How to Create Custom Pipe in Angular: Simple Guide

To create a custom pipe in Angular, define a class with the @Pipe decorator and implement the PipeTransform interface with a transform method. Then, declare the pipe in your module and use it in templates with the pipe symbol |.

Syntax

A custom pipe in Angular requires a class decorated with @Pipe that has a unique name. The class must implement PipeTransform and define a transform method which takes input value and optional arguments, returning the transformed output.

• @Pipe({name: 'pipeName'}): Declares the pipe and its name for template use.
• implements PipeTransform: Ensures the class has a transform method.
• transform(value: any, ...args: any[]): any: Method to change the input value.

typescript

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

@Pipe({
  name: 'customPipeName'
})
export class CustomPipe implements PipeTransform {
  transform(value: any, ...args: any[]): any {
    // transformation logic here
    const transformedValue = value; // example placeholder
    return transformedValue;
  }
}

Example

This example shows a custom pipe named exclaim that adds exclamation marks to a string. It demonstrates how to create the pipe, declare it in a module, and use it in a template.

typescript

import { NgModule, Component, Pipe, PipeTransform } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';

@Pipe({
  name: 'exclaim'
})
export class ExclaimPipe implements PipeTransform {
  transform(value: string, times: number = 1): string {
    return value + '!'.repeat(times);
  }
}

@Component({
  selector: 'app-root',
  template: `<p>{{ 'Hello Angular' | exclaim:3 }}</p>`
})
export class AppComponent {}

@NgModule({
  declarations: [AppComponent, ExclaimPipe],
  imports: [BrowserModule],
  bootstrap: [AppComponent]
})
export class AppModule {}

platformBrowserDynamic().bootstrapModule(AppModule);

Output

Hello Angular!!!

Common Pitfalls

• Forgetting to add the custom pipe to the declarations array of your Angular module causes the pipe to be unknown in templates.
• Not implementing PipeTransform interface can lead to missing the required transform method.
• Using a pipe name that conflicts with built-in pipes or other custom pipes can cause unexpected behavior.
• Returning null or undefined unintentionally from transform can break template rendering.

typescript

/* Wrong: Pipe not declared in module */
import { Pipe, PipeTransform } from '@angular/core';

@Pipe({ name: 'wrongPipe' })
export class WrongPipe implements PipeTransform {
  transform(value: string): string {
    return value.toUpperCase();
  }
}

/* Right: Declare pipe in module declarations */
import { NgModule } from '@angular/core';

@NgModule({
  declarations: [WrongPipe],
  // other module metadata
})
export class AppModule {}

Quick Reference

Remember these key points when creating custom pipes:

• Use @Pipe decorator with a unique name.
• Implement PipeTransform and define transform method.
• Declare the pipe in your module's declarations.
• Use the pipe in templates with | pipeName syntax.
• Pass optional parameters to transform for flexible transformations.

Key Takeaways

Create a class with @Pipe decorator and implement PipeTransform interface.

Define a transform method to specify how input values change.

Declare your custom pipe in the Angular module declarations array.

Use the pipe in templates with the pipe symbol | followed by the pipe name.

Avoid name conflicts and always return valid transformed values.