Option to disallow "var" keyword with esnext: true

[fluidsonic]

esnext: true adds support for let and const which make var obsolete in most cases, esp. in node.js environments. It would be great to have an option to warn about variables declared with var since its usage is most likely a mistake in ES6 files.

[rwaldron]

The let and const that are available in v8 --harmony and subsequently node.js are obsolete implementations.

[fluidsonic]

Why are they obsolete?

[rwaldron]

The implementation is limited to strict-mode only, which is incorrect. I suppose for node.js purposes, this is not a problem, but it remains an issue in code that is expected to run in browsers: IE11 is implemented correctly, Chrome (w/ flags on) is limited to strict-mode and Firefox is an ancient implementation that's so wrong it doesn't even count.

To be clear, I definitely support a future "novar" (or whatever) flag, but it's important to understand the consequences before jumping to something like that.

[fluidsonic]

You're right that it depends on the environment whether you can fully rely on let and const. But as with most options it should be up to the developers to decide which flags they want use since they know the targeted runtime environment best.

As usual there should be a sane default.

[rwaldron]

I find that compelling enough to +1

[callumlocke]

relevant discussion:
Is there any reason to use the “var” keyword in ES6?

[rwaldron]

@caitp @jugglinmike are either of you interested in this one?

[caitp]

I guess if it's a linting rule people want, why not?

[callumlocke]

thanks for this @caitp 👍