There are two sample methods in Crockford's library (as raised by @Anonymous):
JSON string to object:
var obj = JSON.parse('{ "property":"value" }');
alert (obj.property);
// value
Object to JSON string:
var str = JSON.stringify({ "property":"value" })
alert (str);
//{ "property":"value" }
There are also built in methods to do this in most of the major frameworks.