Tslint: Allow fast null checks in no-unused-expression rule does not cover some cases

[gnapse]

The allow-fast-null-checks option for the no-unused-expression rule, which was added in #1638, is currently not covering all cases. Today it's only meant for allowing expressions that check if a variable is truth before calling a method on that variable:

// This is what's currently supported
e && e.preventDefault()

I propose that it should also cover cases that check if a variable is truth before calling it as a function:

// This is not currently supported, but it should be.
callback && callback();

And perhaps there are other cases as well. If someone can think of any, please add them here so it can be covered in a future PR for this issue as well.

[alamothe]

This pattern does not work either:

this.props.onClick && this.props.onClick()

[gnapse]

@alamothe that is effectively the same pattern the issue raised. It doesn't matter if the nullable thing is a variable, or a property inside an object.

[alamothe]

Right, sorry about that, I guess my point was that the pattern is fairly common inside React.

[XeniaSiskaki]

x && y() also doesn't work

[gnapse]

@XeniaSiskaki why would you expect that to work? If x is not null, that does not tell you anything about the ability to call y as a function.

[XeniaSiskaki]

It's the short way of saying if (x) y(); which is pretty useful. I'm just saying that there should be a way to not have tslint complain.

[gnapse]

tslint complains if the type of y makes it potentially null or undefined. And the if (x) at the front of it does not remove the possibility of y not being a function at the moment of it being called. If you really want not to be warned about it, then why use TS at all? Or if you're sure you know what you're doing and want TS out of your way in this particular case (for instance if you know that if x is not null, then y is also not null for sure, but TS has no way of knowing), then do this:

if (x) y!();

But my point is, this is not an issue with TS, and it's not related to the issue I brought up here, which is a real thing tslint could improve.

[JoshuaKGoldberg]

☠️ TSLint's time has come! ☠️

TSLint is no longer accepting most feature requests per #4534. See typescript-eslint.io for the new, shiny way to lint your TypeScript code with ESLint. ✨

It was a pleasure open sourcing with you all!

[JoshuaKGoldberg]

🤖 Beep boop! 👉 TSLint is deprecated 👈 _(#4534)_ and you should switch to typescript-eslint! 🤖

🔒 This issue is being locked to prevent further unnecessary discussions. Thank you! 👋