How validate old password with current password exist on db

Question

problem

How validate old password with current password exist on db ?

I have user on database his name 

michel 

and password 1234567

I need when write old password on angular 7

check password exist on database for user michel as 1234567

with 

password he write 

if two is same not problem

but if he write password different for this 

display error message

<div class="form-group">
  <label >OldPassword</label><br>
  <input type="password" formControlName="oldPassword" class="textboxclass"   [ngClass]="{ 'is-invalid': submitted && c.oldPassword.errors }" />
  <div *ngIf="submitted && c.oldPassword.errors" class="invalid-feedback">
      <div *ngIf="c.oldPassword.errors.required">Password is required</div>
      <div *ngIf="c.oldPassword.errors.minlength">Password must be at least 6 characters</div>
  </div>
</div>

on ts
oldPassword = new FormControl('', [Validators.required, Validators.minLength(6), Validators.maxLength(10)]);
  ngOnInit() {
    this.createFormResetPassword();
  }
createFormResetPassword() {
    this.ChangePasswordForm = this.formBuilder.group({

      oldPassword: this.oldPassword,

    }
  }
onSubmit() {
    this.submitted = true;


    if (this.ChangePasswordForm.invalid) {
      console.log("status failed ");
        return;
    }
    this.loading = true;
    this.userservice.changePassword()
        .pipe(first())
        .subscribe(
            data => {
             });
               console.log("data values" + data);
               this.toastr.success('Users  Added Successfully');
            },
            error => {
                this.error = error;
                this.loading = false;
            });
  }