Support for eg Array<string>?

[ShardPhoenix]

Hi,

Flowtypes apparently supports annotations like Array, which checks that only string values are included in the array. Currently this plugin ignores the type parameter and just checks that the value is an array (so eg you can pass ["abc", 1] to a function that takes Array). Are there any plans to support type parameters?

[phpnode]

@ShardPhoenix I'd like to but there's a tension here because the typechecks need to be fast and small enough that people can leave them enabled in production if they want to. Right now they incur almost zero overhead. This feature, along with things like type aliases are probably too slow for many production systems. Probably there needs to be a way to set the strictness level, but that would be blocked by https://github.com/babel/babel/issues/1833

[ShardPhoenix]

Thanks for the response. I can understand why performance might be an issue. It would be good to go with your idea for a strictness level option if and when possible.

[cesarandreu]

People truly leave typechecks enabled in production?

[phpnode]

@cesarandreu why not? The performance impact is laughable in 99.9999% of cases and it can protect against things that testing didn't catch. Of course this won't work for everyone, but most projects can safely leave them on.

[timoxley]

@phpnode I've seen runtime type checking add severe negative impact to webpage performance, the difference between usable with them off and not usable with them on. It entirely depends on how much data your app needs to crunch and it's not uncommon for apps to already be running on a tight performance budget without any additional laughable overheads.

[phpnode]

@timoxley you've seen dramatic performance impact from using this plugin in particular or typecheck libraries in general? I can see this causing performance issues when the expansion causes the function to exceed V8's max-inlined-source-size threshold, or in very very tight loops, but we're in micro optimization territory - typeof and instanceof are fast!

That being said, I totally agree that there are scenarios where this matters, and I definitely want a nice way to turn them off in production - waiting on babel for this at the moment. But the fact is that most apps are not written for speed at all and those apps will not generally suffer from leaving this turned on.

[timoxley]

@phpnode I've seen dramatic performance impact from a bunch of typecheck libs. Was considering trying this out, and being able to toggle the checks on and off easily is a big +1 for peace of mind knowing I'm not locking myself into a decision too tightly.

[phpnode]

@timoxley we could look for an environment variable and make the plugin a no-op if it's set, e.g.

DISABLE_TYPECHECKS=1 babel ./src

[phpnode]

@timoxley for a number of reasons this plugin should be a lot faster than most typecheck libraries, I'd encourage you to see for yourself :)

By the way, there's no lockin - just remove the relevant line from your .babelrc and everything will work as normal but without typechecks.

[STRML]

FYI, in your .babelrc, just do something like:

"env": {
  "development": {
    "plugins": [
      "typecheck"
    ]
  }
}

if you're concerned about performance (I am, until I am sure this doesn't slow us down).

[jthomaschewski]

EDIT: Sorry, just saw that I posted this comment in the wrong issue...