Develop Reference

How to throw a javascript error during runtime via browser (Chrome)?

My objective: Test out my error handling functionality.
Temporary solution: Have a custom route: /error, which contains code which purposefully produces fatal error.
var a = undefined;
a.b.c // Breaks.
The above works, but I can't use it to test production site as the page is not required.
I was looking for a way to test it via the browser. I tried simply adding"
throw new Error("Custom error thrown here") to the console. That doesn't actually break it during runtime.
I tried adding a break point and adding the same code: throw new Error("Custom error thrown here"). That didn't work either.
Any other easier ways to do this rather than the above?
I was looking for a way where I can do it via browser only.
Thanks.

You did not clearly mention how and where the error should be thrown. I will assume that you can use a modified copy of your JavaScript file to throw errors. The modified file will reside on your computer and only be used when you're using Chrome developer tools. This feature is called Local Overrides. The steps are as follows:
Open the webpage
Open Chrome developer tools for that webpage
In Sources panel go to Overrides tab
Click Select folder for overrides and choose a folder on your computer
A warning appears on the webpage which reads "DevTools requests full access to ..." which you must allow
In Sources panel go to Page tab
Locate the file in which you need to inject the "throw error" code
Right click and choose Save for overrides
Now you can edit the copy of the file on your computer or from within developer tools. Insert the code that produces the error at the desired location. When you reload the page with developer tools open, Chrome will load the local copy of the JavaScript file and throw the error. The error thrown that way will contain the context from where it originated e.g. call stack. If the developer tools are closed then live copy will be used.

If I got your question right, this is How you can do it from the console:
var script_tag = document.createElement('script');
script_tag.type = 'text/javascript';
script_tag.text = 'throw new Error("Custom error thrown here")';
document.body.appendChild(script_tag);
Or if you want you can trigger it on click:
var script_tag = document.createElement('script');
script_tag.type = 'text/javascript';
script_tag.text = 'window.document.onclick = function() { throw new Error("Custom error thrown here")}';
document.body.appendChild(script_tag);
And then you click anywhere on the page, to throw the error;

I would use the exec function which actually takes string and runs the code within at compile time.
exec('a.b.c')

You won't be able to throw an error inside your application from the console, since you are out of scope of the app.
Having said that, one slightly awkward way you could do this is by adding a breakpoint at the start of the javascript file.
Reload the page and your app will pause at the breakpoint - you can then modify the code as you need - like adding a throw new Error("something...") - and save your edits.
Then allow the code to run and you will see your error.
A downside is if you reload the changes will be gone, but I believe it's as close as you can get to modifying code at runtime.

Add this code to your production code
window.addEventListener('err', () => {
throw new Error('break it');
})
and when you want to create an error simply
dispatchEvent(new Event('err'))
in the console

You can use a global variable, which is accessible from your app and from debug console.
if (window.shouldThrow) {
throw new Error("Custom error thrown here");
}
This way you can turn on/off the exception throwing using the window.shouldThrow variable.

Try this way to catch error detail on run time
try
{
var a = undefined;
a.b.c // Breaks.
}
catch ( e )
{
alert("Error: " + e.description );
}

Swift JavaScriptCore not working with large files?

https://github.com/gg2001/monero/blob/master/monero/NewWallet.js
I have a js file that is quite large 6000 lines and JavaScript core does not seem to be able to retrieve variable values whereas running the same file in any web browser works fine for me. When I try to retrieve the value of a variable it shows up as undefined, but when I use a js console in a browswer it shows up fine. I am speculating that this is due to the size of the file because when I put
var helloWorld = "Hello World";
in the front of the js file this swift code can retrieve it
func helloWorld() {
if let variableHelloWorld = self.jsContext.objectForKeyedSubscript("helloWorld") {
print(variableHelloWorld.toString())
}
}
but when I put it at the end it cannot.

Normally this indicates a parsing error. Try adding an error handler to self.jsContext before calling objectForKeyedSubscript() and see if it outputs anything insightful.
self.jsContext.exceptionHandler = { context, exception in
print("JS Error: \(exception?.description ?? "unknown error")")
}
Although your JS code may be valid in a browser console, iOS Safari doesn't support as many Javascript features as newer browsers.
I did see a line in your JS source code beginning with just a semicolon (followed immediately by (function). I wonder if the parser might complain about an empty line without a statement..? Maybe nothing, though.

