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How to access c# variables from a JavaScript file?

I have a C# file that pulls various config settings from different config files. A JavaScript application I'm writing needs some of those settings to run. I'm having trouble bringing the C# variables into the js file. Not sure what is the best approach. They need to end up as a JS object on the page. Would adding them to a JSon object in the C# file & calling that from the JS file work? Not even sure how to do that to be honest. Any help appreciated.

You can just declare a global variable in one of your root pages (say, _layout.cshtml):
<script>
var settings = {
foo: #(IsFoo ? "true", "false"),
bar: #SomeNumber,
baz: "#ImportantString"
}
</script>
Or, if you're writing a Web API, you can just add a /settings endpoint you can query like so (Assuming you're using jQuery):
$.get("/settings", response => {
// Store the `response` in a global variable.
});

If it's an independent javascript project and .net is more of an API, you need to make an API call which will send you those config settings.
If your javascript is part of .net MVC application, i.e., you are adding your javascript through an tag at then end of a .cshtml/.aspx file, you can easily pass it through a global variable, but even then the C# code need to pass those values through Modal to the .cshtml/aspx file.
what you can do is:
var pageConfig = pageConfig || {};
pageconfig.settings = #Html.Raw(Model.Settings); // This model object is part of the C# code and my assumption is that Settings will have array of configurations.
This can also be done globally, depending upon how the C# code is written.

Assuming that when you say 'various config settings' you're referring to .NET's ApplicationSettings (defined in Visual Studio's Properties > Settings), we've done something similar, in a generic manner, as follows:
public void WriteSettings(TextWriter writer)
{
// Declare the nameSpace for the DLL you want to pull settings from
var nameSpace = "foo.Bar"
ApplicationSettingsBase properties = (System.Configuration.ApplicationSettingsBase)Activator.CreateInstance(nameSpace, string.Format("{0}.Properties.Settings", nameSpace)).Unwrap();
foreach (SettingsProperty property in properties.Properties)
{
writer.Write(string.Format("{0}=\"{1}\", property.Name, properties[property.Name]);
}
}
I've ignored a few issues in your case:
I've not bothered trying to deal with non-strings; consider looking at property.PropertyType and casting your values appropriately
I've wrapped this in a function that accepts a TextWriter; you can pass this method Response.Output

PHP logic to Javascript front into html tags

I am currently trying to convert a lot of backend code to front end (to lighten the load on a small system).
The code at the moment calls a PHP function to return specific information. (e.g. image locations, strings, styling)
I am converting this code to its js equivalent, the content from Mysql was converted to JSON and stored in a read only file and I am accessing that file using this code:
<script>
function jsread(tag) {
$.getJSON("/strings.json", function(result){
document.write(result[tag]['value']);
});
}
</script>
I want the function to "print" where ever it is invoked. document write writes the value to the page but stops all other loading and write only the value.
Let me be very clear on this: I DO NOT want to use anything that needs extra calls or references out side of this function, that will take months of work so no getting elements by their IDs I have already view many questions on this subject and none are what I can work with. I need something that can be applied to every situation. Other wise I will just have to read the JSON using PHP as a middle compromise.

The problem here is, document.write()'s behaviour is crazy across all the browsers, because, it directly modifies the document object and messes up with the events attached. So it is always better to avoid this function as each browser defines it differently and has a different effect on the same code, with different browsers.
Is there a way to use them without a direct reference?
Solution
The wise thing is, as I said in the comments, it is better to use one of the jQuery functions safely, which create a textNode and insert it the right way, without affecting the others:
<script>
function jsread(tag) {
$.getJSON("/strings.json", function(result){
$("body").append(result[tag]['value']);
});
}
</script>
In case, if you wanna do something like having a placeholder and doing stuff, then you can try giving something like this:
$(function () {
var data = "Dummy Data, that would probably get returned from the getJSON";
// Inside the Success function, do this:
$("span.placeholder-of-the-json").replaceWith(data);
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<span class="placeholder-of-the-json">This gets replaced</span>

In my asp.net-mvc file, how can I move my javascript to a seperate js file?

I have a bunch of javascript inside my view and its getting quite large so I want to move it to a separate js file. The one issue I have is that I have this line:
var tags = <%= new JavaScriptSerializer().Serialize(Model.Tags) %>;
which I obviously can't just copy over since it has the server side asp.net-mvc tags. What is the recommended way to deal with that:
Keep this one function inside the aspx page and have the javascript from the seperate js file call that function?
Other??

What you could do is have a JavaScript object contain all the information you get from the controller and pass it through a javascript function which is located in the external javascript file. You can also pass other information through the options variable.
Example:
var options = {
tags: <%= new JavaScriptSerializer().Serialize(Model.Tags) %>
};
initPage(options);
Usage:
function initPage(options) {
console.log(options.tags);
}

This depends a lot on the way you want to use it and the level of repeating.
Some thoughts:
The tags are static. Then I guess then don't really need to be in the model. So you can move them into a new Controller action which outputs the scipt. You can call this action in the <script> part of your view.
The tags are changing very frequently (maybe even at every page load). Then there is no gain in moving this in a separate script file.

