ViewController.swift

import UIKit
import WebKit

class ViewController: UIViewController, WKUIDelegate, WKNavigationDelegate, WKScriptMessageHandler {
    var webView: WKWebView!

    override func loadView() {
        let webConfiguration = WKWebViewConfiguration()
        let contentController = WKUserContentController()
        // Inject JavaScript which sending message to App
        let js: String = "window.webkit.messageHandlers.callbackHandler.postMessage('Hello from JavaScript');"
        let userScript = WKUserScript(source: js, injectionTime: WKUserScriptInjectionTime.atDocumentEnd, forMainFrameOnly: false)
        contentController.removeAllUserScripts()
        contentController.addUserScript(userScript)
        // Add ScriptMessageHandler
        contentController.add(
            self,
            name: "callbackHandler"
        )

        webConfiguration.userContentController = contentController

        webView = WKWebView(frame: .zero, configuration: webConfiguration)
        webView.uiDelegate = self
        webView.navigationDelegate = self
        view = webView
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        let myURL = URL(string: "http://localhost:3000")
        let myRequest = URLRequest(url: myURL!)
        webView.load(myRequest)
    }

    // Implement `WKScriptMessageHandler`，handle message which been sent by JavaScript
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        if(message.name == "callbackHandler") {
            print("JavaScript is sending a message \(message.body)")
        }
    }
}