I'm trying to compare lists using an extension method that calls a comparer like so:
type HasDiff<T> = (object: T, row: any) => boolean;
export const isListEqualToRows = <T>(objects: T[], rows: any[], hasDiffFunction: HasDiff<T>): boolean => {
if (objects.length !== rows.length)
return true;
return rows.every((row, index) => {
const object = objects[index];
if (row.isNew) {
return false;
}
return !hasDiffFunction(object, row);
});
};
The comparison is being triggered by Angular's value changed in a ReactiveForm, not sure if relevant, but here's it:
import { isListEqualToRows } from '@core/helpers';
formGroup.valueChanges.pipe(debounceTime(debounceValue)).subscribe((changes) => {
this.areChangesDetected = !isListEqualToRows<Person>(this.items, changes, this.hasDiff);
});
My issue is that for one object, I have a list within a list. So I'm trying to call the isListEqualToRows() method again inside:
hasDiff(item: Person, row: any): boolean {
return item.description !== row.description
|| !isListEqualToRows<Address>(item.addresses, row.addresses, this.hasDiffAddress);
}
hasDiffAddress(item: Address, row: any): boolean {
return item.AddressId !== row.id
|| item.AddressStreet !== row.street;
}
But I end up getting this error while running the second line of the hasDiff() method:
Cannot read properties of undefined (reading 'hasDiffAddress')
How to solve this issue?
You are calling hasDiff without binding it to the correct this object. Since you have strict mode enabled, the this that hasDiff is referring to when invoked outside of a someObject.hasDiff(...) syntax is then undefined (without strict mode it would be the global object).
Instead of passing this.hasDiff to isListEqualToRows, you need to pass either this.hasDiff.bind(this) or (item, row) => this.hasDiff(item, row).
Alternatively, add a thisArg argument to isListEqualToRows where you pass this from the caller's scope and then invoke hasDiffFunction as hasDiffFunction.call(thisArg, object, row) instead of hasDiffFunction(object, row).
See also: How does the "this" keyword work?