Creating Custom Pipes in Angular

Pipes in Angular 2+ are a great way to transform and format data right from your templates. Out of the box you get pipes for dates, currency, percentage and character cases, but you can also easily define custom pipes of your own. Here for example we create a pipe that takes a string and reverses the order of the letters. Here's the code that would go into a reverse-str.pipe.ts file inside of your app folder:

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

@Pipe({name: 'reverseStr'})
export class ReverseStr implements PipeTransform {
  transform(value: string): string {
    let newStr: string = "";
    for (var i = value.length - 1; i >= 0; i--) {
      newStr += value.charAt(i);
    }
    return newStr;
  }
}

Then you’d include the custom pipe as a declaration in your app module:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { HttpModule } from '@angular/http';

import { AppComponent } from './app.component';
import { ReverseStr } from './reverse-str.pipe.ts';

@NgModule({
  declarations: [
    AppComponent,
    ReverseStr
  ],
  imports: [
    BrowserModule,
    FormsModule,
    HttpModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

And finally in your templates this is how you would use this custom pipe:

{{ user.name | reverseStr }}

Pipe with parameters

You can also define any amount of parameters in the pipe, which enables you to pass parameters to it. For example, here’s a pipe that adds a string before and after our provided string:

@Pipe({name: 'uselessPipe'})
export class uselessPipe implements PipeTransform {
  transform(value: string, before: string, after: string): string {
    let newStr = `${before} ${value} ${after}`;
    return newStr;
  }
}

And you would call it like that:

{{ user.name | uselessPipe:"Mr.":"the great" }}

Notice how we used ES6’s string interpolation to construct the new string with such ease: `${before} ${value} ${after}`

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