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Russ Hammett
Russ Hammett

Posted on • Originally published at Medium on

Angular and TypeScript basics — Custom Pipes

#angular #typescript #programming

Last post, we added some basic pipe information to my http://www.kritner.com/solar-projection page. I noticed that due to the way my array worked, starting at index 0, the data in the table shows the first year as “year 0”. Yes, I could change several places in the code to start with index 1, update the calls to the compounding interest to use index — 1, etc, but that seems like a PITA. Surely we could just do a custom pipe to apply to the year column of our table, that returns the original number +1, right?

So custom pipes, will allow me to write my first function in TypeScript, woohoo! I’ve never been a fan of JavaScript, so TypeScript sounds pretty great considering it (seemingly) C#-ifies JavaScript. More info at http://www.typescriptlang.org/. Note — I say the first function I write, because it really is — the bit of typescript displayed in previous posts was just interface declarations and/or information that was a part of the prebuild dotnet net angular template.

Starting from https://angular.io/guide/pipes#custom-pipes as a base, I can see the custom pipe needs to implement a PipeTransform The complete example is (as of writing):

import { Pipe, PipeTransform } from '[@angular/core](http://twitter.com/angular/core)'; /* * Raise the value exponentially * Takes an exponent argument that defaults to 1. * Usage: * value | exponentialStrength:exponent * Example: * {{ 2 | exponentialStrength:10 }} * formats to: 1024 */ [@Pipe](http://twitter.com/Pipe)({name: 'exponentialStrength'}) export class ExponentialStrengthPipe implements PipeTransform { transform(value: number, exponent: string): number { let exp = parseFloat(exponent); return Math.pow(value, isNaN(exp) ? 1 : exp); } }
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Seems simple enough, let’s adapt the above code to get our pipe — a pipe that simply takes in a number, and outputs a number +1 over the input number:

import { Pipe, PipeTransform } from '[@angular/core](http://twitter.com/angular/core)'; [@Pipe](http://twitter.com/Pipe)({name: 'indexOffset'}) export class IndexOffsetPipe implements PipeTransform { transform(value: number): number { return value + 1; } }
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Easy peasy! Now to apply it to the table:

<td>{{ projection.purchaseYear }}</td>
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Becomes:

<td>{{ projection.purchaseYear | indexOffset }}</td>
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Let’s try it ou — RUNTIME ERROR. Oh noes, what doesn’t it work? Oh, apparently this pipe needs to be registered in my components, that should be an easy fix.

Under app-module.ts I need to make sure to import the file:

import { SolarProjectionComponent } from './solar-projection/solar-projection.component';
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and reference the pipe within the @NgModule declaration:

@NgModule({ declarations: [ IndexOffsetPipe, // etc } })
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Boom! Now when we view the page we can see:

Hurray! The year starts with 1 instead of 0!

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