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");             

Answer 2

How about this -- just use the message inline on success and don't even bother to make it part of the JSON. On an error, do include the entire message and use it directly. Also, I'd have your server return something like:

{ "status": true }

or

{ "status": false, "msg": "The mail server is down." }

Then you can just evaluate it as a boolean without comparing it to a string value.

$.ajax({
    url: "ajax-share-email.php",
    type: "POST", 
    dataType: "json", 
    data: {},
    success: function(data) {
        if (data.status) {
            msg.text('Your email has been sent successfully!').addClass("email-msg-success");                   
        } else {
            msg.text(data.msg).addClass("email-msg-error");
        }
    }
});

If, and only if, you start reusing your messages for multiple functions, then refactor to a message dictionary and reference it from there. Note your messages object would likely need to be a global variable, or at least in the outer scope of all the functions that use it.

 var messages = {};
 messages.mail_success = 'Your email has been sent successfully!';
 messages.post_success = 'Your data has been updated!';

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

Answer 3

var predefined = "hello world";
var foo = {"msg":predefined}; // JSON string
alert(foo.msg)

?

Answer 4

var out = eval(foo.msg); // out is now "hello world"

Note: do not use eval() if you are not sure what the content of foo.msg is.

or

var out = foo.msg=="predefined" ? predefined : foo.msg;

Answer 5

IFF I understand what you are asking, I think I have all of the pieces here.

You have a variable predefined and you want to be able to return that in your json and have the resulting parsed object contain the value in predefined

JSON.parse will not work for you (at least not in Chrome), but eval will.

var predefined = "Hi! I'm predefined";
// ...

var json = '{"foo": predefined}'; // notice no quotes
var response = eval("(" + json + ")");
alert(response.foo);