Selenium-Java-geckodriver : Test Execution halts with JavaScript error as "document.getElementById(...) is null"

I had been encountering this issue for a while now. Browsed through a lot of threads on SO & other forums but still clueless.
While automating a simple flow on a Web Application with Selenium 3.4.0, geckodriver v0.16.1 & Mozilla Firefox 53.0 within Eclipse Neon v2 IDE, I am facing an error intermittently on the console as:
JavaScript error: https://www.url.com/my, line 1715: TypeError: document.getElementById(...) is null
Though using chromedriver v2.29/Google Chrome 58.0 or using Python I don't face any such issue.
Once this error appears, the Test Execution halts and finally shows TimeoutException as follows:
Exception in thread "main" org.openqa.selenium.TimeoutException: Timeout loading page after 300000ms
URL of the website is: https://www.shareinvestor.com/my
The HTML DOM is:
<div id="sic_sitemap">
<div id="sic_container">
<div id="sic_header">
<h1 id="sic_siteTitle">
<div id="sic_headerMembershipLink">
<a id="sic_mobileEdition" href="/mobile">
<div id="sic_loginContainer" class="sic_logIn" style="">
<div class="sic_login-wrap">
<div class="sic_logIn-subscribe">
<div class="sic_logIn-bg">
<a href="/user/login.html">
</div>
</div>
</div>
</div>
<div id="sic_subHeader">
<div id="sic_mainNav" class="sic_withRightCorner">
<div id="sic_sideBar" class="sic_expanded { selected_market_suffix: 'MY'}">
<div class="sic_superBanner sic_superBannerTop">
<div id="sic_content" class="sic_collapsed">
<div id="sic_footer" class="si_fixed">
</div>
As of now, I have tried out the following options but of no avail:
Java Click
JavascriptExecutor Click
Actions Click
Here is my code:
import org.openqa.selenium.By;
import org.openqa.selenium.JavascriptExecutor;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.openqa.selenium.interactions.Actions;
import org.openqa.selenium.remote.DesiredCapabilities;
public class 78644072022 {
public static void main(String[] args) {
System.setProperty("webdriver.gecko.driver", "C:\\Utility\\BrowserDrivers\\geckodriver.exe");
DesiredCapabilities dc = DesiredCapabilities.firefox();
dc.setCapability("marionette", true);
WebDriver driver = new FirefoxDriver(dc);
driver.manage().window().maximize();
driver.get("https://www.shareinvestor.com/my");
WebElement login_button = driver.findElement(By.xpath("//div[#id='sic_loginContainer']/div/div[#class='sic_logIn-bg']/a"));
//Java Click
login_button.click();
//JavascriptExecutor Click
/*JavascriptExecutor jse = (JavascriptExecutor)driver;
jse.executeScript("arguments[0].click();", login_button);*/
//Actions Click
/*Actions act = new Actions(driver);
act.moveToElement(login_button).click().build().perform();*/
driver.findElement(By.xpath("//input[#id='sic_login_header_username']")).sendKeys("debanjan");
driver.findElement(By.xpath("//input[#id='sic_login_header_password']")).sendKeys("5786");
driver.findElement(By.xpath("//input[#id='sic_login_submit']")).click();
}
}
I am looking for a Java Solution with geckodriver to overcome the error JavaScript error:TypeError: document.getElementById(...) is null
In one of the SO threads I saw a solution like:
You need to do a null check in updateHTML like this:
function updateHTML(elmId, value) {
var elem = document.getElementById(elmId);
if(typeof elem !== 'undefined' && elem !== null) {
document.getElementById(elmId).innerHTML = value;
}
}
Can we implement this?
Any suggestions & pointers will be helpful.

I am looking for a Java Solution with geckodriver to overcome the error JavaScript error:TypeError: document.getElementById(...) is null
To answer your question, I don't believe there's anything you can do to "fix" this via Java/Selenium. This is a JavaScript error, which originates from the website that you are trying to access. You might want to consider contacting their support team, maybe one of their developers can look at this issue.
Instead of clicking on the login button, maybe consider navigating directly to the login page?
driver.get("https://www.shareinvestor.com/user/login.html");

