Option Proposal: for no-unused-vars, add exceptions and exceptionsIgnorePattern options

[SpadeAceman]

Following from the discussion in #3543, I propose the addition of options to the no-unused-vars rule, allowing us to specify how we want to handle exception variables defined in the catch() clause of a try/catch statement.

Currently, the documentation doesn't address this, and the rule allows caught exception variables to remain unused, even when the "all" settings are used for the current options (tested via ESLint 1.4.3, though this behavior was also present in previous versions):

/*eslint no-unused-vars: [2, {"vars": "all", "args": "all"}]*/
function example(){
  try {
    doSomething();
  } catch (err) { // unused exception variable isn't flagged as an error
    return false;
  }
  return true;
}

Therefore, I propose the addition of exceptions and exceptionsIgnorePattern options, mirroring the functionality of the vars/varsIgnorePattern and args/argsIgnorePattern options. Valid settings for exceptions would be:

• none - the default, matches the current behavior of not reporting any unused exception variables
• all - reports all unused exception variables, aside from those matching exceptionsIgnorePattern

/*eslint no-unused-vars: [2, {"exceptions": "all", "exceptionsIgnorePattern": "^ignore"}]*/
function hypotheticalExample(){
  try {
    doSomething();
  } catch (err) { // unused exception variable IS flagged as an error
    return false;
  }
  try {
    doSomethingElse();
  } catch (ignoreErr) { // unused exception variable matches ignore pattern, is NOT flagged as an error
    return false;
  }
  return true;
}

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[eslintbot]

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[nzakas]

Looks good, let's do it.

[gyandeeps]

@nzakas Should we just make the default behavior to flag args inside catch statements and People who do not want to flag that can use argsIgnorePattern option. Since err is also an argument to the catch block. I never knew this rule was missing this as I would expect it to flag things inside catch statement also, unless I am missing something which I am not thinking of.
We can release this with 2.0 with under the breaking section.

[nzakas]

I don't see a reason to introduce a breaking change for this. It flags things inside catch it just didn't flag the exception because it's required.

[mischah]

I know you hate those +1 comments.

… but I love to see this 😘

[platinumazure]

@mischah Issue is accepted, so if you want to write a pull request, have at it! It's like somebody said (I forget who, maybe Gandhi?), "Be the change you wish to see in the world". So he probably would have also said "If you want something, write it yourself" 😁

[pvamshi]

I am working on this , will create a pull request for review shortly

[ryanbriscall]

Thanks for this feature, but I needed this for function names. It works for variables, given the provided "catch (ignoreErr)" example. But doesn't work for functions, say for example: function ignoreTest() { }. I was expecting this "no-unused-vars" exceptions solution to work for functions too, since eslint is showing my unused function error as a "no-unused-vars" error.