How does evaluateJavascript work?

Question

I\'m trying to use the new evaluateJavascript method in Android 4.4, but all I ever get back is a null result:

webView1.evaluateJavascript(\"return \\\"test\         


        

Answer 1

To summarize the answer of @GauntFace and provide an alternative solution without using JSON parser:

If your JS function returns just a String and you're wondering about why the string is mangled in Java, it's because it's JSON-escaped.

mWebView.evaluateJavascript("(function() { return 'Earvin \"Magic\" Johnson'; })();", new ValueCallback() {
    @Override
    public void onReceiveValue(String s) {
        Log.d("LogName", s);
        // expected: s == Earvin "Magic" Johnson
        // actual:   s == "Earvin \"Magic\" Johnson"
    }
});

(note that onReceiveValue always provides a String while JS function may return a null, a Number literal etc.)

To get the string value the same as in JS, if you're 100% sure you're expecting a proper String returned, you'd need to JSON-unescape it, for example like that:

String unescaped = s.substring(1, s.length() - 1)  // remove wrapping quotes
                     .replace("\\\\", "\\")        // unescape \\ -> \
                     .replace("\\\"", "\"");       // unescape \" -> "

However, note that s might be a string "null" if JS returns proper null, so you obviously need to check that as the very first step.

if ("null".equals(s)) {
   ...
} else {
   // unescape JSON
}