TruncatePipe fails with undefined or null input value to truncate

[cdoremus]

When value to truncate is undefined or null, the TruncatePipe.transform() call throws a TypeError. This error points to line 12 of truncate.pipe.ts where value.length is evaluated. See below for a spec to replicate this error.

Examining the source code of standard Angular 2 pipes that work with strings (e.g. lowercase or uppercase. see: https://github.com/angular/angular/blob/2.1.2/modules/angular/common/src/pipes/uppercase_pipe.ts), shows that the pipe transform() returns null in this case. However, another option is to have transform() return an empty string in the case when value is null or undefined. My opinion is to have TruncatePipe.transform() conform to the standard Angular 2 pipe behavior and return null when the value to truncate is null or undefined. Other opinions are welcome.

/**
 * Spec to replicate this issue
 */
import { Component } from '@angular/core';
import { TestBed } from '@angular/core/testing';

import { TruncatePipe } from './truncate.pipe';


describe('TruncatePipe', () => {
    let pipe;

    beforeEach(() => {
        TestBed.configureTestingModule({
            declarations: [TruncatePipe, TestComponent, Test2Component]
        })
        pipe = new TruncatePipe();
    });

    it('should return null if value is undefined', () => {
        let value = undefined;

        let actual = pipe.transform(value);

        expect(actual).toBeNull();
    });

    it('should return null if value is null', () => {
        let value = null;

        let actual = pipe.transform(value);

        expect(actual).toBeNull();
    });

    it('should return a null string inside a real component template when value to truncate is undefined', () => {
        // TestComponent defined below whose template uses the truncate pipe
        let fixture = TestBed.createComponent(TestComponent);
        let component = fixture.componentInstance;
        let message = component.message;
        let expected = '';
        fixture.detectChanges();

        let actual = fixture.nativeElement.querySelector('h2').innerText;

        expect(actual).toBeNull();
    });

    it('should return a null string inside a real component template when value to truncate is null', () => {
        // Test2Component defined below whose template uses the truncate pipe
        let fixture = TestBed.createComponent(TestComponent);
        let component = fixture.componentInstance;
        let message = component.message;
        fixture.detectChanges();

        let actual = fixture.nativeElement.querySelector('h2').innerText;

        expect(actual).toBeNull();
    });

});

@Component({
    selector: 'test',
    template: `<h2>{{ message | truncate }}</h2>`
})
class TestComponent {
    message = undefined;
}

@Component({
    selector: 'test2',
    template: `<h2>{{ message | truncate }}</h2>`
})
class Test2Component {
    message = null;
}

[lukad03]

@cdoremus Taking the Angular approach sounds like the right call. Would you be willing to make a PR with a fix? (Thanks for all of your other PRs so far - super helpful!)

[cdoremus]

Yes, I will create a PR to fix this issue including updating truncate.pipe.spec.ts.

[lukad03]

Closed by #166