Selenium and asynchronous JavaScript calls

Question

i'm quite new to Selenium and JavaScript-callback functions and i have a big issue I can't solve myself. I need a specified variable using JavaScript. If I open the page with GoogleChrome and use the console to enter my JavaScript-Code I can get the variable using something like this:

1. var myId;
2. getSomeIdStuffInfo("somestuff",function(docId)(myId = docId));
3. return myId;

If I enter this lines step by step I easily get the correct value myId. But, of course, if i execute the three lines as fast as possible, i get null as return value because the callback function is not finished when i return myId. SOOOO.. if I use selenium like this:

JavascriptExecutor js = (JavascriptExecutor) driver; 
    String docInfoVal = (String) js.executeScript("" +
            "var myId; " +
            "getCurrentDocumentInfo(\"somestuff\"," +
                "function(docId) {" +
                    "myId = docId;" +
                "}" +
            ");" +
            "return myId;");

I only get null as result. So... somehow I have to "wait" for the callback function until i return myId. Do I have to use executeAsyncScript and how? I'm sitting on it for hours and tried different things but I just can't find the answer.

Thanks in advance for any help!

score 8 · Accepted Answer · answered Jul 03 '15 at 16:11

For asynchronous code you have to use executeAsyncScript:

JavascriptExecutor js = (JavascriptExecutor) driver; 
String docInfoVal = (String) js.executeAsyncScript("" +
        "var done = arguments[0]; " +
        "getCurrentDocumentInfo(\"somestuff\"," +
            "function(docId) {" +
                "done(docId);" +
            "}" +
        ");");

The script you call with executeAsyncScript will have a callback added to the list of arguments passed to it. Since you pass no arguments to your script, then arguments[0] contains the callback. Your code must call this callback when it is done working. The value you give to the callback is the value that executeAsyncScript returns.

In the code above, I've spelled out the call to done by putting it in an anonymous function but the code could be written more concisely as:

JavascriptExecutor js = (JavascriptExecutor) driver; 
String docInfoVal = (String) js.executeAsyncScript("" +
        "var done = arguments[0]; " +
        "getCurrentDocumentInfo(\"somestuff\", done);");

Or even:

JavascriptExecutor js = (JavascriptExecutor) driver; 
String docInfoVal = (String) js.executeAsyncScript(
        "getCurrentDocumentInfo('somestuff', arguments[0]);");

Thanks for the easy explanation, it works and more important, I think I understood how :) — spcial, Jul 06 '15 at 06:28

score 2 · Answer 2 · answered Jul 03 '15 at 17:00

Though this is almost same as what @Louis said.You have to set the setScriptTimeout beforehand for the script to pass.

The default timeout for a script to be executed is 0ms. In most cases, including the examples below, one must set the script timeout WebDriver.Timeouts.setScriptTimeout(long, java.util.concurrent.TimeUnit) beforehand to a value sufficiently large enough.

Below is an example for which I'm waiting for 10 seconds to return a string

  driver.manage().timeouts().setScriptTimeout(20, TimeUnit.SECONDS);//important

    JavascriptExecutor executor = (JavascriptExecutor) driver;
    String val = (String) executor.executeAsyncScript(""
            + "var done=arguments[0]; "
            + "setTimeout(function() {"
            + "   done('tada');"
            + "  }, 10000);");

    System.out.println(val);

You saved me hours of mindless debugging! Thank you! – César Alberca Jul 10 '17 at 15:58 — César Alberca, Jul 10 '17 at 15:58

Selenium and asynchronous JavaScript calls

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