Jest/Istanbul unit test branch coverage on var declaration

Question

Why is var declaration being flagged for missing branch coverage in Istanbul coverage report?

Answer 1

|| operator is kind of if...else.. conditional statement. You need to test each branch.

E.g.

index.ts:

export class SomeClass {
  INCREMENT = 1;
  MIN_SCALE = 2;
  public zoomOut(this: any): void {
    const scaleVal = this.getFloorVar() || this.INCREMENT || this.MIN_SCALE;
    this.updateZoom(scaleVal);
  }

  public getFloorVar() {
    return 0;
  }

  public updateZoom(scaleVal) {
    console.log(scaleVal);
  }
}

index.spec.ts:

import { SomeClass } from './';

describe('SomeClass', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  it('should pass - 1', () => {
    const instance = new SomeClass();
    jest.spyOn(instance, 'getFloorVar');
    jest.spyOn(instance, 'updateZoom');
    instance.zoomOut();
    expect(instance.getFloorVar).toBeCalledTimes(1);
    expect(instance.updateZoom).toBeCalledWith(1);
  });

  it('should pass - 2', () => {
    const instance = new SomeClass();
    jest.spyOn(instance, 'getFloorVar').mockReturnValueOnce(22);
    jest.spyOn(instance, 'updateZoom');
    instance.zoomOut();
    expect(instance.getFloorVar).toBeCalledTimes(1);
    expect(instance.updateZoom).toBeCalledWith(22);
  });

  it('should pass - 3', () => {
    const instance = new SomeClass();
    instance.INCREMENT = 0;
    jest.spyOn(instance, 'getFloorVar');
    jest.spyOn(instance, 'updateZoom');
    instance.zoomOut();
    expect(instance.getFloorVar).toBeCalledTimes(1);
    expect(instance.updateZoom).toBeCalledWith(2);
  });
});

Unit test result with 100 coverage report:

PASS  src/stackoverflow/59330476/index.spec.ts (11.68s)
  SomeClass
    ✓ should pass - 1 (30ms)
    ✓ should pass - 2 (3ms)
    ✓ should pass - 3 (2ms)

  console.log src/stackoverflow/59330476/index.ts:407
    1

  console.log src/stackoverflow/59330476/index.ts:407
    22

  console.log src/stackoverflow/59330476/index.ts:407
    2

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.ts |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       3 passed, 3 total
Snapshots:   0 total
Time:        12.984s

Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/59330476