[javascript] JavaScript alert not working in Android WebView

In my application I am using WebView and in that I am using JavaScript alert( ) method but its not working, no pop-up appears.

in my manifest file I have added

<uses-permission android:name="android.permission.INTERNET"></uses-permission>

and in activity file I have added

mWebView = (WebView) findViewById(R.id.webview);
mWebView.getSettings().setJavaScriptEnabled(true);
mWebView.loadUrl("file:///android_asset/demo.html");

In layout xml file I have added

<WebView 
    android:id="@+id/webview"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
/>

Any clue how to enable full JavaScript in WebView?

Update

Thanks mark
the alert() method in the html file are working now :) .

Now there are two issues in WebView :
1: I am using a <textarea> in the html file that i am loading in WebView , and trying to write in Hindi language font in it, but when i try to write Hindi text it displays as symbols ( rectangle symbols like [] ) .

when i do the same in firefox browser on desktop it works fine. any clue how to give support for multiple language in textarea in WebView ?

2: When I am clicking submit and trying to open the value of text in alert() method in another java script it doesn't work , does it mean even after using WebChromeClient its applicable only for current loaded html page and not javascripts called from that page ?

The following code will work:

private WebView mWebView;
final Activity activity = this;

// private Button b;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mWebView = (WebView) findViewById(R.id.webview);
    mWebView.getSettings().setJavaScriptEnabled(true);
    mWebView.getSettings().setDomStorageEnabled(true);
    mWebView.setWebChromeClient(new WebChromeClient() {
        public void onProgressChanged(WebView view, int progress) {
            activity.setProgress(progress * 1000);
        }
    });

    mWebView.loadUrl("file:///android_asset/raw/NewFile1.html");
}

You can try with this, it worked for me

WebView wb_previewSurvey=new WebView(this); 


       wb_previewSurvey.setWebChromeClient(new WebChromeClient() {
        @Override
        public boolean onJsAlert(WebView view, String url, String message, JsResult result) {
            //Required functionality here
            return super.onJsAlert(view, url, message, result);
        }

    });

As others indicated, setting the WebChromeClient is needed to get alert() to work. It's sufficient to just set the default WebChromeClient():

mWebView.getSettings().setJavaScriptEnabled(true);
mWebView.setWebChromeClient(new WebChromeClient());

Thanks for all the comments below. Including John Smith's who indicated that you needed to enable JavaScript.

if you wish to hide URL from the user, Show an AlertDialog as below.

myWebView.setWebChromeClient(new WebChromeClient() {

            @Override
            public boolean onJsAlert(WebView view, String url, String message, JsResult result) {
                Log.d(TAG, "onJsAlert url: " + url + "; message: " + message);
                AlertDialog.Builder builder = new AlertDialog.Builder(
                        mContext);
                builder.setMessage(message)
                        .setNeutralButton("OK", new OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int arg1) {
                                dialog.dismiss();
                            }
                        }).show();
                result.cancel();
                return true;
            }
    }

webView.setWebChromeClient(new WebChromeClient() {
    @Override
    public boolean onJsAlert(WebView view, String url, String message, JsResult result) {
        return super.onJsAlert(view, url, message, result);
    }
});