Parameterized pipes in Angular

Introduction

Parameterized pipes let you change how data looks by giving extra information. This helps show data in different ways without changing the original data.

You want to format a date differently based on user choice.

You need to show a number with a specific number of decimal places.

You want to shorten a long text to a certain length.

You want to add a currency symbol to a number dynamically.

You want to convert text to uppercase or lowercase based on a setting.

Syntax

Angular

{{ value | pipeName:parameter1:parameter2 }}

Parameters come after the pipe name, separated by colons (:).

You can pass multiple parameters to customize the pipe's behavior.

Examples

Formats the birthday date as a full date string like 'Tuesday, June 15, 2021'.

Angular

{{ birthday | date:'fullDate' }}

Shows the price with the Euro currency symbol.

Angular

{{ price | currency:'EUR' }}

Shows only the first 10 characters of text.

Angular

{{ text | slice:0:10 }}

Formats score as a number with at least 1 digit before decimal and 2 digits after.

Angular

{{ score | number:'1.2-2' }}

Sample Program

This Angular component shows how to use parameterized pipes for currency, slicing text, and formatting dates. It displays the original values and their transformed versions using pipes with parameters.

Angular

import { Component } from '@angular/core';

@Component({
  selector: 'app-parameterized-pipe',
  template: `
    <h2>Price Display</h2>
    <p>Original price: {{ price }}</p>
    <p>Price in USD: {{ price | currency:'USD' }}</p>
    <p>Price in GBP: {{ price | currency:'GBP' }}</p>

    <h2>Text Slice</h2>
    <p>Original text: {{ text }}</p>
    <p>First 5 chars: {{ text | slice:0:5 }}</p>

    <h2>Date Format</h2>
    <p>Original date: {{ today }}</p>
    <p>Short date: {{ today | date:'shortDate' }}</p>
    <p>Full date: {{ today | date:'fullDate' }}</p>
  `
})
export class ParameterizedPipeComponent {
  price = 1234.56;
  text = 'Hello Angular Pipes!';
  today = new Date();
}

Important Notes

Angular pipes can take multiple parameters separated by colons.

Parameters can be strings, numbers, or expressions.

Use parameterized pipes to keep templates clean and reusable.

Summary

Parameterized pipes let you customize how data is shown.

Use colons to add parameters after the pipe name.

They help keep your code simple and flexible.

Practice

1. What is the main purpose of parameterized pipes in Angular?
{{ value | pipeName:param }}

easy

A. To define routes in the application

B. To customize how data is displayed by passing extra information

C. To handle HTTP requests

D. To create new components dynamically

Solution

1. Step 1: Understand what pipes do

   Pipes transform data in templates to display it differently.

2. Step 2: Recognize parameterized pipes

   Parameterized pipes accept extra values to customize the transformation.

3. Final Answer:

   To customize how data is displayed by passing extra information -> Option B

4. Quick Check:

   Parameterized pipes = customize display [OK]

Hint: Remember pipes change display; parameters customize it [OK]

Common Mistakes:

• Confusing pipes with components
• Thinking pipes handle logic like HTTP
• Mixing pipes with routing

2. Which of the following is the correct syntax to pass a parameter to a pipe named customPipe with parameter 5?

easy

A. {{ value | customPipe:5 }}

B. {{ value | customPipe(5) }}

C. {{ value | customPipe;5 }}

D. {{ value | customPipe->5 }}

Solution

1. Step 1: Recall Angular pipe parameter syntax

   Angular pipes use colon : to separate parameters.

2. Step 2: Check each option

   Only {{ value | customPipe:5 }} uses colon correctly: | customPipe:5.

3. Final Answer:

   {{ value | customPipe:5 }} -> Option A

4. Quick Check:

   Pipe parameters use colon : [OK]

Hint: Use colon : to pass parameters to pipes [OK]

Common Mistakes:

• Using parentheses instead of colon
• Using semicolon or arrow instead of colon
• Omitting the colon

3. Given the pipe usage {{ 1234.5678 | number:'1.2-2' }}, what will be the displayed output?

medium

A. 1234.56

B. 1234.5678

C. 1,234.57

D. 1,234.5678

Solution

1. Step 1: Understand the number pipe parameter

   The parameter '1.2-2' means minimum 1 digit before decimal, 2 minimum and 2 maximum digits after decimal.

2. Step 2: Format the number 1234.5678 accordingly

   The number is rounded to 2 decimal places and includes thousand separator: '1,234.57'.

3. Final Answer:

   1,234.57 -> Option C

4. Quick Check:

   number pipe rounds decimals as per parameter [OK]

Hint: Number pipe rounds decimals between min and max digits [OK]

Common Mistakes:

• Ignoring thousand separators
• Not rounding decimals
• Confusing min and max decimal digits

4. What is wrong with this pipe usage?
{{ today | date:'yyyy-mm-dd' }}
Assuming today is a valid Date object.

medium

A. The pipe must be used with parentheses

B. The pipe name should be uppercase 'Date'

C. Date pipe does not accept format strings

D. The format string uses lowercase 'mm' which shows minutes, not months

Solution

1. Step 1: Check date format string

   In Angular date pipe, 'mm' means minutes, 'MM' means months.

2. Step 2: Identify the error

   Using 'yyyy-mm-dd' will show minutes instead of months, causing wrong output.

3. Final Answer:

   The format string uses lowercase 'mm' which shows minutes, not months -> Option D

4. Quick Check:

   Date pipe format is case-sensitive [OK]

Hint: Use uppercase MM for months in date pipe format [OK]

Common Mistakes:

• Using lowercase mm for months
• Capitalizing pipe names incorrectly
• Adding parentheses to pipes

5. You want to create a custom pipe truncate that shortens a string to a given length passed as a parameter. Which of these pipe transform method signatures correctly supports a parameter for length?

hard

A. transform(value: string, length: number): string

B. transform(value: string): string

C. transform(length: number, value: string): string

D. transform(): string

Solution

1. Step 1: Recall pipe transform method signature

   The first argument is the value, followed by optional parameters.

2. Step 2: Check parameter order and types

   transform(value: string, length: number): string correctly has value first, then length parameter.

3. Final Answer:

   transform(value: string, length: number): string -> Option A

4. Quick Check:

   Pipe transform(value, params...) signature [OK]

Hint: First argument is value, next are parameters in transform() [OK]

Common Mistakes:

• Swapping parameter order
• Omitting parameters
• Using no parameters at all