Pipes

Overview

Pipes are a special operator in Angular template expressions that allows you to transform data declaratively in your template. Pipes let you declare a transformation function once and then use that transformation across multiple templates. Angular pipes use the vertical bar character (|), inspired by the Unix pipe.

Here is an example using some built-in pipes that Angular provides:

      
import { Component } from '@angular/core';import { CurrencyPipe, DatePipe, TitleCasePipe } from '@angular/common';@Component({  selector: 'app-root',  standalone: true,  imports: [CurrencyPipe, DatePipe, TitleCasePipe],  template: `    <main>       <!-- Transform the company name to title-case and       transform the purchasedOn date to a locale-formatted string --><h1>Purchases from {{ company | titlecase }} on {{ purchasedOn | date }}</h1>        <!-- Transform the amount to a currency-formatted string -->      <p>Total: {{ amount | currency }}</p>    </main>  `,})export class ShoppingCartComponent {  amount = 123.45;  company = 'acme corporation';  purchasedOn = '2024-07-08';}
    

When Angular renders the component, it will ensure that the appropriate date format and currency is based on the locale of the user. If the user is in the United States, it would render:

      
<main>  <h1>Purchases from Acme Corporation on Jul 8, 2024</h1>  <p>Total: $123.45</p></main>
    

See the in-depth guide on i18n to learn more about how Angular localizes values.

Built-in Pipes

Angular includes a set of built-in pipes in the @angular/common package:

Using pipes

Angular's pipe operator uses the vertical bar character (|), within a template expression. The pipe operator is a binary operator– the left-hand operand is the value passed to the transformation function, and the right side operand is the name of the pipe and any additional arguments (described below).

      
<p>Total: {{ amount | currency }}</p>
    

In this example, the value of amount is passed into the CurrencyPipe where the pipe name is currency. It then renders the default currency for the user’s locale.

Combining multiple pipes in the same expression

You can apply multiple transformations to a value by using multiple pipe operators. Angular runs the pipes from left to right.

The following example demonstrates a combination of pipes to display a localized date in all uppercase:

      
<p>The event will occur on {{ scheduledOn | date | uppercase }}.</p>
    

Passing parameters to pipes

Some pipes accept parameters to configure the transformation. To specify a parameter, append the pipe name with a colon (:) followed by the parameter value.

For example, the DatePipe is able to take parameters to format the date in a specific way.

      
<p>The event will occur at {{ scheduledOn | date:'hh:mm' }}.</p>
    

Some pipes may accept multiple parameters. You can specify additional parameter values separated by the colon character (:).

For example, we can also pass a second optional parameter to control the timezone.

      
<p>The event will occur at {{ scheduledOn | date:'hh:mm':'UTC' }}.</p>
    

How pipes work

Conceptually, pipes are functions that accept an input value and return a transformed value.

      
import { Component } from '@angular/core';import { CurrencyPipe} from '@angular/common';@Component({  selector: 'app-root',  standalone: true,  imports: [CurrencyPipe],  template: `    <main>      <p>Total: {{ amount | currency }}</p>    </main>  `,})export class AppComponent {  amount = 123.45;}
    

In this example:

1. CurrencyPipe is imported from @angular/common
2. CurrencyPipe is added to the imports array
3. The amount data is passed to the currency pipe

Pipe operator precedence

The pipe operator has lower precedence than other binary operators, including +, -, *, /, %, &&, ||, and ??.

      
<!-- firstName and lastName are concatenated before the result is passed to the uppercase pipe -->{{ firstName + lastName | uppercase }}
    

The pipe operator has higher precedence than the conditional (ternary) operator.

      
{{ (isAdmin ? 'Access granted' : 'Access denied') | uppercase }}
    

If the same expression were written without parentheses:

      
{{ isAdmin ? 'Access granted' : 'Access denied' | uppercase }}
    

It will be parsed instead as:

      
{{ isAdmin ? 'Access granted' : ('Access denied' | uppercase) }}
    

Always use parentheses in your expressions when operator precedence may be ambiguous.

