Javascript array destructuring assignment gives strange errors [duplicate]

问题

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.)

The following code:

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

When run generates an very confusing error:

ReferenceError: can't access lexical declaration 'next_col_time_step' before initialization

Despite (apparently) having been just initialized before the error line.

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

I see the expected value displayed and a different error:

next_col_time_step 2,52,[object Object] 
TypeError: console.log(...) is undefined

I.e., console.log(...) apparently works as expected, then is reported as undefined. What's going on here?

回答1:

The problem here is a confusing ambiguity in Javascript syntax.

Notice that I wasn't using ; statement terminators?

It appears that the array destructuring assignment is being parsed as an array indexing operation applied to the previous statement.

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

And, voila!, all is well :)

来源：https://stackoverflow.com/questions/57006526/javascript-array-destructuring-assignment-gives-strange-errors

标签

javascript

ecmascript-6

destructuring