Migrating AngularJS filters to Angular pipes

Migrating AngularJS filters to Angular pipes is an essential part of the migration process when moving from AngularJS to Angular. AngularJS filters are commonly used to transform or format data within templates. However, Angular (2+) does not have filters; instead, it uses pipes to perform similar data transformations.

Here’s a detailed step-by-step guide on how to migrate AngularJS filters to Angular pipes:

Step 1: Understanding the AngularJS Filter

In AngularJS, filters are used to format, transform, or filter data directly in templates. Filters can be applied to expressions in the view, and they are often used to format dates, currencies, or numbers, or to filter arrays.

Example of an AngularJS filter:

angular.module('app')
  .filter('uppercase', function() {
    return function(input) {
      return input ? input.toUpperCase() : input;
    };
  });

You can apply the uppercase filter in your template:

htmlCopyEdit<p>{{ 'hello world' | uppercase }}</p>

This would display “HELLO WORLD” in the view.

Step 2: Create an Angular Pipe

In Angular, you use pipes instead of filters. A pipe in Angular is a simple function that accepts an input and returns a transformed output. Angular provides built-in pipes for various common tasks like date, currency, and uppercase, but you can also create custom pipes.

Create a Custom Pipe in Angular

To migrate the AngularJS filter to an Angular pipe, follow these steps:

Generate the Pipe using Angular CLI:

bashCopyEditng generate pipe uppercase

This will create a new pipe file, usually in the src/app directory, like uppercase.pipe.ts.

Define the Pipe Logic:

Open the uppercase.pipe.ts file and write the transformation logic in the pipe’s transform method.

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

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

@Pipe decorator: This decorator marks the class as a pipe, and the name property is the name you use in templates.
PipeTransform interface: The transform method implements the transformation logic that will be applied to the input value.

Step 3: Using the Angular Pipe in Templates

Once the pipe is defined, you can use it in Angular templates, just like AngularJS filters. The syntax is very similar to the AngularJS filter syntax, but instead of the | character, Angular uses the | symbol with a pipe name.

Example Usage:

htmlCopyEdit<p>{{ 'hello world' | uppercase }}</p>

This would display “HELLO WORLD” in the view, just like the AngularJS filter.

Step 4: Migrating Complex Filters to Pipes

For more complex AngularJS filters that contain logic (e.g., filtering arrays), you can migrate them into Angular pipes with additional functionality.

Example of a Complex AngularJS Filter (Array Filtering):

angular.module('app')
  .filter('searchFilter', function() {
    return function(items, searchText) {
      return items.filter(item => item.name.includes(searchText));
    };
  });

This filter takes an array of items and filters them based on a searchText.

Migrating to Angular Pipe:

To migrate this to an Angular pipe, you would need to handle input transformation with multiple parameters. In this case, the search text is another input besides the array of items.

Generate the Pipe:

bashCopyEditng generate pipe searchFilter

Define the Pipe Logic:

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

@Pipe({
  name: 'searchFilter'
})
export class SearchFilterPipe implements PipeTransform {
  transform(items: any[], searchText: string): any[] {
    if (!items || !searchText) {
      return items;
    }
    return items.filter(item =>
      item.name.toLowerCase().includes(searchText.toLowerCase())
    );
  }
}

The transform method in the pipe filters the array based on the searchText.
Null or undefined checks: Before filtering, the code checks if the items or searchText is falsy (e.g., null or undefined). If either is falsy, it simply returns the original list.

Use the Pipe in a Template:

htmlCopyEdit<ul>
  <li *ngFor="let item of items | searchFilter: searchText">
    {{ item.name }}
  </li>
</ul>

Here, the searchText is bound to a variable in the component, and the pipe filters the items array based on this value.

Step 5: Using Multiple Pipes in Angular

Angular allows you to chain multiple pipes together. For example, you can combine the uppercase pipe with a searchFilter pipe to display only uppercase filtered results.

htmlCopyEdit<ul>
  <li *ngFor="let item of items | searchFilter: searchText | uppercase">
    {{ item.name }}
  </li>
</ul>

Angular allows the application of multiple pipes in a sequence, where the output of one pipe is passed to the next.

Step 6: Performance Considerations

While migrating filters to pipes, it’s important to note that pipes in Angular can impact performance, especially if they are used on large datasets or in frequently updated components. Angular pipes are pure by default, meaning that they will only execute when their input values change. However, if you create impure pipes (i.e., pipes that execute on every change detection cycle), this can degrade performance.

Example of an Impure Pipe:

@Pipe({
  name: 'searchFilter',
  pure: false  // Impure pipe
})
export class SearchFilterPipe implements PipeTransform {
  transform(items: any[], searchText: string): any[] {
    if (!items || !searchText) {
      return items;
    }
    return items.filter(item =>
      item.name.toLowerCase().includes(searchText.toLowerCase())
    );
  }
}

pure: false: By setting pure: false, the pipe becomes impure and will run on every change detection cycle, even if its inputs haven’t changed.

Tip: Use impure pipes sparingly, and ensure you’re optimizing performance where necessary by reducing the frequency of change detection.