问题描述
var a = [1,2,3,4];
var b = [10,20,30,40];
console.log([a,b].length)
[a,b].some(function(x){ x.push(x.shift()) });
今天这个代码导致我非常惊讶
I was extremely surprised today when this code caused
[a,b].some(function(x){ x.push(x.shift()) });
^
TypeError: Cannot call method 'some' of undefined
显然JavaScript'自动分号插入'在这里没有按预期工作。但是为什么？
Obviously the JavaScript 'auto semicolon insertion' is not working as expected here. But why?
我知道你可能会建议在任何地方使用; 以避免类似的事情，但问题是不管是否更好地使用; 。我很想知道这里究竟发生了什么？
I know you might recommend to use ; everywhere to avoid something like that, but the question is not about whether it is better to use ; or not. I would love to know what exactly happens here?
推荐答案
当我担心分号插入时，我想到了什么线条有问题的看起来没有任何空格。在你的情况下，那将是：
When I'm worried about semicolon insertion, I think about what the lines in question would look like without any whitespace between them. In your case, that would be:
console.log([a,b].length)[a,b].some(function(x){ etc });
这里你告诉Javascript引擎调用 console.log ，长度为 [a，b] ，然后查看索引 [a，b] 该调用的结果。
Here you're telling the Javascript engine to call console.log with the length of [a,b], then to look at index [a,b] of the result of that call.
console.log 返回一个字符串，因此您的代码将尝试查找属性 b 该字符串未定义，并且对 undefined.some（）的调用失败。
console.log returns a string, so your code will attempt to find property b of that string, which is undefined, and the call to undefined.some() fails.
有趣的是， str [a，b] 将解析为 str [b] 假设str是一个字符串。正如Kamil指出的那样， a，b 是一个有效的Javascript表达式，该表达式的结果只是 b 。
It's interesting to note that str[a,b] will resolve to str[b] assuming str is a string. As Kamil points out, a,b is a valid Javascript expression, and the result of that expression is simply b.
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