This article details how to correctly use DatePipe to format dates in Angular applications. The content covers the import of DatePipe, configuring it as a provider in the component, obtaining the instance through dependency injection, and the specific steps and code examples for applying DatePipe in HTML templates, aiming to help developers avoid common date formatting problems.

Angular DatePipe Overview

Angular's DatePipe is a powerful built-in pipe for formatting date values ??into user-friendly strings in HTML templates. It can handle Date objects, numbers (Unix timestamps), and various date string formats, and display them locally according to the application's locale. However, developers often encounter situations where DatePipe "doesn't work" when using it, usually due to not properly configuring or understanding its scope.

The core function of DatePipe is to convert a date or time value into a readable string according to the specified format or default locale. For example, format new Date() as "June 12, 2024" or "2024/06/12".

Correct configuration and use of DatePipe

To ensure that DatePipe works properly in your Angular component, there are a few key steps to follow.

1. Import the DatePipe module

First, you need to import DatePipe in your component's TypeScript file. DatePipe is located in the @angular/common module.

 import { Component, OnInit } from '@angular/core';
import { DatePipe } from '@angular/common'; // Import DatePipe

2. Provide DatePipe in the component

In order for Angular's dependency injection system to recognize and provide an instance of DatePipe, you need to add it to the providers array in the component's @Component decorator. This makes the DatePipe available in the injector hierarchy of the current component and its subcomponents.

 // list-todos.component.ts
import { Component, OnInit } from '@angular/core';
import { DatePipe } from '@angular/common';

// ...other code...

@Component({
  selector: 'app-list-todos',
  templateUrl: './list-todos.component.html',
  styleUrls: ['./list-todos.component.css'],
  providers: [DatePipe] // Add DatePipe as a provider of the component})
export class ListTodosComponent implements OnInit {
  // ...component properties and methods...
}

Note: If your application has imported CommonModule through AppModule or other shared modules, and you are just using DatePipe in the template without dependency injection, there is usually no need to declare DatePipe again in the component's providers. However, when you need to inject and use DatePipe in a component's TypeScript logic, or when DatePipe is having problems in a specific component, adding it to the providers array is a reliable solution.

3. (Optional) Obtain a DatePipe instance through dependency injection

If you need to programmatically format a date in the component's TypeScript logic (rather than just in the template), you can obtain an instance of DatePipe via constructor dependency injection.

 // list-todos.component.ts
import { Component, OnInit } from '@angular/core';
import { DatePipe } from '@angular/common';

// ...other code...

@Component({
  selector: 'app-list-todos',
  templateUrl: './list-todos.component.html',
  styleUrls: ['./list-todos.component.css'],
  providers: [DatePipe]
})
export class ListTodosComponent implements OnInit {
  testDate: string = new Date(2010, 1, 1).toDateString();
  testDate2: string = new Date(2010, 1, 2).toDateString();

  todos = [
    // ...todo list...
  ];

  constructor(private datePipe: DatePipe) {
    // At this point, you can use the this.datePipe.transform() method in the TS logic of the component // For example: const formattedDate = this.datePipe.transform(new Date(), 'shortDate');
  }

  ngOnInit() {}
}

4. Apply DatePipe in HTML template

Once the DatePipe is properly provided and available, you can apply it in your HTML template using the pipe operator |.

 <!-- list-todos.component.html -->
<h1>My todos</h1>

Fun times ahead

{{testDate}}

{{testDate2 | date}}

Description	Target Completion Date	Is it done?
{{todo.description}}	{{ todo.targetDate | date }}	Yes	No

In the above example, {{ todo.targetDate | date }} will display the value of the todo.targetDate property formatted by DatePipe. You can optionally provide a format parameter to a DatePipe, such as {{ todo.targetDate | date:'shortDate' }} or {{ todo.targetDate | date:'yyyy-MM-dd HH:mm' }}.

Complete code example

For a clearer presentation, below is the complete code of modified list-todos.component.ts and list-todos.component.html.

list-todos.component.ts

 import { Component, OnInit } from '@angular/core';
import { DatePipe } from '@angular/common'; // Import DatePipe

export class Todo {
  constructor(
    public id: number,
    public description: string,
    public done: boolean,
    public targetDate: string // targetDate can be string or Date type) {}
}

@Component({
  selector: 'app-list-todos',
  templateUrl: './list-todos.component.html',
  styleUrls: ['./list-todos.component.css'],
  providers: [DatePipe] // Add DatePipe as a provider of the component})
export class ListTodosComponent implements OnInit {
  testDate: string = new Date(2010, 1, 1).toDateString();
  testDate2: string = new Date(2010, 1, 2).toDateString();

  todos = [
    new Todo(1, 'ex1', true, new Date().toDateString()),
    new Todo(2, 'ex2', false, new Date().toDateString()),
    new Todo(3, 'ex3', false, new Date().toDateString()),
    new Todo(4, 'ex4', false, new Date().toDateString()),
    new Todo(5, 'ex5', false, new Date().toDateString()),
    new Todo(6, 'ex6', false, new Date().toDateString()),
  ];

  constructor(private datePipe: DatePipe) {
    //Inject DatePipe into the constructor for use in TS logic}

  ngOnInit() {}
}

list-todos.component.html

 <h1>My todos</h1>

Fun times ahead

Original date string: {{testDate}}

Use DatePipe formatting: {{testDate2 | date}}

Description	Target Completion Date	Is it done?
{{todo.description}}	{{ todo.targetDate | date:'yyyy-MM-dd' }}	Yes	No

Things to note when using DatePipe

1. Input data type: DatePipe works best for Date objects or date strings in ISO 8601 format (for example, "2024-06-12T10:30:00Z"). Although it can parse a variety of date string formats, using Date objects provides the best compatibility and robustness, avoiding parsing failures caused by non-standard string formats. If your targetDate is already of type Date, no toDateString() conversion is required.
2. Default locale: DatePipe formats dates according to the application's default locale (configured via LOCALE_ID). This means that the same date pipe may produce different output in different locales.
3. Custom format: You can provide multiple predefined format strings (such as 'shortDate', 'mediumDate', 'longDate', 'fullDate') or custom format strings (such as 'yyyy-MM-dd HH:mm:ss') for DatePipe.
   • 'short' (eg 6/15/15, 9:03 AM)
   • 'medium' (eg Jun 15, 2015, 9:03:01 AM)
   • 'long' (eg June 15, 2015 at 9:03:01 AM GMT 1)
   • 'full' (eg Monday, June 15, 2015 at 9:03:01 AM GMT 01:00)
   • 'shortDate' (eg 6/15/15)
   • 'mediumDate' (eg Jun 15, 2015)
   • 'longDate' (eg June 15, 2015)
   • 'fullDate' (eg Monday, June 15, 2015)
   • 'shortTime' (eg 9:03 AM)
   • 'mediumTime' (eg 9:03:01 AM)
   • 'longTime' (eg 9:03:01 AM GMT 1)
   • 'fullTime' (eg 9:03:01 AM GMT 01:00)
4. Pure pipe characteristics: DatePipe is a pure pipe. This means it will only recalculate and rerender when its input value changes. This helps improve the performance of your application because it avoids unnecessary double calculations.

Summarize

Correctly using Angular's DatePipe involves several key steps: ensuring that the DatePipe is imported and provided in the component, doing dependency injection if needed for use in TypeScript logic, and finally applying it via pipe syntax in the HTML template. Follow these steps to format dates efficiently and resolve common issues with DatePipe "not working" in your app. Understanding its input types and formatting options will help you make better use of DatePipe to improve the user experience and application readability.

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