Simplest Ever Custom Pipe In Angular 2

1/5/2017

This is a post meant to help you understand pipes in Angular 2, and what better place to start than the simplest pipe possible? :)

From Filters To Pipes

If you're familiar with AngularJS then you might be wondering why there are no filters in Angular 2. Well, you can pretty much think of pipes as the Angular 2 equivalent for filters. Although creating each uses a different syntax, they provide the same purpose; to transform data in the view while preserving the data in it's original form. For example, the currency filter allows you to work with pure numbers that can be combined with ordinary mathematical operations while at the same time displaying the data in a nice format with a currency symbol and at most only 2 digits past the decimal.

Angular CLI Pipe Scaffolding

If you're using the Angular CLI you can scaffold out a stub for a pipe super easily with the generate command (shorthand is just g):

ng g pipe derpy-derp

This will generate a file named derpy-derp.pipe.ts (and also a unit test file named derp-derp.spec.ts) in the app/ directory. You could also prefix the name "derpy-derp" with folder names and slashes in order to generate your files further down in your directories. This command will generate some boilerplate code for a pipe that should look something like this:

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

@Pipe({
  name: 'derpyDerp'
})
export class DerpyDerpPipe implements PipeTransform {

  transform(value: any, args?: any): any {
    return null;
  }

}

The above code is what's generated by the Angular CLI, but a pipe that just returns null is no fun. Let's instead have it return something simple, like the number 42!

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

@Pipe({
  name: 'derpyDerp'
})
export class DerpyDerpPipe implements PipeTransform {

  transform(value: any, args?: any): any {
    return 42;
  }

}

Using A Pipe In Your Template

In the HTML markup we can make use of this pipe. Normally we're going to want to transform some dynamic piece of data that is represented by a variable in our component. Inside our our Angular 2 data-binding double curlies we can add the pipe symbol followed by our pipe's name:

<div>
  {{ points | derpyDerp }}
<div>

We can also pass arguments to our pipe like so:

<div>
  {{ points | derpyDerp: arg1:arg2:arg3... }}
<div>

And then we can reference these arguments in our pipe's transform method like this:

  transform(value:any, arg1:any, arg2:any, arg3:any):any

Using Your Pipe In The TypeScript

Although you will often see pipes being used in the HTML with the pipe symbol, you can also make use of your pipes directly from the TypeScript code. Since our pipe is just a TypeScript class we can treat it like a regular old class. That means if we want to use our pipe to transform some data all we need to do is instantiate the pipe and call it's trandform method! Wow!

new DerpyDerpPipe().transform(myData, arg1, arg2, arg3)

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