Wait for Asynchronous javascript function

Question

I've seen similar questions asked regarding this problem. I'm pretty new to javascript and I couldn't figure it out.

I have a function which calls another function.

sayHello() has an async call.

var hello_message = null;

function invokeSayHello(msg) {
    sayHello(msg);
    //next action
    return hello_message;
} 

function sayHello(msg) {
    // simulate async call 
    setTimeout(function(){hello_message = msg + " World";},1000);
}

In this case hello_message is returned as null. How would I wait for that async call to complete before the next action line executes in invokeSayHello() function so the returned hello_message would not be null.

I think I'm supposed to use a callback but not sure how to do it.. Also, I call invokeSayHello() from a java file using executeScript()/Selenium

Appreciate all the help.

Darin Dimitrov · Accepted Answer · 2014-04-13 08:09:18Z

0

You should use the result inside the callback. You cannot/should not wait for the asynchronous function to complete. The asynchronous pattern assumes that you use the callback.

Like this:

function invokeSayHello(msg) {
    sayHello(msg);
} 

function sayHello(msg) {
    // simulate async call 
    setTimeout(function(){
        var hello_message = msg + " World";

        // Here you can process the result, like alerting it for example or
        // passing it to another function
        alert(hello_message);
    }, 1000);
}

So basically in asynchronous programming you forget about the keyword return and start passing callbacks to your javascript functions so that the caller can subscribe to those callbacks and whatever he wanted to do with the results inside the callback..

answered Apr 13, 2014 at 8:09

Darin Dimitrov

1.0m275 gold badges3.3k silver badges3k bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

ecthelion84 Over a year ago

Thanks for the quick response. What if I had other statements instead of returning a value? I need to figure out a way to wait until that async call completes before //next action line executes.

Darin Dimitrov Over a year ago

You write those statements INSIDE the callback and not after the function call.

ecthelion84 Over a year ago

The driver program I use is written in Java. I have 2 seperate .js files. invokeSayHello() is in the wrapper.js and it calls sayHello() in api.js. The java driver calls the functions in wrapper.js. I also have get() methods in the wrapper file which I execute from the java driver. If I don't put a Thread.sleep between those two calls, my test fails. Am I approaching this problem the wrong way ?

Darin Dimitrov Over a year ago

I am not sure what test you are talking about and what does Java has to do with javascript but in javascript when you use asynchronous functions nobody should be sleeping. You don't even have threads in javascript.

ecthelion84 Over a year ago

Maybe this would be a better explanation: I call these methods from a Java program, which I test the values of assigned variables. By the time sayHello() finishes executing, my Java driver keeps executing the next line, which is some getOutput() method. If I don't put a Thread.sleep between sayHello() and getOutput(), my test case fails, because getOutput is executed before a value assigned to the hello_message variable. How do I solve this problem? Is there a way I can take care of this using a feature in Selenium ? Thanks.

|

Collectives™ on Stack Overflow

Wait for Asynchronous javascript function

1 Answer 1

9 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

9 Comments

Your Answer

Sign up or log in

Post as a guest

Related