[Angular Unit Testing] Testing Component methods

import {ComponentFixture, TestBed} from '@angular/core/testing';
import {BrowserDynamicTestingModule, platformBrowserDynamicTesting} from '@angular/platform-browser-dynamic/testing';

import {StockCounterComponent} from './stock-counter.component';

TestBed.initTestEnvironment(
  BrowserDynamicTestingModule,
  platformBrowserDynamicTesting()
);

describe('StockCounterComponent', () => {

  let component: StockCounterComponent;
  let fixture: ComponentFixture<StockCounterComponent>;

  beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [
        StockCounterComponent
      ]
    });

    fixture = TestBed.createComponent(StockCounterComponent);
    component = fixture.componentInstance;

    component.value = 0;
  });

  it('should increment correctly', () => {
    component.increment()
    expect(component.value).toBe(1);
  });

  it('should decrement correctly', () => {
    component.increment()
    expect(component.value).toBe(1);
    component.decrement()
    expect(component.value).toBe(0);
  });

  it('should not decrement below the minimum value', () => {
    component.increment()
    expect(component.value).toBe(1);
    component.decrement()
    expect(component.value).toBe(0);
    component.decrement()
    expect(component.value).toBe(0);
  });

  it('should not increment below the maximum value', () => {
    for (let i = 0; i < 200; i++) {
      component.increment();
    }
    expect(component.value).toBe(100);
  });
});

component:

import { Component, Input, ChangeDetectionStrategy, EventEmitter, Output } from '@angular/core';

@Component({
  selector: 'stock-counter',
  changeDetection: ChangeDetectionStrategy.OnPush,
  template: `
    <div class="stock-counter">
      <div>
        <div 
          (keydown)="onKeyUp($event)"
          (blur)="onBlur($event)"
          (focus)="onFocus($event)"
          tabindex="0">
          <p>{{ value }}</p>
          <div tabindex="-1">
            <button type="button" tabindex="-1" (click)="increment()" [disabled]="value === max">
              +
            </button>
            <button type="button" tabindex="-1" (click)="decrement()" [disabled]="value === min">
              -
            </button>
          </div>
        </div>
      </div>
    </div>
  `
})
export class StockCounterComponent {
  @Input() step: number = 1;
  @Input() min: number = 0;
  @Input() max: number = 100;

  @Output() changed = new EventEmitter<number>();

  value: number = 0;
  focused: boolean;

  increment() {
    if (this.value < this.max) {
      this.value = this.value + this.step;
      this.changed.emit(this.value);
    }
  }

  decrement() {
    if (this.value > this.min) {
      this.value = this.value - this.step;
      this.changed.emit(this.value);
    }
  }

  private onBlur(event: FocusEvent) {
    this.focused = false;
    event.preventDefault();
    event.stopPropagation();
  }

  private onKeyUp(event: KeyboardEvent) {
    let handlers = {
      ArrowDown: () => this.decrement(),
      ArrowUp: () => this.increment()
    };

    if (handlers[event.code]) {
      handlers[event.code]();
      event.preventDefault();
      event.stopPropagation();
    }
  }

  private onFocus(event: FocusEvent) {
    this.focused = true;
    event.preventDefault();
    event.stopPropagation();
  }

}