Unit Testing — Complete Guide

Unit Testing — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of MEAN Stack Tutorial on Toolliyo Academy.

8 min read Updated 8/19/2026

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MEAN Stack Tutorial · Lesson 76 of 100

Unit Testing

Stack ✓ → Projects

Projects · 2 — Apps · ~10 min · MEAN — Performance & Testing

What is this?

Unit tests verify isolated functions, Angular components, and Express services with mocks — fast feedback in MeanVerse CI.

Why should you care?

Transfer fee calculation bugs cost money; tests catch regressions before merge.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

// Jest + Angular TestBed
describe('TransferFormComponent', () => {
  it('disables submit when amount invalid', () => {
    TestBed.configureTestingModule({
      imports: [TransferFormComponent],
      providers: [{ provide: AccountService, useValue: { transfer: jest.fn() } }]
    });
    const fixture = TestBed.createComponent(TransferFormComponent);
    fixture.componentInstance.transferForm.patchValue({ amount: 0 });
    fixture.detectChanges();
    expect(fixture.nativeElement.querySelector('button[type=submit]').disabled).toBe(true);
  });
});

// Node service
expect(calculateFee(10000, 'express')).toBe(250);