在JSFiddle中，为什么未在`window`上定义此全局变量? [英] In JSFiddle, why is this global variable not defined on `window`?
问题描述
我有此代码在小提琴中:
var a = 1;
function b() {
var a = 10;
alert(window.a);
}
b();
为什么在这里未定义a?它已经在全局名称空间(即window)中定义. (有关这种意外行为的示例，请参见小提琴.)
Why is a is undefined here? It's already defined in global namespace, i.e., window. (See the fiddle for an example of this unexpected behavior.)
推荐答案
如果您正在未将位置设置为不自动换行"的小提琴中运行此代码，或者在任何情况下在顶级范围内，您的外部a不是不是全局变量window.a.考虑一个简单的示例，其中代码包装在名为wrappingFunc的函数中:
If you're running this code in a fiddle that does not have the location set to "No wrap", or any circumstance in which you're not at the top-level scope, your outer a is not the global variable window.a. Consider a simple example where your code is wrapped inside a function called wrappingFunc:
// THIS would be the global `a`, outside `wrappingFunc`
var a = "now the global a is defined";
function wrappingFunc() {
// this is NOT the global `a`
var a = 1;
function b() {
var a = 10;
alert(window.a);
}
b();
}
wrappingFunc();
当您将位置设置为onLoad或onDomready时，这正是JSFiddle所做的. (请参阅什么是onLoad，onDomready，< head中没有换行和< body>中没有换行之间的区别?)我的wrappingFunc示例实际上是onload或ondomready侦听器函数，这会阻止Keep在全局上下文中运行.
This is exactly what JSFiddle does when you set the location to onLoad or onDomready. (See What is the difference between onLoad, onDomready, No wrap - in <head>, and No wrap - in <body>?) My wrappingFunc example is in actuality an onload or ondomready listener function, which prevents the keep from running in a global context.
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