no-undef: ignorePattern option

With no-undef: [2] and this code:

var x;
[_, x] = [1, 2];
console.log(x);

one gets

2:2  error  '_' is not defined  no-undef

There are two possible ways for the author to solve this:

1. var [_, x] = [1, 2];. Not an option if one wants to re-use existing variables in scope.
2. [, x] = [1, 2];. Arguably ugly.

I think it would be great if no-undef would gain a ignorePattern option for cases like this, similar to the options available in no-unused-vars.

[ilyavolodin]

I’m not sure why [ , x] = [1, 2] is considered ugly? That seems like a significantly better and much more readable solution then proposed [_, x] with a throw-away variable.

[silverwind]

Sure:

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What rule do you want to change?

Add a ignorePattern option to no-undef, which takes a RegExp pattern for variable names that are ignored by the rule. This is useful for unavoidable “trash” variables like _ during array destructuring, where a pattern ^_ can match them.

Does this change cause the rule to produce more or fewer warnings?

Fewer warnings based on the user’s preference.

How will the change be implemented? (New option, new default behavior, etc.)?

New option.

Please provide some example code that this change will affect:

var x;
[_, x] = [1, 2];
console.log(x);

What does the rule currently do for this code?

2:2  error  '_' is not defined  no-undef

What will the rule do after it’s changed?

With ignorePattern: ^_, the code would pass.