Where is a custom pipe used?

It is used in Angular templates to transform displayed values using the | pipe syntax.

How Do You Create a Custom Pipe in Angular | FullStackPrep.Dev

Q: How do you create a custom pipe in Angular?

To create a custom pipe, define a class using the @Pipe decorator, implement the PipeTransform interface, and provide logic inside the transform() method.

💡 Concept Name

Custom Pipe – A user-defined transformation function that formats data in Angular templates.

📘 Quick Intro

Pipes in Angular transform displayed values within templates. When built-in pipes are insufficient, you can create your own custom pipe by implementing the PipeTransform interface.

🧠 Analogy / Short Story

Think of a pipe as a coffee filter—it transforms raw coffee into a drinkable form. Similarly, a custom pipe filters or formats raw data into human-friendly output.

🔧 Technical Explanation

🔨 Custom pipes are created using @Pipe decorator.
📦 Must implement the PipeTransform interface.
🚀 The transform() method performs the transformation logic.
🔄 Pipes are used in templates via the | pipeName syntax.
📁 Register the pipe in the module’s declarations array.

🎯 Purpose & Use Case

✅ Formatting text (e.g., capitalize, truncate).
✅ Converting status codes to user-friendly labels.
✅ Filtering arrays or mapping enums.

💻 Real Code Example

// create a pipe using Angular CLI:
// ng generate pipe capitalize

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

@Pipe({ name: 'capitalize' })
export class CapitalizePipe implements PipeTransform {
  transform(value: string): string {
    if (!value) return '';
    return value.charAt(0).toUpperCase() + value.slice(1).toLowerCase();
  }
}

// usage in template:
{{ 'angular' | capitalize }} // Output: Angular

❓ Interview Q&A

Q1: What is a custom pipe in Angular?
A: A user-defined transformation used to format data in Angular templates.

Q2: Which interface must a custom pipe implement?
A: PipeTransform.

Q3: What method must be defined inside a custom pipe?
A: The transform() method.

Q4: How do you use a pipe in an Angular template?
A: With the | pipeName syntax (e.g., {{ value | capitalize }}).

Q5: Where should the custom pipe be declared?
A: In the module’s declarations array.

📝 MCQs

Q1. Which interface is implemented by a custom pipe?

OnInit
PipeTransform
NgPipe
PipeModule

Q2. Which method defines transformation logic?

convert
apply
format
transform

Q3. How is a pipe used in templates?

With * syntax
With & syntax
With | syntax
With # syntax

Q4. What does the @Pipe decorator define?

Component class
Pipe metadata
Directive
Service logic

Q5. Where do you declare custom pipes?

providers
imports
exports
declarations

Q6. Which CLI command creates a pipe?

ng new pipe
ng make pipe
ng generate pipe
ng pipe create

Q7. What type does transform() return?

void
string
number
Any type

Q8. Which Angular core package contains Pipe?

@angular/router
@angular/core
@angular/common
@angular/forms

Q9. Can a pipe accept arguments?

No
Yes
Only one
Only if pure

Q10. What is the default purity of custom pipes?

Pure
Impure
Manual
Stateful

💡 Bonus Insight

Pipes can be marked as pure: false in the decorator to respond to every change, but this may impact performance. Always prefer pure pipes unless mutation tracking is necessary.

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How Do You Create a Custom Pipe in Angular