Javascript Promise return value

I am trying to make a crawler, and as the data is not showing in the page source, I can only execute the javascript with the web driver and get the response, and then do the data analysis. The script is simplified, like this, use Promise.

var res = ""

function f1() {
    p = window.Promise
    a = p.resolve(5).then(function(value) {
        console.log(value)
        res = value
        return res
    })
    return a
}
console.log(f1()) // Promise object
console.log("result = ", res) // res is empty

My program is like this, writing with c#:

 public void GetParameters(string url)
        {
            IWebDriver driver = new ChromeDriver();
            driver.Navigate().GoToUrl(url);
            IJavaScriptExecutor js = driver as IJavaScriptExecutor;
            string script = readFile(@"C:\myscript.js", Encoding.UTF8).TrimEnd(); // The script is a bit long, so I save it to a local file.
            var json = js.ExecuteScript(script);
            Console.WriteLine("Get the return value");
            Console.WriteLine(json);
            driver.Close();
        }

I want to get the value, and I know then always return Promise object, so I define a variable, and want to store the value to it. But seems that, the Promise is executed asynchronous, so the res always be empty.

OK, I'm writing a crawler, and use Selenium to get the response from the server (Selenium.webdriver can open a web browser, and then execute script), I need get the result, as it will use by another program. So, I can not just add another .then, just output the value. Maybe I can save to a local file, and then read it, but I think it is inefficient.

Anyone can help?

回答1:

When you try to log the global res, the result hasn't been computed yet. When using Promises, you have to get the result asynchronously, using .then, like this:

f1().then(res => console.log("result =", res));

回答2:

Try this You're using promises incorrectly.

f1().then(res => console.log('result=', res)).