Node.js 全局 eval，抛出 ReferenceError [英] Node.js Global eval, throwing ReferenceError

问题描述

我正在尝试从 Rhino 书中学习 JavaScript.我试图执行书中关于 eval() 的以下代码.我正在使用 node.js (v0.10.29) 来执行示例.

I am trying to learn JavaScript from the Rhino book. I was trying to execute the following code from the book with regards to eval(). I am using node.js (v0.10.29) to execute the examples.

var geval = eval;                  // aliasing eval to geval
var x = 'global';                  // two global variables
var y = 'global';

function f () {
  var x = 'local';                 // define a local variable
  eval('x += "changed";');         // direct eval sets the local variable
  return x;
}

function g () {
  var y = 'local';                 // define a local variable
  geval('y += "changed";');        // indirect eval sets global variable
  return y;
}

console.log(f(), x);               // => expected 'localchanged global'
console.log(g(), y);               // => expected 'local globalchanged'

但是，当尝试使用 geval() 别名时，我在 g() 函数中得到一个 ReferenceError:

However, I get an ReferenceError inside the g() function when there is an attempt to use the geval() alias:

undefined:1
y += "changed";
^
ReferenceError: y is not defined
    at eval (eval at g (/Users/codematix/Learning/learnjs/expressions.js:148:3), <anonymous>:1:1)
    at eval (native)
    at g (/Users/codematix/Learning/learnjs/expressions.js:148:3)
    at Object.<anonymous> (/Users/codematix/Learning/learnjs/expressions.js:153:3)
    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) 

据我所知，当我将 eval() 别名为 geval() 时，传递的字符串中的代码将按照 ES5 在全局范围内进行评估.但是，我遇到了 ReferenceError 并且无法理解为什么.

From what I understand, when I alias eval() as geval(), the code in string passed is evaluated in global-scope as per ES5. However, I am running into the ReferenceError and unable to understand why.

虽然我不认为 eval() 是一个关键特性，但我绝对想了解为什么我会遇到这种行为.

Although I would not consider eval() to be a critical feature, I definitely would like to understand why I am encountering this behavior.

附言当我尝试在 Google Chrome 中执行相同的代码时，它似乎很有魅力！奇怪！

P.S. When I attempt to execute the same code in Google Chrome, it seems to work like a charm! Strange!

推荐答案

问题是你从一个模块运行这段代码，其中 var y = global; 实际上定义了 y 在模块作用域，而不是全局作用域.

The issue is that you are running this code from a module, wherein var y = global; actually defines y in module scope, not global scope.

在浏览器中，顶级作用域是全局作用域.这意味着在浏览器中，如果您在全局范围内 var 将定义一个全局变量.在 Node 中这是不同的.顶级范围不是全球范围；节点模块中的 var 某些内容将是本地的那个模块.

In browsers, the top-level scope is the global scope. That means that in browsers if you're in the global scope var something will define a global variable. In Node this is different. The top-level scope is not the global scope; var something inside a Node module will be local to that module.

http://nodejs.org/api/globals.html#globals_global

因此，在 Node 中实现此功能的两种可能方法是:

So, two possible ways to get this to work in Node are:

1. 在节点 REPL
中按原样运行
2. 在模块中运行它，但将 var y = global; 替换为 y = global;

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