HelloWord.m

// Objective-C

#import "RCTBridgeModule.h"

@interface MyCustomModule : NSObject <RCTBridgeModule>

@end

@implementation MyCustomModule

RCT_EXPORT_MODULE();

// Available as NativeModules.MyCustomModule.processString
RCT_EXPORT_METHOD(processString:(NSString *)input callback:(RCTResponseSenderBlock)callback)
{
  callback(@[[input stringByReplacingOccurrencesOfString:@"Goodbye" withString:@"Hello"]]);
}
@end