在 JavaScript 中指定 eval() 的范围? [英] Specify scope for eval() in JavaScript?

问题描述

有什么方法可以在特定范围(但不是全局)上执行 eval()?

is there any way I can execute eval() on a specific scope (but NOT global)?

例如，以下代码不起作用(第二个语句中的 a 未定义)，因为它们在不同的范围内:

for example, the following code doesn't work (a is undefined on the second statement) because they are on different scope:

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

如果可能，我想即时创建一个范围.例如(语法肯定是错误的，但只是为了说明这个想法)

If possible, I would like to create a scope on the fly. for example (the syntax is definitely wrong, but just to illustrate the idea)

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
}

知道如何实现这样的目标吗?谢谢！

Any idea on how to achieve something like this? Thanks!

推荐答案

你可以使用"use strict" 在 eval 本身中包含 eval 的代码.

you can use the "use strict" to contain the eval'ed code within the eval itself.

其次，严格模式代码的eval不会在周围的作用域中引入新的变量.在普通代码中，eval("var x;") 将一个变量 x 引入到周围的函数或全局作用域中.这意味着，通常，在包含对 eval 调用的函数中，每个不引用参数或局部变量的名称都必须在运行时映射到特定定义(因为 evalcode> 可能引入了一个新变量来隐藏外部变量).在严格模式下 eval 只为被评估的代码创建变量，所以 eval 不能影响一个名字是指外部变量还是某个局部变量

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

如果一个函数声明为use strict"，那么里面的所有东西都会以严格模式执行.以下将执行与上述相同的操作:

If a function is declared with "use strict", everything in it will be executed in strict mode. the following will do the same as above:

function foo(){
    "use strict";

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

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