JavaScript数组解构分配产生奇怪的错误 [英] Javascript array destructuring assignment gives strange errors
问题描述
在使用Javascript解构分配时,我刚刚注意到一个奇怪的错误,这使我不得不做出一些猜测.我在这里发布,以便我可以展示我学到的东西.(我接受 JavaScript自动分号插入(ASI)的规则是什么?,但是我的问题是,当ASI的工作方式与我预期的不同时,如何诊断一个奇怪的错误;这是我希望能在何时找到的答案搜索数组解构错误"或类似内容.)
I've just noticed a strange error when using Javascript destructuring assignment, which took me some guesswork to resolve. I'm posting here so I can show what I learned. (I accept that the question about Javascript semicolon insertion is answered by What are the rules for JavaScript's automatic semicolon insertion (ASI)?, but my problem was about how to diagnose a strange error when ASI works differently than I was expecting; it's an answer that I would have hoped to find when searching for "array destructuring error" or similar.)
以下代码:
let next_col_time_step = head_steps.reduce(
choose_next_step, [-1, -1, null]
)
[next_step_col, next_step_time, next_step] = next_col_time_step
运行时会产生一个非常混乱的错误:
When run generates an very confusing error:
ReferenceError: can't access lexical declaration 'next_col_time_step' before initialization
尽管(显然)只是在错误行之前被初始化.
Despite (apparently) having been just initialized before the error line.
或者,如果我尝试查看已分配的值,则:
Or, if I try to see what value has been assigned, thus:
let next_col_time_step = head_steps.reduce(
choose_next_step, [-1, -1, null]
)
console.log("next_col_time_step %s", next_col_time_step)
[next_step_col, next_step_time, next_step] = next_col_time_step
我看到显示的期望值和另一个错误:
I see the expected value displayed and a different error:
next_col_time_step 2,52,[object Object]
TypeError: console.log(...) is undefined
即 console.log(...)
显然可以按预期工作,然后报告为未定义.这是怎么回事?
I.e., console.log(...)
apparently works as expected, then is reported as undefined. What's going on here?
推荐答案
这里的问题是Javascript语法中令人困惑的歧义.
The problem here is a confusing ambiguity in Javascript syntax.
是否注意到我没有使用;
语句终止符?
Notice that I wasn't using ;
statement terminators?
似乎数组解构分配被解析为应用于上一条语句的数组索引操作.
It appears that the array destructuring assignment is being parsed as an array indexing operation applied to the previous statement.
快速解决方案:在前面的语句之后添加;
(尽管不幸的是,如果通常省略这些样式,则会强制产生不一致的样式):
Quick fix: add ;
after the preceding statement (though this unfortunately forces an inconsistent style if these are generally omitted):
let next_col_time_step = head_steps.reduce(
choose_next_step, [-1, -1, null]
);
[next_step_col, next_step_time, next_step] = next_col_time_step
然后,瞧!,一切都很好:)
And, voila!, all is well :)
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