What are pipes and how do you use them in Angular?

Q: What are pipes and how do you use them in Angular?

Learn the answer to "What are pipes and how do you use them in Angular?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a frontend engineer at a social media company. You are building a new component that needs to display a date in a user-friendly format.

You could format the date in the component’s class, but this would not be a very reusable solution.

The Challenge

Explain what pipes are in Angular and how you would use them to solve this problem. What are the key benefits of using pipes?

Wrong Approach

A junior engineer might try to solve this problem by formatting the date in the component's class. This would work, but it would not be a very reusable solution.

Addresses symptoms, not root cause

Right Approach

A senior engineer would know that a pipe is the perfect tool for this job. They would be able to explain what a pipe is and how to use it to format a date in a user-friendly way.

Step 1: Understand What Pipes Are

A pipe is a way to transform data in your template. It takes in data as input and returns a transformed version of that data.

Step 2: The Different Types of Pipes

Type	Description
Pure	A pure pipe is only called when its input changes. This is the default.
Impure	An impure pipe is called on every change detection cycle.

Step 3: Use a Built-in Pipe

Angular has a number of built-in pipes that you can use, such as:

DatePipe
UpperCasePipe
LowerCasePipe
CurrencyPipe
DecimalPipe
PercentPipe

Here’s how we can use the DatePipe to format a date:

{{ myDate | date:'medium' }}

Step 4: Create a Custom Pipe

You can also create your own custom pipes. Here’s how we can create a custom pipe to truncate a string:

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

@Pipe({
  name: 'truncate'
})
export class TruncatePipe implements PipeTransform {
  transform(value: string, limit: number): string {
    return value.length < limit ? value : value.slice(0, limit) + '...';
  }
}

We can then use the custom pipe in our template:

{{ myString | truncate:10 }}

Systematic, production-ready debugging

Practice Question

You are building a component that displays a list of items that are being filtered in real-time. Which type of pipe would you use for the filter?

Questions