为什么"3 [1，2];"返回未定义的JavaScript? [英] Why does "3 [1, 2];" return undefined in JavaScript?

问题描述

我正在查看GMail所做的一些AJAX调用，并且我发现有时调用的返回值以数字开头.示例:(请注意，第一行之后没有分号)

I was looking at some of the AJAX calls that GMail does, and I noticed that sometimes the return value of the calls started with a number. Example: (note that there's is no semi-colon after the first line)

3 
[1, 2];

如果我将其输入到JavaScript控制台中，我将得到未定义的返回.但是，如果第二个参数是数字或字符串，则可以返回第二个参数.示例:

If I were to enter this into a JavaScript console, I'd get undefined returned back. However, if the second parameter is a number or a string, I'd get the second parameter returned back. Example:

3
4

返回4.

我的猜测是，他们正在这样做以阻止jsonp型攻击，但是，有人知道正在进行什么基础评估吗?在第二种情况下，我认为在第一行之后插入"了分号，这将使返回4有意义.但是，我想不出第一个表达式返回未定义的原因.

My guess is that they're doing this to stop jsonp-type attacks, however, does anyone know what underlying evaluation is being done? In the 2nd case I believe that a semi-colon is "inserted" after the first line, which would make returning 4 make sense. However, I can't think of a reason why the first expression would return undefined.

推荐答案

这是因为ASI(自动分号插入)的工作方式.第一条语句解释为

This is because how ASI ( Automatic Semicolon Insertion ) works. The first statement is interpreted as

3[1,2];

所以它是undefined.第二个被ASI解释为3;4;，即4.

so it is undefined. The second one is interpreted by ASI as 3;4; which is 4.

在某些情况下，ASI规则是违反直觉的，例如，您可能想知道为什么数字和括号之间没有分号?好吧，这是有原因的.阅读这些资源以获取更多详细信息:

ASI rules are counterintuitive in some cases, for example you might wonder why there is no semicolon between number and bracket? Well, there is a reason for that. Read these resources for more details:

> JavaScript自动分号插入(ASI)的规则是什么)?

http://bclary.com/2004/11/07/#a-7.9.1

Google可能会为您带来更多结果. :)这就是为什么我们永无止境的无分号JavaScript 战争.

Google will probably give you more results. :) That's why we have neverending semicolon-free JavaScript war.

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