Javascript ||或具有未定义变量的运算符 [英] Javascript || or operator with a undefinded variable

问题描述

我最近一直在读一篇我读过的文章来自Opera。

I have been doing some reading lately one article I read was from Opera.

http://dev.opera.com/articles/view/javascript-best-practices/

在那篇文章中他们写道：

In that article they write this:

JavaScript
中的另一种常见情况是为$提供预设值b $ b变量如果未定义，例如
so：

Another common situation in JavaScript is providing a preset value for a variable if it is not defined, like so:

if(v){
  var x = v;
} else {
  var x = 10;
}

这个的快捷符号是
双管字符：

The shortcut notation for this is the double pipe character:

var x = v || 10;

出于某种原因，我无法让这个为我工作。是否真的可以检查是否定义了v，如果不是x = 10？

For some reason I cant get this to work for me. Is it really possible to check to see if v is defined, if not x = 10?

- 谢谢。
Bryan

--Thanks. Bryan

推荐答案

Opera文章对发生的事情描述不佳。

That Opera article gives a poor description of what is happening.

虽然 x 确实会得到 10 如果 v 是 undefined 。如果 v x ， 10 也是如此有任何falsey值。

While it is true that x will get the value of 10 if v is undefined. It is also true that x will be 10 if v has any "falsey" value.

javascript中的falsey值为：

The "falsey" values in javascript are:

• 0


• null


• undefined


• NaN


• （空字符串）


• false

• 0
• null
• undefined
• NaN
• "" (empty string)
• false

所以你可以看到很多情况下 x 将设置为 10 除了 undefined 。

So you can see that there are many cases in which x will be set to 10 besides just undefined.

这是一些文档，详细说明了逻辑运算符。 （这是逻辑OR。）它给出了几个用于这种赋值的例子。

Here's some documentation detailing the logical operators. (This one is the "logical OR".) It gives several examples of its usage for such an assignment.

快速示例：http://jsfiddle.net/V76W6/

var v = 0;

var x = v || 10;

alert( x ); // alerts 10

分配 v 任何我在上面指出的假值，你会得到相同的结果。

Assign v any of the falsey values that I indicated above, and you'll get the same result.

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