Parse the JSON string using the Eval function of JavaScript, and hint "not avalible" what to do?

In the simplest form, a "name/value pair" can be represented by a JSON such as the following:

var jsonstring = ' {' firstName ': ' Brett '} ';

If such a simple JSON string using the Eval function is not resolvable (tested in Chrome browser 37.0)

var a = eval (jsonstring);

Execute this statement is the system will prompt: "Not avalible";

What about this problem, we actually need to transfer the values of multiple parameters.

The solution is to add the simplest JSON string "[]" to the list price as the data, the modified code is as follows:

var jsonstring = ' [{' FirstName ': ' Brett '}] ';

var a = eval (jsonstring);

Console.log (a[0].firstname);//output: Brett

Parse the JSON string using the Eval function of JavaScript, and hint "not avalible" what to do?