Angular Reactive forms : how to get just changed values

You can check all controls for dirty-flag. See https://angular.io/api/forms/FormControl

getDirtyValues(form: any) {
        let dirtyValues = {};

        Object.keys(form.controls)
            .forEach(key => {
                let currentControl = form.controls[key];

                if (currentControl.dirty) {
                    if (currentControl.controls)
                        dirtyValues[key] = this.getDirtyValues(currentControl);
                    else
                        dirtyValues[key] = currentControl.value;
                }
            });

        return dirtyValues;
}