Question about new `new-cap` rule

[dandean]

• Original discussion: https://github.com/airbnb/javascript/issues/1089#issuecomment-249409943
• Patch: https://github.com/airbnb/javascript/pull/1090

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I don't want to bikeshed on this, but I do have some concerns about this rule change I'd like to bring up.

My biggest concern is that it will lead to more people overriding the new-cap rule than in the past because this new rule is so limited in scope. This would be the opposite of the intended outcome of #1089.

• This rule creates exceptions for Immutable.{Map,Set,List}
• The Immutable API contains many more types (Record, Seq, Collection, OrderedMap, etc)

The result is that eslint will tell people to use new for new Immutable.Record() AND to NOT use new for Immutable.Map(). The only way to make this consistent is to override the rule in your local environment, and this will be incredibly common.

My suggestion is to either:

• revert the rule change
• re-implement the rule to use capIsNewExceptionPattern instead of capIsNewExceptions in order to target the entire Immutable.js API with a regular expression.

If this interests you I'm happy to submit a patch for either choice.

[ljharb]

I think using the pattern for Immutable.js (as long as it requires you omit the new in those cases) is the ideal approach.

[dandean]

👍

A quick test shows this working:

    "new-cap": [
      "error", {
        "capIsNewExceptionPattern": "^Immutable.\\w"
      }
    ],

BUT it does not require the developer to omit new. I don't see any obvious way to enforce the omission of new for Immutable.* functions. I'll keep digging.

[ljharb]

What about also adding newIsCapExceptionPattern that's the same?

[dandean]

I tried that as well, but it leads to allowing constructs like:

new Immutable.foo();

In other words, that configuration allows the name of the constructor used with new to be lower case.

allows any lowercase-started function names that match the specified regex pattern to be called with the new operator.