Query about JSON in JavaScript

Question

I have been playing around with JSON a little, but I found something I don't quite understand, for example, I have this piece of code:

var str = "{'name':'vvv'}";                    
var cjson = eval ("(" + str + ")");
alert(cjson.name);

It works fine, but with this piece of code

var str = "{'name':"+'vvv'+"}";    
var cjson = eval ("(" + str + ")");
alert(cjson.name);

It does not, I got the following firebug error : ReferenceError: vvv is not defined.

Why doesnt it works with the second way, isnt str a valid string in both cases?

You need to add quotes around 'vvv', after string concatenation you get var str = "{'name':vvv}"; You should have something like var str = "{'name':"+ "'vvv'" +"}"; — uthark
– uthark, Commented Aug 27, 2013 at 1:38

Paul Roub · Accepted Answer · 2013-08-27 01:21:18Z

3

It's a string, yes. But not the same string, and not valid JSON.

 var str = "{'name':" + 'vvv' + "}";

is the same as:

 var str = "{'name':" + "vvv" + "}";

which is the same as:

 var str = "{'name':vvv}";

When you try to evaluate that, it's as if you'd declared:

 var cjson = { 'name': vvv };

and you have no variable named vvv.

answered Aug 27, 2013 at 1:21

Paul Roub

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1 Comment

bfavaretto Over a year ago

For the record, valid JSON would be var str = '{"name": "vvv"}';

Arun P Johny · Accepted Answer · 2013-08-27 01:21:24Z

0

when you do var str = "{'name':"+'vvv'+"}";, the generated string is "{'name':vvv}" where vvv is not a string literal. So the eval will try to resolve it in the execution scope, since it is not found there it throws an error

answered Aug 27, 2013 at 1:21

Arun P Johny

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