Node.js Global 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。我试图从书中执行以下代码，关于 eval（）。我使用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'

然而，我得到一个 ReferenceError geval（）别名时，$ c>在 g（）函数内：

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) 

根据我的理解，当我将 eval（）别名为 geval（）时，代码传递的字符串在全局范围内按照ES5进行评估。但是，我遇到 ReferenceError 并且无法理解原因。

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.

虽然我不会考虑 eval（）是一个关键功能，我当然想了解为什么我遇到这种行为。

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

PS当我尝试在谷歌浏览器中执行相同的代码时，它似乎就像一个魅力！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; 实际上在模块范围中定义 y ，而非全局范围。

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，顶级范围是全局范围。这意味着如果您在全局范围内，浏览器中的
会定义
全局变量。在Node中，这是不同的。顶级范围不是全局范围
; Node模块中的var内容将是
该模块的本地内容。

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中使用它的两种可能方法是：

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

1. 在节点 REPL


2. 在模块中运行它，但替换 var y =全球; 只有 y =全球;

1. Run it as is in the node REPL
2. Run it in a module, but replace var y = global; with just y = global;

这篇关于Node.js Global eval，抛出ReferenceError的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！