JS+iOS=🤯.swift

private let sandboxURL = URL(string: "https://example.web.app/")!

class SomeViewController: UIViewController {
	
	...

	private lazy var webView: WKWebView = {
    
		let configuration = WKWebViewConfiguration()
		// configuration.userContentController.add(self, name: signInWithCustomToken)
		let rect = CGRect(x: 0, y: 0, width: view.frame.width, height: view.frame.height)
		let webView = WKWebView(frame: rect, configuration: configuration)

		view.addSubview(webView)

		webView.allowsBackForwardNavigationGestures = true
		webView.scrollView.contentInsetAdjustmentBehavior = .never
		webView.navigationDelegate = self
		return webView
	}()
}

extension SomeViewController: WKNavigationDelegate {

	func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {

		let token = "eyJhbGciOiJSUzI1Ni...GeFbBt6R7AxsEKfMFf4Ngq6f091pSMBx7o7mGw"

		let javaScript = "window.app.signInWithCustomToken('\(token)')"
		// let javaScript1 = "setTimeout(window.app.signInWithCustomToken('\(token)'), 1000);"
		webView.evaluateJavaScript(javaScript) { (result, error) in
			print(result)
			print(error?.localizedDescription)
		}

// JS web code
//		window.app.signInWithCustomToken('token')
//			.then(user => {
//				// это будет выполнено если все ок и произодет логин
//			})
//			.catch(err => {
//				// это если будет ошибка
//			})

}