Change detection with pipes

By default, all pipes are considered pure, which means that it only executes when a primitive input value (such as a String, Number, Boolean, or Symbol) or a changed object reference (such as Array, Object, Function, or Date). Pure pipes offer a performance advantage because Angular can avoid calling the transformation function if the passed value has not changed.

As a result, this means that mutations to object properties or array items are not detected unless the entire object or array reference is replaced with a different instance. If you want this level of change detection, refer to detecting changes within arrays or objects.

Creating custom pipes

You can define a custom pipe by implementing a TypeScript class with the @Pipe decorator. A pipe must have two things:

• A name, specified in the pipe decorator
• A method named transform that performs the value transformation.

The TypeScript class should additionally implement the PipeTransform interface to ensure that it satisfies the type signature for a pipe.

Here is an example of a custom pipe that transforms strings to kebab case:

      
// kebab-case.pipe.tsimport { Pipe, PipeTransform } from '@angular/core';@Pipe({  name: 'kebabCase',  standalone: true,})export class KebabCasePipe implements PipeTransform {  transform(value: string): string {    return value.toLowerCase().replace(/ /g, '-');  }}
    

Using the @Pipe decorator

When creating a custom pipe, import Pipe from the @angular/core package and use it as a decorator for the TypeScript class.

      
import { Pipe } from '@angular/core';@Pipe({  name: 'myCustomTransformation',  standalone: true})export class MyCustomTransformationPipe {}
    

The @Pipe decorator requires two configuration options:

• name: The pipe name that will be used in a template
• standalone: true - Ensures the pipe can be used in standalone applications

Naming convention for custom pipes

The naming convention for custom pipes consists of two conventions:

• name - camelCase is recommended. Do not use hyphens.
• class name - PascalCase version of the name with Pipe appended to the end

Implement the PipeTransform interface

In addition to the @Pipe decorator, custom pipes should always implement the PipeTransform interface from @angular/core.

      
import { Pipe, PipeTransform } from '@angular/core';@Pipe({  name: 'myCustomTransformation',  standalone: true})export class MyCustomTransformationPipe implements PipeTransform {}
    

Implementing this interface ensures that your pipe class has the correct structure.

Transforming the value of a pipe

Every transformation is invoked by the transform method with the first parameter being the value being passed in and the return value being the transformed value.

      
import { Pipe, PipeTransform } from '@angular/core';@Pipe({  name: 'myCustomTransformation',  standalone: true})export class MyCustomTransformationPipe implements PipeTransform {  transform(value: string): string {    return `My custom transformation of ${value}.`  }}
    

Adding parameters to a custom pipe

You can add parameters to your transformation by adding additional parameters to the transform method:

      
import { Pipe, PipeTransform } from '@angular/core';@Pipe({  name: 'myCustomTransformation',  standalone: true})export class MyCustomTransformationPipe implements PipeTransform {  transform(value: string, format: string): string {    let msg = `My custom transformation of ${value}.`    if (format === 'uppercase') {      return msg.toUpperCase()    else {      return msg    }  }}
    

Detecting change within arrays or objects

When you want a pipe to detect changes within arrays or objects, it must be marked as an impure function by passing the pure flag with a value of false.

Avoid creating impure pipes unless absolutely necessary, as they can incur a significant performance penalty if used without care.

      
import { Pipe, PipeTransform } from '@angular/core';@Pipe({  name: 'featuredItemsImpure',  pure: false,  standalone: true})export class FeaturedItemsImpurePipe implements PipeTransform {  transform(value: string, format: string): string {    let msg = `My custom transformation of ${value}.`    if (format === 'uppercase') {      return msg.toUpperCase()    else {      return msg    }  }}
    

Angular developers often adopt the convention of including Impure in the pipe name and class name to indicate the potential performance pitfall to other developers.