How to set default selected value of ion-option?

If you deal with default value xor objects, the cleanest way is to provide a compare function to the [compareWith] attribute. This function takes 2 objects in parameters and returns true if they are equals. This way, existing values in model will be selected at start. This works too if new data are added in model outside of the select.

Following example is taken from official Ionic doc

<ion-item>
  <ion-label>Employee</ion-label>
  <ion-select [(ngModel)]="employee" [compareWith]="compareFn">
    <ion-option *ngFor="let employee of employees" [value]="employee"></ion-option>
  </ion-select>
</ion-item>

compareFn(e1: Employee, e2: Employee): boolean {
  return e1 && e2 ? e1.id == e2.id : e1 == e2;
}

EDIT : using double equals make it work for Ionic 4 (as Stan Kurdziel suggests in comment)

The problem seems to be that ion-option don't like objects in rc3. I have to work with only the id part of the object and write a seperate changehandler that find the needed object and set it as a value.

  <ion-select [ngModel]="company.form.id" (ngModelChange)="companyFormSelected($event)" okText="Ok" cancelText="Mégsem">
    <ion-option *ngFor="let form of forms" [value]="form.id" [selected]="form.id == company.form.id">{{form.name}}</ion-option>
  </ion-select>

And the changehandler:

companyFormSelected(newform) {
    let selectedForm = this.forms.find((f)=>{
      return f.id === newform;
    });
    this.company.form=selectedForm;
}

This seems to be a bug in rc3 but I don't know where can I report bugs. I did open a topic on ionic forum.