even or odd and return accordingly.

3. Final Answer:

   transform(value: number): string { if (value === 0) return 'Zero'; return value % 2 === 0 ? 'Even' : 'Odd'; } -> Option B

4. Quick Check:

   Zero check first, then even/odd [OK]

Hint: Check zero before even/odd to avoid wrong output [OK]

Common Mistakes:

• Not handling zero separately
• Returning wrong labels for zero
• Using incorrect conditions for even/odd