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How do I solve the ReferenceError in my API calls from my UI?

I have done a microsoft tutorial called Web API with Javascript
I now have a UI made with Javascript and HTML which looks like this:
How do I use the UI? I keep getting a Reference Error. Is there a specific syntax I am supposed to follow when I add and edit something via APIs?

In the future, please copy and paste error messages into your question. I would normally copy and paste the error message in my answer, but I don't want to type it all out :)
The errors (red) mean that you're trying to use JavaScript that is not defined yet. The warnings (yellow) are the reason why.
The second warning says that it could not load the JavaScript. That explains the errors. The first warning might be the reason why. It's saying that the MIME type is empty, when it should be application/javascript.
But you said in the comments that the site.js file is empty when you try to access it directly. Did you save all the JavaScript in step 4 of that tutorial to site.js?
And what are you using as a web server? IIS Express?

Debugging JavaScript in client

I'm facing an issue while debugging my application. Following is the architecture:
Server: Java (Servlet)
Client: React+D3
Problem: Whenever, I change some react or d3 code and if an error occurs then it just shows me that some react (or d3) error has occurred but never tells me which function the error occurred (as seen in snapshot). Now, I know that simply debugging it by having the information like variable name and searching where I defined that variable. However, situation becomes tough when I use same object multiple times (say window) and had made several changes in the code. In this case, a specific line number where the error occured can be handy and quick. Let me know if I'm missing some basics about debugging such applications?
EDIT1:
1. In the snapshot, http://localhost:8080/..../Server Server is the main servlet application, kind of launchpad, which triggers several other react-based js files.
2. The mentioned ReferenceError is inside a function updateWindow() but the console never mentions this (and that's my problem).
PS: I'm using Eclipse tomcat on server-side

I think there's no straight forward solution to this problem. So, I'll post the method that worked for me with few additional points:
Problem: It doesn't gives a nice error trace like standard Java application, probably because it mixes with JavaScript code.
At every line of the error trace, line:column specifies the error line. I used this as a reference and started manual debugging from where my application launches i.e. Server.java and look where I defined the createChart() in the JS file and drill-down until I found the referenced variable.
In case of ReactJS' error (an error after resolving reference issue), I debugged it with normal react.js (not minified version react.min.js) so that it shows me exact line of error. Minified version is cluttered and is useless while debugging.
PS: If someone has better answer I'll edit this in future.

Not able to access variable in google chrome console

I am using the latest version of Meteor js. I am a newbie and couldn't find the solution. I have surfed for this problem a lot.
Before going to the problem, please have look at the directory structure first.
My directory structure of the project is like this :
\client
\main.html
\main.css
\main.js
\imports
\lib
\todos
\todos.js
\server
\main.js
I have created a Mongo Object in todos.js.
export const Todos = new Mongo.Collections('todos');
This is working fine.
Now in client\main.js, I am importing this object,
import { Todos } from '\imports\lib\todos\todos.js';
Note: I tried relative and absolute both type of addressing.
I put a debugger after that and checked and the variable is there. I can access that variable on the console. But as soon as I pass the statement and all the code is rendered on the browser(google chrome), I am not able to use Todos. It is giving me a ReferenceError. The error is
Todos is not defined.
I know there is no problem till the browser is loading because I checked that. I have surfed a lot. Please help me.
Thank you in advance.
Edit 1: I am using windows 7 if that is necessary.

As the comments above said, variables declared or imported in a file/module don't end up on the developers console.
What you can do to get stuff on the console is import it to the console using require
> require('/imports/lib/todos/todos.js')
Any valid absolute path or package will work here
Note: the path separators are always *nix style /

I had a similar problem while trying to evaluate moment from Moment.js in Chrome console. It was imported by the script being debugged/under breakpoints but does not work in console. import or require Moment.js in console gave me errors and did not work for me.
I ended up switching to Firefox Developer Edition, where I can put breakpoints and then evaluate moment in its console out of the box without any problems.

Uncaught TypeError: undefined is not a function?

I am getting this error in Chrome's Developer Tools:
Uncaught TypeError: undefined is not a function
This error is referencing this line of my main.js file. These two lines are both causing this error to happen:
jQuery("#signupfrm").fadeToggle('fast',function(){
jQuery("#loginfrm").fadeToggle('fast',function(){
I am getting this error when working on the website on my local computer, but it's from a theme I purchased that has a demo available online.
Here's a working copy of this same template:
https://www.whmcsdesigns.com/demo/cart.php?a=add&pid=1&systpl=flex
Just select "I will use my existing domain and update my nameservers" and enter any domain name. You'll be taken to a page where personal info can be entered. You will see that you have the option to select between "New Customer" and "Existing Customer". It works at the link above.
However, on my local server it's not letting me switch between new and existing. It just gives the error show above and is referencing those lines with signupfrm and loginfrm.
I'm guessing since it's working on the link aboving, those functions must be defined. Can anyone else find where those are being defined at?

Things that I suspect to be the problem:
jQuery hasn't been loaded correctly so the jQuery function can't be performed. It might not have been loading at all or it might be called $, but if other calls to jQuery work it should be OK.
fadeToggle is a part of jQuery so I expect that to work if jQuery is loaded
jQuery("#signupfrm").fadeToggle('fast',function(){
sometimes if you load dynamically and you test on C:\ or whatever local path things don't work the same, especially AJAX calls. Set up a local webserver to work on always, just using localhost on a simple Apache is a good start

Programatically retrieve count of javascript errors on page

I'd like to write a test case (using Selenium, but not the point of this question) to validate that my web application has no script errors\warnings or unhanded exceptions at certain points in time (like after initializing a major library).
This information can easily be seen in the debug consoles of most browsers. Is it possible to execute a javascript statement to get this information programatically?
It's okay if it's different for each browser, I can deal with that.

not so far read about your issue (as far as I understood your problem) here
The idea be the following:
I found, however, that I was often getting JavaScript errors when the page first loaded (because I was working on the JS and was introducing errors), so I was looking for a quick way to add an assert to my test to check whether any JS errors occurred. After some Googling I came to the conclusion that there is nothing built into Selenium to support this, but there are a number of hacks that can be used to accomplish it. I'm going to describe one of them here. Let me state again, for the record, that this is pretty hacky. I'd love to hear from others who may have better solutions.
I simply add a script to my page that will catch any JS errors by intercepting the window.onerror event:
<script type="text/javascript">
window.onerror=function(msg){
$("body").attr("JSError",msg);
}
</script>
This will cause an attribute called JSError with a value corresponding to the JavaScript error message to be added to the body tag of my document if a JavaScript error occurs. Note that I'm using jQuery to do this, so this specific example won't work if jQuery fails to load. Then, in my Selenium test, I just use the command assertElementNotPresent with a target of //body[#JSError]. Now, if any JavaScript errors occur on the page my test will fail and I'll know I have to address them first. If, for some strange reason, I want to check for a particular JavaScript error, I could use the assertElementPresent command with a target of //body[#JSError='the error message'].
Hope this fresh idea helps you :)

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