Run some Javascript in Flutter Webview

flutterWebviewPlugin.evalJavascript('<script language="JavaScript" type="text/javascript">alert("Hello World")</script>')

expects JavaScript, not HTML

<script language="JavaScript" type="text/javascript">alert("Hello World")</script>

is HTML.

Try

flutterWebviewPlugin.evalJavascript('alert("Hello World")')

You can try my plugin flutter_inappwebview, which is a Flutter plugin that allows you to add inline WebViews or open an in-app browser window and has a lot of events, methods, and options to control WebViews.

To run some js, you can use:

• Future<dynamic> evaluateJavascript({@required String source}): Evaluates JavaScript code into the WebView and returns the result of the evaluation.
• Future<void> injectJavascriptFileFromUrl({@required String urlFile}): Injects an external JavaScript file into the WebView from a defined url.
• Future<void> injectJavascriptFileFromAsset({@required String assetFilePath}): Injects a JavaScript file into the WebView from the flutter assets directory (see more here on how to load a file from the assets folder).

Full example:

import 'dart:async';

import 'package:flutter/material.dart';

import 'package:flutter_inappwebview/flutter_inappwebview.dart';

Future main() async {
  runApp(new MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> {

  @override
  void initState() {
    super.initState();
  }

  @override
  void dispose() {
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: InAppWebViewPage()
    );
  }
}

class InAppWebViewPage extends StatefulWidget {
  @override
  _InAppWebViewPageState createState() => new _InAppWebViewPageState();
}

class _InAppWebViewPageState extends State<InAppWebViewPage> {
  InAppWebViewController webView;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
            title: Text("InAppWebView")
        ),
        body: Container(
            child: Column(children: <Widget>[
              Expanded(
                child: Container(
                  child: InAppWebView(
                    initialUrl: "https://www.example.org/",
                    initialHeaders: {},
                    initialOptions: InAppWebViewWidgetOptions(
                      inAppWebViewOptions: InAppWebViewOptions(
                        debuggingEnabled: true,
                      ),
                    ),
                    onWebViewCreated: (InAppWebViewController controller) {
                      webView = controller;
                    },
                    onLoadStart: (InAppWebViewController controller, String url) {

                    },
                    onLoadStop: (InAppWebViewController controller, String url) async {
                      int result1 = await controller.evaluateJavascript(source: "10 + 20;");
                      print(result1); // 30

                      String result2 = await controller.evaluateJavascript(source: """
                        var firstname = "Foo";
                        var lastname = "Bar";
                        firstname + " " + lastname;
                      """);
                      print(result2); // Foo Bar

                      // inject javascript file from an url
                      await controller.injectJavascriptFileFromUrl(urlFile: "https://code.jquery.com/jquery-3.3.1.min.js");
                      // wait for jquery to be loaded
                      await Future.delayed(Duration(milliseconds: 1000));
                      String result3 = await controller.evaluateJavascript(source: "\$('body').html();");
                      print(result3); // prints the body html

                      // inject javascript file from assets folder
                      await controller.injectJavascriptFileFromAsset(assetFilePath: "assets/myJavascriptFile.js");
                    },
                  ),
                ),
              ),
            ]))
    );
  }
}

Well, nothing happened because flutter webview does not support javascript alert function. Try writing a javascript function that changes the value of the innerText of an HTML element and then call the function using .evalJavascript to see the result.