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How to make Cross Domain Request to an API from Javascript Client when I don't have access to modify the other domain API?

I have a scenario where I need to fetch JSON response data from an API in a different domain. Lets say the requesting domain is ClientDomain and the API domain in APIDomain.
Now I know that this can be achieved using Access-Control-Allow-Origin header in the API server and then using a client ajax marking the CrossDomain attribute as true, etc.
All my search leads either to modify the API settings to add the Access-Control-Allow-Origin header or do a proxy coding in backend server code. But in my case, I don't have access to the API code and I can't change anything there. I'm able to do a proxy read from C# controller code and fetch data from the APIDomain. I want to know how this can be achieved only with Javascript/Jquery in my current scenario.
Edit: Is there a way to do proxy-ing via Javascript/Jquery?
Any help would be great!

Make an API/page on ClientDomain and access it via Javascript/Jquery. In that API/page call the API on APIDomain and return its data.

Cross domain request with JSON response

I am trying cross domain request from my js file.
First,I was trying JSONP but my target domain URL is not support it. It return plain JSON object.
I am authorize person to access my target domain URL. but i can not modify it as per JSONP response.
SO how can i get JSON response from my target domain URL?

Without modifying a bit the server side there is not much you can do. The general policy is to not to allow cross domain requests.
There are few things worth mentioning though:
Try changing the server side so it will support JSONP.
If the HTTP response contains Access-Control-Allow-Origin header then you can communicate with it with normal AJAX. This feature is supported in modern browsers only. Check this out for more info.
You can do the cross domain requests with Flash and/or WebSockets. However server does have to support them.

I have always done it with jsonp, by passing a callback b/c services return json, if call back is passed then it will wrap all json in callback else they will simple return json.
But in your case
You can look up with this article
http://www.webdevdoor.com/jquery/cross-domain-browser-json-ajax/

Don't know what type application you are developing. But in ASP.NET you can do it by using a proxy page
These links may be helpful:
http://www.codeproject.com/Articles/667611/ASP-NET-Proxy-Page-Used-for-Cross-Domain-Requests
http://encosia.com/use-asp-nets-httphandler-to-bridge-the-cross-domain-gap/
https://gist.github.com/jkresner/3982746

Make REST call in JavaScript without using JSON?

(extremely ignorant question, I freely admit)
I have a simple web page with a button and a label. When I click the button, I want to make a REST call to an entirely different domain (cross-domain, I know that much) and display the results (HTML) in the label.
With other APIs, I've played around with using JSON/P and adding a element on the fly, but this particular API doesn't support JSON so I'm not sure how to go about successfully getting through.
The code I have is:
function getESVData() {
$.get('http://www.esvapi.org/v2/rest/passageQuery?key=IP&passage=John+1', function (data) {
$('#bibleText').html(data);
app.showNotification("Note:", "Load performed.");
});
}
I get an "Access denied." Is there anyway to make this call successfully without JSON?

First off, JSON and JSONP are not the same. JSON is a way of representing information, and JSONP is a hack around the same-origin policy. JSONP works by requesting information from another domain, and that domain returns a script which calls a function (with the name you provided) with the information. You are indeed executing a script on your site that another domain gave to you, so you should trust this other domain.
Now when trying to make cross domain requests you basically have 3 options:
Use JSONP. This has limitations, including the fact that it only works for GET requests, and the server you are sending the request to has to support it.
Make a Cross Origin Resource Sharing (CORS) request. This also must be supported by the server you are sending the request to.
Set up a proxy on your own server. In this situation you set an endpoint on your site that simply relays requests. ie you request the information from your server, your server gets it from the other server and returns it to you.
For your situation, it the other server doesn't have support for other options, it seems like you will have to go with options 3.

Google Geocoding ajax request with Yahoo YUI 3

I understand the way to make an ajax call in YUI 3 is using the IO utility.
I want to get the address of a location from Google's geocoding API.
<script type="text/javascript"><!--
YUI().use('io-base', function(Y) {
function complete(id, o) {
var data = o.responseText; // Response data.
alert(o.responseText);
};
Y.on('io:complete', complete, Y);
var request = Y.io("http://maps.googleapis.com/maps/api/geocode/json?language=en&sensor=false&latlng=12,34);
});
//-->
</script>
I get a reply with method OPTIONS and status code 405 Method Not Allowed.
I believe this is because of some "preflight" permission check. I do not receive the desired response. If I copy and paste the url into the browser, I see the json data.
I could post the ajax request to a php script on my own domain and get the json response with curl.
But why have this extra step if I could just get the data in javascript?
So what can I do to solve this? Is the IO utility not the right library to use?

You're making a cross-domain XHR request, and running into the "Same origin policy", a generic restriction in client-side JavaScript. See for example Why do I still receive 405 errors even though both URLs are from XXXX.com?
There are various ways to work around this problem:
1) Make a server-side request in PHP, as you suggest
2) Use the YUI jsonp module
3) Use the YUI YQL module, which proxies your request through Yahoo! servers and handles JSONP housekeeping for you
There are many other ways to tackle this problem, but those three should get you started.

Y.io has support for cross domain requests. See http://yuilibrary.com/yui/docs/io/#cross-domain-transactions
You need to properly config it with the "xdr" property, and load the "io-xdr" module, etc. This example uses it as well: http://yuilibrary.com/yui/docs/io/weather.html

How can I get HTTP response header using JS?

I tried so many tutorials online but everything I try is really old and fails - I can't even create an XMLHTTPRequest object!
I just want to get the header from google.com - how can I do that?

You would use xhr.getResponseHeader() to get a single header, or xhr.getAllResponseHeaders() to read all of the headers from an XMLHttpRequest response.
The reasons this won't work for you:
XMLHttpRequest is case sensitive. If you are using HTTP in all caps, it will fail.
Unless you are a google employee adding code to google.com, your request falls victim to the same origin policy. You'll have to use your server as a proxy to get headers from a google request.

You cant simply do this by JS. You'll have to use AJAX and do a server request to PHP,ASP, Java or whatever. The XMLHTTPRequest should do it - if you really want to do it manually. But it will really not work with foreign domains, so you are forced to do the XMLHTTTPRequest to a page on your server which will deliver the header.