Jest test creates empty blob object

Question

I have a working function that builds a new Blob() in my app

_buildBlobForProperties(app): Blob {
   return new Blob(
        [
          JSON.stringify({
            name: app.name,
            description: app.description,
          }),
        ],
        {
          type: 'application/json',
        }
      );
}

And I have the following test:

it('should return properties for update',() => {
      const blob: Blob = appService._buildBlobForProperties(app);
      expect(blob.text()).toBe(updateBlob.text());
    });

This test works fine in Jasmin/Karma but when migrating the test to jest I get:

TypeError: blob.text is not a function

And when I print the content of the return Blob I get

console.log
    --->  Blob {}

Any suggestions?

_buildBlobForProperties() does not return a promise from what i see so why the await ? — HenriDev, Sep 10 '21 at 16:28
Good point, it's obvious it's not needed once I simplified it for the example (I'm only migrating our tests, didn't write most of them :P ) — Jason Rogers, Sep 13 '21 at 08:42

score 1 · Answer 1 · answered Sep 15 '21 at 04:46

Jest test environment doesn't support the Blob object very well, see issue#2555, I suggest you use blob-polyfill package to global patch Blob object in the test environment. So whether the testEnvironment is jsdom or node test environment, your test cases can pass.

service.ts:

export class AppService {
  _buildBlobForProperties(app): Blob {
    return new Blob([JSON.stringify({ name: app.name, description: app.description })], { type: 'application/json' });
  }
}

service.test.ts:

import { Blob } from 'blob-polyfill';
import { AppService } from './service';

globalThis.Blob = Blob;

describe('69135061', () => {
  test('should pass', async () => {
    const appService = new AppService();
    const app = { name: 'teresa teng', description: 'best singer' };
    const blob: Blob = appService._buildBlobForProperties(app);
    const text = await blob.text();
    expect(text).toEqual(JSON.stringify(app));
  });
});

jest.config.js:

module.exports = {
  preset: 'ts-jest/presets/js-with-ts',
  testEnvironment: 'jsdom',
  // testEnvironment: 'node'
};

test result:

 PASS  examples/69135061/service.test.ts (8.044 s)
  69135061
    ✓ should pass (3 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        8.092 s, estimated 9 s
Ran all test suites related to changed files.

package versions:

"ts-jest": "^26.4.4",
"jest": "^26.6.3",
"blob-polyfill": "^5.0.20210201"

score 1 · Answer 2 · answered Nov 06 '22 at 15:35

You can ju test how Blob is being called.

const TEST_BLOB_CONSTRUCTOR = jest.fn();

jest.spyOn(global, 'Blob').mockImplementationOnce((...args) => {
  TEST_BLOB_CONSTRUCTOR(...args);
});

expect(TEST_BLOB_CONSTRUCTOR).toHaveBeenCalledWith(
  [JSON.stringify({ name: EXPECTED_NAME, description: EXPECTED_APPLICATION })], 
  { type: 'application/json' }
)

Jest test creates empty blob object

2 Answers2