This error occurs when using a *ngFor loop on an object in Angular.

You can fix this by creating the following pipe. Using this, you transform the object into an array on runtime

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
  name: 'objectToArray',
})
export class ObjectToArrayPipe implements PipeTransform {
  // The object parameter represents the values of the properties or index.
  transform = (objects: any = []) => {
    return Object.values(objects);
  }
}

You can now use:

<div *ngFor="let review of (book.reviews|objectToArray)">
{{ review.id }}
</div>

I changed this original fix on Stackexchange a bit, using the newer strict typescript syntax.