JavaScript is bound to use of Five eval

eval

eval (parse) parse: With the parameter string inside, we know that executing javascript compiles execution.

Change the value of the global variable:

var x = ; // Defined global variables 
alert(x);//
var g = eval("x="); //eval  Resolves based on the current context x
alert(x);// 

Reference eval in global scope, change the value of global scope, do not change the value of local scope

var g = eval; // Global Reference eval
var x = "global";// Define global variables 
(function f() {
var x = "local";
g("x+='changed'");
alert(x);// local variable local
})();//IIFE  Execute the expression immediately 
alert(x);// Values of global variables globalchanged 

Reference within a local scope changes the value of a local variable without changing the value of a global variable

var g = eval; // Global Reference eval
var x = "global";// Define global variables 
(function f() {
var x = "local";
eval("x+='changed'");
alert(x);// local variable localchanged
})();//IIFE  Execute the expression immediately 
alert(x);// Values of global variables global 　 

summary

eval execution will determine whether the changed variable is local or global based on the context, so the key to using the eval function is to see the scope of the reference to eval!