If you set up your javascript file with proper closures, you can expose a property (or better yet, a parameter to the object/method) with which to pass that information along when you call that function.
Your Javascript would need to be something like this:
var JsFile = (function() {
var tags;
// list all of your methods here.
return {
var setTags = function(_tags) {
tags = _tags;
}
};
})();

Express/Node.js : Render custom javascript as response

In my application, I need to provide an API (something like the Google Maps javascript API), through which I can send some custom javascript (with some session and request related information) as the response. The javascript is then used to plot some graphs on the UI. I'm using Express with Jade as my templating engine. The code that I'm currently using is:
app.use('/graph',function(req, res){
//send out graph data
var var_name = req.session.var_name //fetch something from session
var graphData = fetchGraphData(req.query.graph); //function that fetches graph data
res.contentType("text/javascript");
res.render(__dirname + '/views/graph.jade', {
title: "Title", queryStr: JSON.stringify({var_name: var_name, graphData: graphData })
});
});
And the jade file:
| some_var_name = {
| initGraph : function(divId){
| //some code here
| var graphData = !{graphData}
| // do something
As a workaround, I have started each line of the jade file with |, so that jade parses the text as plain text, and doesn't add any html tags! It works fine, but is there a cleaner way to do this? The solution may or may not use Jade!

You should look into underscore templates. I think that for generating arbitrary text output it would be somewhat cleaner. Jade is purpose-built for rendering HTML.
You could also try Mustache or Handlebars.
Based on your comment, I see you'd like to keep using res.render for rendering the template. consolodate.js adds support for all major template engines to Express. Including Underscore templates, Handlebars, Mustache and Dust, mentioned by #TheHippo.

You may try to define JavaScript functions you need to send to the browser in a separate module, outside of the template, which is probably more correct way from the "concerns separation" point of view. Also if functions are defined in a separate module they can be used both in the server and in the browser.
Then you can convert the functions to strings using its toString() method either in a function that invokes the template or right inside the template, if it supports plain JavaScript which is the case with underscore, EJS and doT templates (I tried both underscore and EJS and ended up using doT which is not only the fastest but very versatile - check it out):
JS code:
// if you send the same functions you may want to convert them to strings in advance
var data = {
funcStr: func.toString();
};
res.render(view, data);
Template (doT):
<script type="text/javascript">
func = {{= it.funcStr }};
// now you can call it here if you want but I would use
// separate JavaScript files
func();
</script>
I use it to send pre-compliled templates to the browser together with the page on the first page load, but I think it can be used in your case too.
As a side question, why can't you just bundle all these functions in a separate JavaScript module and load them as normal script file?

You can use https://www.npmjs.com/package/rendercustomjs package, it works fine but in ejs templating

Insert JavaScript data chunk (JSON) into an HTML page

I need to include a JavaScript object (JSON) in my HTML page.
JSON is rendered at the same time page HTML is rendered on server. Data is not retrieved using AJAX call.
I can think of two ways of doing this, and looking for feedback and recommendations.
What are good practices for passing JavaScript (JSON) blob with a page?
Option 1
HTML:
<script type='text/javascript'>
var model = { <JSON> };
</script>
.js:
function doSomething() { <use this.model here> }
Option 2
HTML:
<script type='text/javascript'>
loadModel({<JSON>});
</script>
.js (included at the top of the html file):
var model = null;
function loadModel(model) { this.model = model; }
function doSomething() { <use this.model here> }
Variation
Instead of including JSON in HTML, JSON can be stored in a separate .js file. Any comments on doing so?
Option 1 lets you include .js file anywhere, and including it at the bottom of the page makes it render faster (good thing), but since JavaScript renders the model on the page, this makes it a moot point. Still not depending on the location of the .js inclusion makes it less error prone.
Also R# complains (reasonably) about model being uninitialized.
Option 2 feels better (it encapsulate details better, for one), but .js must be included before call to loadModel.
I have seen and done both ways, but didn't notice any significant advantages of one way over the other.
Server platform should be irrelevant, but it is IIS 7.5/ASP.NET MVC 3/Razor

Forget your two suggestions - both are extremely vulnerable to XSS. NEVER PUT UNTRUSTED TEXT IN A SCRIPT TAG.
Instead, use the owasp recommendation.
Stick your (HTML encoded) JSON in the DOM like so:
<div id="init_data" hidden>
<%= html_escape(data.to_json) %>
</div>
Then read it in JavaScript like so:
// external js file
var dataElement = document.getElementById('init_data');
// decode and parse the content of the div
var initData = JSON.parse(dataElement.textContent);

There would be ever so slightly more overhead with option two. As you have the overhead of a function call, and an extra variable (your parameter), which will be allocated and deallocated.
As you said, there is little advantage/disadvantage to either way.

Can you use jQuery? Then you can use the DOM ready event instead of including javascript in your HTML.
EDIT:
Hmm, in that case you could include the JSON inside a hidden element when the page is generated. Then inside the DOM ready event you could read and parse it from the page using jQuery.
Another alternative might be to use HTML 5 data attributes and including the data in one of those.
If it were me I'd probably just use an ajax call since it is easier and seems a little cleaner.