As a newcomer to AngularJS, I encountered a frustrating CORS issue that almost drove me crazy! Fortunately, after some trial and error, I found a solution to resolve it. Here's how...

The problem arose when I was using AngularJS $resource to send API requests and kept receiving the error message

XMLHttpRequest cannot load http://website.com. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost' is therefore not allowed access.

Despite adding callback="JSON_CALLBACK", the issue persisted.

To overcome this obstacle, instead of sticking with the GET method or relying on $http.get, I switched to JSONP. By replacing the GET method with JSONP and adjusting the API response format to JSONP, I was able to successfully tackle the problem.

    myApp.factory('myFactory', ['$resource', function($resource) {

           return $resource( 'http://website.com/api/:apiMethod',
                        { callback: "JSON_CALLBACK", format:'jsonp' }, 
                        { 
                            method1: { 
                                method: 'JSONP', 
                                params: { 
                                            apiMethod: 'hello world'
                                        } 
                            },
                            method2: { 
                                method: 'JSONP', 
                                params: { 
                                            apiMethod: 'hey ho!'
                                        } 
                            }
            } );

    }]);

I hope this workaround proves helpful to others encountering a similar challenge. :)

When working with Express, I've found success in adjusting the res.header values. While my setup is similar to yours, one key difference is how I handle the Allow-Headers, as shown below:

res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");

My project involves utilizing Angular alongside Node/Express. Interestingly, I only specify headers in the Node/Express code and not in Angular.

If you're looking to solve this issue, consider implementing this middleware!

app.use(function(req, res, next) {
res.header("Access-Control-Allow-Origin", "*");
res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
next();
});

To learn more, check out http://enable-cors.org/server_expressjs.html

Here is the solution I found for resolving my issue in server.js:

    server.post('/your-rest-endpt/*', function(req,res){
    console.log('');
    console.log('req.url: '+req.url);
    console.log('req.headers: ');   
    console.dir(req.headers);
    console.log('req.body: ');
    console.dir(req.body);  

    var options = {
        host: 'restAPI-IP' + ':' + '8080'

        , protocol: 'http'
        , pathname: 'your-rest-endpt/'
    };
    console.log('options: ');
    console.dir(options);   

    var reqUrl = url.format(options);
    console.log("Forward URL: "+reqUrl);

    var parsedUrl = url.parse(req.url, true);
    console.log('parsedUrl: ');
    console.dir(parsedUrl);

    var queryParams = parsedUrl.query;

    var path = parsedUrl.path;
    var substr = path.substring(path.lastIndexOf("rest/"));
    console.log('substr: ');
    console.dir(substr);

    reqUrl += substr;
    console.log("Final Forward URL: "+reqUrl);

    var newHeaders = {
    };

    //Perform a deep copy of the headers
    for (var headerKey in req.headers) {
        newHeaders[headerKey] = req.headers[headerKey];
    };

    var newBody = (req.body == null || req.body == undefined ? {} : req.body);

    if (newHeaders['Content-type'] == null
            || newHeaders['Content-type'] == undefined) {
        newHeaders['Content-type'] = 'application/json';
        newBody = JSON.stringify(newBody);
    }

    var requestOptions = {
        headers: {
            'Content-type': 'application/json'
        }
        ,body: newBody
        ,method: 'POST'
    };

    console.log("server.js : routes to URL : "+ reqUrl);

    request(reqUrl, requestOptions, function(error, response, body){
        if(error) {
            console.log('The error from Tomcat is --> ' + error.toString());
            console.dir(error);
            //return false;
        }

        if (response.statusCode != null 
                && response.statusCode != undefined
                && response.headers != null
                && response.headers != undefined) {
            res.writeHead(response.statusCode, response.headers);
        } else {
            //404 Not Found
            res.writeHead(404);         
        }
        if (body != null
                && body != undefined) {

            res.write(body);            
        }
        res.end();
    });
});

@Samantha Johnson

After implementing an ajax request and converting the data to 'jsonp' format, I was able to successfully resolve the issue at hand.

$.ajax({
          method: 'GET',
          url: endpoint,
          defaultHeaders: {
              'Content-Type': 'application/json',
              "Access-Control-Allow-Origin": "*",
              'Accept': 'application/json'
           },

          dataType: 'jsonp',

          success: function (response) {
            console.log("Request successful");
            console.log(response);
          },
          error: function (xhr) {
            console.log("An error occurred during the request");
            console.log(xhr);
          }
});

Discovering a new approach to implement the JSONP method within the $http service directly, along with utilizing the params in the config object has been a breakthrough:

params = {
  'a': b,
  'callback': 'JSON_CALLBACK'
};

$http({
  url: url,
  method: 'JSONP',
  params: params
})