Creating custom pipes in Angular - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a custom pipe class with the correct decorator.

Angular

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

@Pipe({
  name: '[1]'
})
export class ExclaimPipe implements PipeTransform {
  transform(value: string): string {
    return value + '!';
  }
}

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

Atransform

BExclaimPipe

CpipeName

Dexclaim

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of a simple pipe name string.

Leaving the name property empty.

The @Pipe decorator requires a 'name' property which is the pipe's name used in templates. Here, 'exclaim' is the correct pipe name.

2fill in blank

medium

Complete the transform method signature to accept a number and return a string.

Angular

transform(value: [1]): string {
  return value.toFixed(2);
}

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

Aboolean

Bnumber

Cany

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using string type which does not have toFixed method.

Using boolean type which is invalid here.

The transform method receives the input value. Here, it should be a number to use toFixed(2).

3fill in blank

hard

Fix the error in the pipe transform method to handle null or undefined input safely.

Angular

transform(value: string | null | undefined): string {
  return value[1] + '!';
}

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

A?.

B!

C|| ''

D?? ''

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!' which asserts non-null but can cause runtime errors if value is null.

Using '?.' which returns undefined if value is null, causing concatenation to fail.

Using '|| ''' ensures that if value is null or undefined, it defaults to an empty string before concatenation.

4fill in blank

hard

Fill both blanks to create a pipe that converts a string to uppercase and appends a suffix.

Angular

transform(value: string, suffix: string = '[1]'): string {
  return value.[2]() + suffix;
}

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

A'!!!'

BtoUpperCase

CtoLowerCase

D'???'

Attempts:

3 left

💡 Hint

Common Mistakes

Using toLowerCase instead of toUpperCase.

Using suffix without quotes.

The default suffix is '!!!' and the string method to convert to uppercase is toUpperCase().

5fill in blank

hard

Fill all three blanks to create a pipe that formats a date with a given format and locale.

Angular

transform(value: Date, format: string = '[1]', locale: string = '[2]'): string {
  return new Intl.DateTimeFormat([3], { dateStyle: format }).format(value);
}

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

A'medium'

B'en-US'

Clocale

D'short'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing locale as a string literal instead of variable.

Using incorrect format strings.

The default format is 'medium', locale is 'en-US', and the Intl.DateTimeFormat uses the locale variable.

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