Firstly, Those javascript errors are not triggered due to any of the selenium code. Ofcourse, the timeout has been triggered by the selenium(will discuss on this on a later point).
You will get that javascript error irrespective of any kind of browser you launch the URL with. But in case of the gecko, you are notified in the eclipse console with but not in case of Chrome. If you need to see the java script error in chrome, just launch the url in chrome and go to devtools/console(F12). You can also see the same in firefox console too.
Chrome Img:
Secondly, We are getting timeout exception because the site is really taking too much time to load. I have been waited for 7 minutes and the page is still loading even now. Selenium won't executes its script unless the page has been completely launched. As a result we are getting timeout exception(not sure about the default page launch time permitted). I thought of bypassing directly to the login page ("https://www.shareinvestor.com/user/login.html") and that's also not taking any finite time to load completely.
Intepolation: Those java script errors are not an issue for automation But those page loads are really. This site doesn't seems like a good candidate for automation with this issue.
Update1: otherwise we can also stop the page loading via another thread like sending "esc" key sequence using Action class.
Update2: I tried the same code today and it works fine. Below is the code snippet that i have tried(There is no change at all)
public static void main(String[] args) throws InterruptedException
{
System.setProperty("webdriver.gecko.driver", "F:\\Softwares\\Selenium\\Webdriver\\geckodriver.exe");
WebDriver driver = new FirefoxDriver();
driver.manage().window().maximize();
driver.manage().timeouts().implicitlyWait(5, TimeUnit.SECONDS);
driver.get("https://www.shareinvestor.com/my");
WebElement login_button = driver.findElement(By.xpath("//div[#id='sic_loginContainer']/div/div[#class='sic_logIn-bg']/a"));
//Java Click
login_button.click();
System.out.println("In Login PAge");
driver.findElement(By.xpath("//input[#id='sic_login_header_username']")).sendKeys("debanjan");
driver.findElement(By.xpath("//input[#id='sic_login_header_password']")).sendKeys("5786");
System.out.println("Entered password");
driver.findElement(By.xpath("//input[#id='sic_login_submit']")).click();
}
Selenium version - 3.4.0
Gecko driver - v0.16.1(mine is 32 bit)
Mozilla - 51.0.1 (Update=>It's working on 53.02 also)
Hope this helps you. Thanks.

I think I've managed to find what is causing this uproar in your script.
I inspected your HTML and it seems javascript method function showRemaining() is causing this problem; because showRemaining() contains this statement
document.getElementById('countdown').innerHTML = '';
where it tries to set innerHTML attribute for element having id as countdown to ''.
But countdown doesn't exist anywhere on your web page hence the error.
TypeError: document.getElementById(...) is null
and somehow selenium isn't able to look past this error. So I think getting it fixed from developers should help you.
UPDATE :
Basically you need to wait all elements to load using implicit wait, once all elements are loaded, then your Javascript error gets resolved and ultimately interaction with the webpage is not hindered by that javascript error:
driver.get("https://www.shareinvestor.com/my");
driver.manage().timeouts().implicitlyWait(20, TimeUnit.SECONDS);
driver.manage().window().maximize();
/*String script = "function updateHTML(countdown, value) {" +
"var elem = document.getElementById('countdown');"+
"if(typeof elem !== 'undefined' && elem !== null) {"+
" document.getElementById('countdown').innerHTML = value;"+
"}"+
"}";
((JavascriptExecutor) driver).executeScript(script);*/
WebElement login_button = driver.findElement(By.xpath("//div[#id='sic_loginContainer']/div/div[#class='sic_logIn-bg']/a"));
login_button.click();
driver.findElement(By.xpath("//input[#id='sic_login_header_username']")).sendKeys("debanjan");
driver.findElement(By.xpath("//input[#id='sic_login_header_password']")).sendKeys("5786");
driver.findElement(By.xpath("//input[#id='sic_login_submit']")).click();

It looks like the XPath for the login_button is incorrect:
By.xpath("//div[#id='sic_loginContainer']/div/div[#class='sic_logIn-bg']/a");
Should be:
By.xpath("//div[#id='sic_loginContainer']/div/div/div[#class='sic_logIn-bg']/a");
This may explain the TimeoutException, since Selenium is unable to locate the non-existent element. EDIT: My mistake, you should see a NoSuchElementException if the element cannot be located.
As for the JavaScript error, unless you were trying to use Selenium to access the web element being manipulated by the JavaScript code (id='countdown' element), Selenium should just ignore the error. Otherwise, there's the potential for other Selenium errors, such as StaleElementReferenceException, but that does not seem to be the case.

