javascript - 在 JavaScript 中指定 eval() 的范围？

Question

有什么方法可以在特定范围(但不是全局)上执行 eval()？

例如，以下代码不起作用(第二条语句中 a 未定义)，因为它们位于不同的范围内:

eval(var a = 1); 
eval(alert(a));

如果可能的话，我想动态创建一个范围。例如(语法肯定是错误的，但只是为了说明想法)

var scope1;
var scope2;
with scope1{
    eval(var a = 1); eval(alert(a));  // this will alert 1
}
with scope2{
    eval(var a = 1); eval(a++); eval(alert(a));  // this will alert 2
}
with scope1{
    eval(a += 2); eval(alert(a)); // this will alert 3 because a is already defined in scope1
}

关于如何实现这样的目标有什么想法吗？谢谢!

Answer 1

您可以使用"use strict"将经过评估的代码包含在评估本身中。

Second, eval of strict mode code does not introduce new variables into the surrounding scope. In normal code eval("var x;") introduces a variable x into the surrounding function or the global scope. This means that, in general, in a function containing a call to eval every name not referring to an argument or local variable must be mapped to a particular definition at runtime (because that eval might have introduced a new variable that would hide the outer variable). In strict mode eval creates variables only for the code being evaluated, so eval can't affect whether a name refers to an outer variable or some local variable

var x = 17;                                       //a local variable
var evalX = eval("'use strict'; var x = 42; x");  //eval an x internally
assert(x === 17);                                 //x is still 17 here
assert(evalX === 42);                             //evalX takes 42 from eval'ed x

如果函数声明为“use strict”，则其中的所有内容都将以严格模式执行。以下将执行与上面相同的操作:

function foo(){
    "use strict";

     var x = 17;
     var evalX = eval("var x = 42; x");
     assert(x === 17);
     assert(evalX === 42);
}