Understanding the usage of executeAsyncScript in Selenium

Question

I am trying to understand the documentation for Selenium's executeAsyncScript here (https://seleniumhq.github.io/selenium/docs/api/java/org/openqa/selenium/JavascriptExecutor.html) , for instance in their first example they have :

  long start = System.currentTimeMillis();
   ((JavascriptExecutor) driver).executeAsyncScript(
       "window.setTimeout(arguments[arguments.length - 1], 500);");
   System.out.println(
       "Elapsed time: " + System.currentTimeMillis() - start);

Now as far as I understand, the first argument is supposed to be a script, while the last one is a callback function, but in this example there is no callback, so what is going on here (arguments[] is empty right?).

If I wanted to have a function that returns a promise, and then print the promise, say doSomething().then(function(result) { return result;)}); how would this work with the executeAsyncScript function?

Thanks

N-ate · Accepted Answer · 2017-12-21 00:03:24Z

3

You have to use the callback that the Selenium function provides you. Imagine that when you use executeAsyncScript that the script is wrapped in a Selenium function like this:

(function(args,,, callbackToServer){
    window.setTimeout(arguments[arguments.length - 1], 500);
})(args,,, callbackToServer);

As you can see arguments[arguments.length - 1] actually references callbackToServer function.

For example your promise would look like this:

doSomething().then(function(result) { 
    arguments[arguments.length - 1](result);
)});

edited Dec 21, 2017 at 0:03

answered Dec 20, 2017 at 23:13

N-ate

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5 Comments

IntegrateThis Over a year ago

sorry but what Selenium function are you referring to?

N-ate Over a year ago

See the example code. The first and third line are what the selenium function are accomplishing. The second line is what you injected.

IntegrateThis Over a year ago

Can you answer the second part of my question, I am horribly confused I'm afraid.

N-ate Over a year ago

I updated your promise example. You will obviously have to keep track of the server callback. Perhaps set it to a variable.

IntegrateThis Over a year ago

So something like ((JavascriptExecutor) driver).executeAsyncScript("var callback = arguments[arguments.length-1]; doSomething().then(function(result){callback(result);}));");

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