javascript string to variable

Question

I am receiving a JSON string from an ajax call and would like to convert a value to a predefined variable:

var predefined = "hello world";
var foo = {"msg":"predefined"}; // JSON string

I want to echo out the standard string accessing it with

alert(foo.msg)

EDIT: to make the answer more clear here is my call:

var success_msg = "Your email is send successfully!";

$.ajax({
    url: "ajax-share-email.php",
    type: "POST", 
    dataType: "json", 
    data: {},
    success: function(data) {
        if (data.status == "success") {
            msg.text(data.msg).addClass("email-msg-success");                   
        } else {
            msg.text(data.msg).addClass("email-msg-error");
        }
    }
})

ajax-share-email.php responds:

{"status":"success", "msg":"success_msg"}

Answer 1

var strings = {"predefined":"hello world"};
alert(strings[foo.msg]);

or e.g.

var messages = {};
messages.success_msg = "Your email is send successfully!";

// ...
            msg.text(messages[data.msg]).addClass("email-msg-success");