How does evaluateJavascript work?

There is an example of the evaluateJavascript method being used in this sample:

https://github.com/GoogleChrome/chromium-webview-samples/tree/master/jsinterface-example

Essentially if the javascript you execute in the WebView returns a value it'll be passed in the callback.

The main thing to note is that the String returned in OnReceiveValue is either a JSON Value, JSON Object or JSON Array depending on what you return.

Things to note about this is if you return a single value, you need to use setLenient(true) on a JSON reader for it to work.

     if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {        // In KitKat+ you should use the evaluateJavascript method        mWebView.evaluateJavascript(javascript, new ValueCallback<String>() {            @TargetApi(Build.VERSION_CODES.HONEYCOMB)            @Override            public void onReceiveValue(String s) {                JsonReader reader = new JsonReader(new StringReader(s));                // Must set lenient to parse single values                reader.setLenient(true);                try {                    if(reader.peek() != JsonToken.NULL) {                        if(reader.peek() == JsonToken.STRING) {                            String msg = reader.nextString();                            if(msg != null) {                                Toast.makeText(getApplicationContext(), msg, Toast.LENGTH_LONG).show();                            }                        }                    }                } catch (IOException e) {                    Log.e("TAG", "MainActivity: IOException", e);                } finally {                    try {                        reader.close();                    } catch (IOException e) {                        // NOOP                    }                }            }        });    }

The reason you may still want to use a parser for a string response is it is converted to a JSON value which means it will be wrapped in quotes.

For example if you went:

mWebView.evaluateJavascript("(function() { return 'this'; })();", new ValueCallback<String>() {    @Override    public void onReceiveValue(String s) {        Log.d("LogName", s); // Prints: "this"    }});

It would print the string this, wrapped in double quotes: "this".

Other examples worth noting:

mWebView.evaluateJavascript("(function() { return null; })();", new ValueCallback<String>() {    @Override    public void onReceiveValue(String s) {        Log.d("LogName", s); // Prints the string 'null' NOT Java null    }});mWebView.evaluateJavascript("(function() { })();", new ValueCallback<String>() {    @Override    public void onReceiveValue(String s) {        Log.d("LogName", s); //s is Java null    }});mWebView.evaluateJavascript("(function() { return ''; })();", new ValueCallback<String>() {    @Override    public void onReceiveValue(String s) {        Log.d("LogName", s); // Prints "" (Two double quotes)    }});

OK, so it turns out the result here is the result of the Javascript call - as if one were entering the command into a Javascript console.

So in order to get a result, it needs to be wrapped in a function:

webView1.evaluateJavascript("(function() { return \"this\"; })();", new ValueCallback<String>() {    @Override    public void onReceiveValue(String s) {        Log.d("LogName", s); // Prints 'this'    }});

This will also work:

webView1.evaluateJavascript("window.variable = \"asd\";", new ValueCallback<String>() {    @Override    public void onReceiveValue(String s) {        Log.d("LogName", s); // Prints asd    }});

The method also handles Javascript objects:

webView1.evaluateJavascript("(function() { return { var1: \"variable1\", var2: \"variable2\" }; })();", new ValueCallback<String>() {    @Override    public void onReceiveValue(String s) {        Log.d("LogName", s); // Prints: {"var1":"variable1","var2":"variable2"}    }});

AndroidJSCore is a good alternative for evaluating JavaScript that does not use a WebView.

If you want to stick with WebView and need to evaluate JavaScript on earlier versions of Android (4+), here is a little library:

https://github.com/evgenyneu/js-evaluator-for-android

jsEvaluator.evaluate("put your JavaScript code", new JsCallback() {  @Override  public void onResult(final String result) {    // get result here (optional)  }});