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 ?

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.

Check this link , and last comment , You have to use WebChromeClient for your purpose.

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

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);
        }

    });

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;
            }
    }