React Native JSI: How to call any javascript function from native?

I need to have a callback function to listen for some events in native module and transfers data from native to javascript and I want to call this javascript function from native directly in React Native iOS app without sending events to NativeEventEmitter.

How to implement this with JSI (JavaScript Interface)?

First, your function must be defined globally in javascript e.g.:

App.js

global.greeting = function(param) {
    return "Hello " + param + "!";
};

Then you should find and call it with React Native Runtime in native:

AppDelegate.mm

#include <jsi/jsi.h>
#import <React/RCTBridge+Private.h>

using namespace facebook::jsi;
using namespace std;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  ...
  
  // Runtime notification
  [NSNotificationCenter.defaultCenter addObserverForName:RCTJavaScriptDidLoadNotification object:nil queue:nil
                                              usingBlock:^(NSNotification* notification) {
    // Get runtime
    RCTCxxBridge* cxxbridge = (RCTCxxBridge*)notification.userInfo[@"bridge"];
    if (cxxbridge.runtime) {
      Runtime& runtime = *(Runtime*)cxxbridge.runtime;
      
      // Get global function
      Function greeting = runtime.global().getPropertyAsFunction(runtime, "greeting");
      
      // Call with param
      Value param = Value(runtime, String::createFromUtf8(runtime, "JSI"));
      Value result = greeting.call(runtime, move(param), 1);
      
      string str = result.asString(runtime).utf8(runtime);
      printf("Result: %s", str.c_str());
    }
  }];
  return YES;
}

Outputs:

Result: Hello JSI!

Note: Since the sample uses JSI for synchronous native methods access, remote debugging (e.g. with Chrome) is no longer possible. Instead, you should use Flipper for debugging your JS code.