About the eval () function handles JSON data returned in the background

for a JSON string returned by the server, if the jquery asynchronous request does not make a type description, or is accepted as a string, then an object processing is required, either in a cumbersome way or by placing the string in eval (). This is also a good way to get JSON objects in a normal Javascipt way, as illustrated below:

var dataobj=eval ("(" +data+")");//Convert to JSON object

Why should Eval be added here ("(" +data+ ")")? 

The reason is that eval itself is a problem. Since JSON starts and ends in the form of "{}", in JS, it is treated as a block of statements, so it must be coerced into an expression. 

The purpose of the parentheses is to force the Eval function to force the expression in parentheses to be converted to an object while processing the JavaScript code, rather than being executed as a statement (statement). For example, if the object literal {} is not enclosed, then eval will recognize the curly brace as the start and end tag of the JavaScript block, and {} will be considered an empty statement. So the following two execution results are different:
Alert (eval ("{}");//return undefined
Alert (eval ("({})");//return Object[object]

About the eval () function handles JSON data returned in the background