This is due to your application HTML page using the async javascript and it uses some variable reference that is not available at the time of execution. we had same issue and asked developer to follow some best practices for javascript in the HTML like putting the script tag at end of the page. I have checked the HTML page source of the site and it contains many script tags in between the code. This will block the DOM rendering. Better, ask the developer to follow some best practices to include script tag in HTML. you refer the link Where should I put <script> tags in HTML markup? for the best practices.

How to catch all javascript warnings and errors to an output div?

i'm using try, catch, for debugging, but warnings is not create exceptions. How to get all javascript warnings and errors to output div?
UPDATED:
If browser supports Afaik logging, how to get that log to string or output div?
UPDATED:
I found the way how to do that:
i can reload console.log function to my custom function an call native console.log function.

First of all, get rid of the try catch. Don't use try catch when you are debugging.
Second, you don't want to out errors to a div, use firebug or inspector for that - console.log();
Third, if you really want to do it: you could use try catch and in the catch, use something like
$('body').append($('div').html('variable for error message goes here'));
if you are using jquery
OR
document.getElementByTagName("body").appendChild( document.createTextNode("variable for error message goes here") );
if you have plain javascript
EDIT: try looking up ie debug bar , ie webDeveloper

I understand myself why someone may want something to actually happen when an error occours in the document. The answers above just say that you would use developer tools, but I needed things to actually happen, and after some searching, here's what I found...
If you wish to catch all errors that come through, you can put the following code into your file, best at the top:
window.onerror = function(errorMsg, url, lineNumber){
// any action you want goes here
// errorMsg is the error message itself.
// url should be the file presenting the error, though i have
// found that it only presents to me the address of the site.
// lineNumber is the line number the error occoured on.
// here is an example of what you could do with it:
alert("Error in " + url + " at " + lineNumber + ":\n" + errorMsg);
}
I, myself, like to output the errors to a div that contains them all, though you can do literally anything to this information that you could do with any other string passed to a function.
Here is an example of what may happen if you throw an error with a button using the code above:
Error in your.site.here at 1:
Uncaught ReferenceError: foo is not defined

For IE javascript debugging you can follow this guide:
http://msdn.microsoft.com/en-au/library/ie/gg699336(v=vs.85).aspx
Keep in mind that the developer tools window must be open prior to loading the page for the warnings and errors to appear in the console.
For webkit, (chrome, safari) developer console - here is a guide:
https://developers.google.com/chrome-developer-tools/docs/console
...Firefox also has a console

Firebug not showing Javascript errors for Rails applications

I have a Rails application, and when I have Javascript errors they are not showing in the Firebug console. I have 'Show javascript errors' and 'Show javascript warnings' selected.
When I insert javascript errors in a basic html file, the errors show as expected.
In the javascript of the Rails app, it only shows errors in rare cases.
For example i can insert nonsense like:
dfghaefb;
and no error is shown in Firebug. But if i insert a space in there Firebug does show the error:
dfgh aefb;
Any ideas? This is driving me nuts.
UPDATE:
Pumbaa80 was right, it's syntax vs runtime.
So I set up onerror:
onerror=errorHandler;
var error="";
function errorHandler(errMessage,url,line){
error="There is an error at this page.\n";
error+="Error: " + errMessage+ "\n";
error+="URL: " + url + "\n";
error+="Line: " + line + "\n\n";
error+="Click OK to continue viewing this page,\n";
alert(error);
return true;
}
And I have a method with an error:
function initForm() {
asdfs;
}
And it works when I call it outside of a method:
initForm();
but not in this case:
document.observe('dom:loaded', function() {
initForm();
});
Why is that?

dfghaefb;
produces a run-time error. Those errors may be suppressed by putting an onerror handler on the window or by trapping them in a try/catch in some way. In that case, Firebug won't show anything.
In contrast,
dfgh aefb;
is a syntax error, which is shown in the error console, regardless of try/catch and onerror.

Did you let the page load completely? I have a rails app running in front of me right now with firebug on and i entered dfghaefb; and yes it does throw an error!
ReferenceError: dfghaefb is not defined { message="dfghaefb is not defined", more...}
Running Firefox 3.6.3, Firebug 1.5.3 and MacOSX 10.5.6 :)

This may appear to be obvious, but be sure you wrap your inline JS using
<script type="text/javascript"> code </script>
Otherwise FireBug won't look at it at all.

I had the same, it wasn't showing anything and it was driving me crazy for ages, even if i had "Show javascript errors" in the console tab selected, it seems that it started working after I did "Enable all panels".

These kinds of problems cannot be answered without a test case.