Angular and TypeScript Basics - Custom Pipes

Angular and TypeScript Basics - Custom Pipes

Last post, we added some basic pipe information to the solar projection page. I noticed that due to the way my array worked, 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):

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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);
}
}

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:

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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;
}
}

Easy peasy! Now to apply it to the table:

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

Becomes:

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<td>{{ projection.purchaseYear | indexOffset }}</td>

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:

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import { SolarProjectionComponent } from './solar-projection/solar-projection.component';

and reference the pipe within the @NgModule declaration:

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@NgModule({
declarations: [
IndexOffsetPipe,
// etc
}
})

Boom! Now when we view the page we can see:

Hurray! The year starts with 1 instead of 0!

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