Angular Jasmine TypeError: Cannot read property 'transform' of null

Question

I am trying to test a Pipe with stars, but i get TypeError: Cannot read property 'transform' of null.

import { StarsPipe } from './stars.pipe';

fdescribe('StarsPipe', () => {
    const inputStars = [
      { 'stars': 5 },
      { 'stars': 4 },
      { 'stars': 3 }
    ];

    afterEach(() => {
        starPipe = null;
    });

    let starPipe: StarsPipe = null;

    it('pipe for five stars', () => {
        const fivestars = starPipe.transform(inputStars, true, false, false);
        const expectedResult = [{ 'stars': 5 }];

        expect(fivestars).toEqual(expectedResult);
    });

    it('pipe for four stars', () => {
        const fourstars = starPipe.transform(inputStars, false, true, false);
        const expectedResult = [{ 'stars': 4 }];

        expect(fourstars).toEqual(expectedResult);
    });

    it('pipe for three stars', () => {
        const threestars = starPipe.transform(inputStars, false, false, true);
        const expectedResult = [{ 'stars': 3 }];

        expect(threestars).toEqual(expectedResult);
    });
});

The Pipe is this:

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

@Pipe({
  name: 'stars',
  pure: true
})
export class StarsPipe implements PipeTransform {
    filter = {
        five: true,
        four: true,
        three: true
    };
    transform(rooms: Array < Room > , five: boolean, four: boolean, three: boolean): any[] {
        if (five === true) {
            return rooms.filter(x => (x.stars === 5));
        } else if (four === true) {
            return rooms.filter(x => (x.stars === 4));
        } else if (three === true) {
            return rooms.filter(x => (x.stars === 3));
        } else {
            return null;
        }
    }
}

I searched with no result, i do not know about it, but i have a similar error testing routing too. On other Projects routing works fine. On routing the error is debugElement of undefined, maybe there is a problem on my testing, not sure.

uminder uminder · Accepted Answer · 2020-01-09T14:27:28

When running the tests, starPipe is null because it is not properly initialized. Therefore, invoking starPipe.transform produces this error.

You can get rid of the afterEach method but should add a beforeEach method instead where you initialize starPipe as follows:

let starPipe: StarsPipe;

beforeEach(() => {
    starPipe = new StarsPipe(); 
});

Angular Jasmine TypeError: Cannot read property 'transform' of null

1 Answers