Question

我正在尝试从JavaScript: The Definitive Guide 6th ed.学习JavaScript第81页，作者解释说，如果您使用原始名称调用eval，而不是创建其他名称时，则会有所不同。作者用以下源代码说明：

var geval = eval;               // Using another name does a global eval
var x = "global", y = "global"; // Two global variables

function f()  // This function does a local eval
{
    var x = "local";            // Define a local variable
    eval("x += 'changed';");    // Direct eval sets local variable
    return x;                   // Return changed local variable
}

function g()  // This function does a global eval
{
    var y = "local";            // A local variable
    geval("y += 'changed';");   // Indirect eval sets global variable
    return y;                   // Return unchanged local variable
}

console.log(f(), x); // Local variable changed: prints "localchanged global":
console.log(g(), y); // Global variable changed: prints "local globalchanged":

我尝试执行代码，然后得到：

/usr/bin/node test.js

localchanged global
undefined:1
y += 'changed';
^
ReferenceError: y is not defined
    at eval (eval at g (/home/martin/Projects/WebPages/test.js:18:5), <anonymous>:1:1)
    at eval (native)
    at g (/home/martin/Projects/WebPages/test.js:18:5)
    at Object.<anonymous> (/home/martin/Projects/WebPages/test.js:23:13)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)

Process finished with exit code 8

请您解释为什么在y的来电中看不到geval变量？

在这种情况下，为什么全局评估不起作用？

0 个答案: