SyntaxError: invalid regular expression flag ajax, Javascript

@Url.Action only returns the action url's string, without quotes around it.

You'll need to wrap that url in quotes.

Replace:

url: @Url.Action("ReturnMethodTest", "HomeController"),

With:

url: '@Url.Action("ReturnMethodTest", "HomeController")',
//   ^                                                 ^

Otherwise, the file returned to the client will contain:

url: /HomeController/ReturnMethodTest,

Which isn't valid JS, nor what you want. The replacement gives the following result:

url: '/HomeController/ReturnMethodTest',

Which is a perfectly valid JavaScript string.

A Javascript regular expression literal looks like this -- a pattern enclosed between slashes:

var re = /pattern/flags;

If you interpolate a variable that begins with a slash but do not put quotation marks around it, it will be interpreted as a regular expression, not a string. Another time this comes up is with JSP expression language, where you should write the first, not the second:

var spinner = "<img src='${contextPath}/images/ajax-loader-small.gif'>"
var spinner = "<img src='" + ${contextPath} + "/images/ajax-loader-small.gif'>"

Please remove the Controller suffix while specifying in url.

Try it this way:

url: '@Url.Action("ReturnMethodTest", "Home")'