Objective C: Is it good practice to check JSON values if it is the type you expect?

Currently, I am checking if the JSON value is an NSString and if it isn't, assign the property a default string. This way I can prevent the app from crashing if the JSON value is null and I can return a User object with a default first name and last name.

// User model

+ (Email *)getEmailInfoWithDictionary:(id)dict {
  Email *email = [[self alloc] init];
  if ([dict isKindOfClass:[NSDictionary class]]) {
    user.firstname = [NSString checkType:dict[@"firstname"] defaultString:@"John"];
    user.lastname = [NSString checkType:dict[@"lastname"] defaultString:@"Smith"];
  }
}
return user;
}

// NSString category method.

+ (NSString *)checkType:(id)obj defaultString:(NSString *)def {
  if (obj == nil || ![obj isKindOfClass:[NSString class]]) {
    return def;
  }
  return obj;
}

I have a couple of concerns, however. Is it a good idea to always check for null values regarding json values so this way you won't get crashes? I've noticed some Objective C tutorials that don't do this. Makes me wonder if I shouldn't worry about this and I should expect the API to return the correct values. My second concern is the method I am using a good idea or is a there better way?

I would say start by leaning into Objective-C's default handling of nil. In most cases, nil will do what you want without crashing. In contrast, [NSNull null] is poorly designed and a constant problem.

@interface NSObject (MyCasting)
- (NSString *)stringOrNil;
- (NSNumber *)numberOrNil;
- (NSDictionary *)dictionaryOrNil;
- (NSArray *)arrayOrNil;
@end

@implementation NSObject (MyCasting)

- (NSString *)stringOrNil {
    return [self isKindOfClass:NSString.class] ? (NSString *)self : nil;
}

- (NSNumber *)numberOrNil {
    return [self isKindOfClass:NSNumber.class] ? (NSNumber *)self: nil;
}

- (NSDictionary *)dictionaryOrNil {
    return [self isKindOfClass:NSDictionary.class] ? (NSDictionary *)self: nil;
}

- (NSArray *)arrayOrNil {
    return [self isKindOfClass:NSArray.class] ? (NSArray *)self: nil;
}

@end

This will provide you with a good level of safety using nil as the default value.

+ (Email *)getEmailInfoWithDictionary:(id)dict {
    Email *email = [[self alloc] init];
    NSDictionary *dictionary = [dict dictionaryOrNil];

    email.firstname = [dictionary[@"firstname"] stringOrNil];
    email.lastname = [dictionary[@"lastname"] stringOrNil];

    return email;
}

If you feel you need the additional safety of object default values, then you can use the ?: operator.

email.firstname = [dictionary[@"firstname"] stringOrNil] ?: "";
email.lastname = [dictionary[@"lastname"] stringOrNil] ?: "";