ng-reactive-form

  import { FormBuilder, FormGroup, FormControl, Validators } from '@angular/forms';
  
  constructor(
    private fb: FormBuilder
  ) {
      this.complexForm = fb.group({
        firstName : [null, Validators.required],
        lastName: [null,  Validators.compose([Validators.required, Validators.minLength(5), Validators.maxLength(10)])],
        gender: ['Female'],
        hiking : false,
        daysOfMonth: this.fb.array([]),
      });
      this.complexForm.get('SOME_FIELD').valueChanges.subscribe( (filed: any) => {
        console.log('form.SOME_FIELD changed to:', filed);
      });
      this.complexForm.valueChanges.subscribe( (form: any) => {
        console.log('form changed to:', form);
      });
      
  }
  setValueOfForm() {
    this.complexForm.setValue({
      ...this.complexForm.value,
      firstName: 'value',
    });
  }
  // Function to clear fb.array -> [];
  // this.clearFormArray(this.complexForm.get('FORM_ARRAY_FIELD_NAME') as FormArray);
  public clearFormArray(formArray: FormArray) {
    while (formArray.length !== 0) {
      formArray.removeAt(0);
    }
  }
    
  // Add/Delete value from/to FormArray  
  public onCheckbox(event: any, formFiledName: string) {
    const formFiled = <FormArray>this.complexForm.get(formFiledName) as FormArray;
    if (event.checked) {
      formFiled.push(new FormControl(event.source.value));
    } else {
      const i = formFiled.controls.findIndex(x => x.value === event.source.value);
      formFiled.removeAt(i);
    }
  }
  ========================================================================================
    
  // HTML Template
  <form [formGroup]="complexForm" (ngSubmit)="submitForm(complexForm.value)">
    <div class="form-group">
      <label>First Name:</label>
      <input class="form-control" type="text" placeholder="John" [formControl]="complexForm.controls['firstName']">
    </div>
    <div class="form-group">
      <label>Last Name</label>
      <input class="form-control" type="text" placeholder="Doe" [formControl]="complexForm.controls['lastName']">
    </div>
    <div class="form-group">
      <label>Gender</label>
    </div>

    <div class="radio">
      <label>
        <input type="radio" name="gender" value="Male" [formControl]="complexForm.controls['gender']">
        Male
      </label>
    </div>
    <div class="radio">
      <label>
        <input type="radio" name="gender" value="Female" [formControl]="complexForm.controls['gender']">
        Female
      </label>
    </div>

    <div class="form-group">
      <label>Activities</label>
    </div>

    <label class="checkbox-inline">
      <input type="checkbox" value="hiking" name="hiking" (change)="onCheckbox($event, 'hiking')" [formControl]="complexForm.controls['hiking']"> Hiking
    </label>

    <div class="form-group">
      <button type="submit" class="btn btn-default">Submit</button>
    </div>
  </form>