Checking for void in logical operations

[tomitrescak]

In many cases it is not desirable to allow comparison with undefined. Is it somehow enforceable that this will be detected during compilation?

function a() { }

if (a()) { } // this should throw error

I guess this is by design not available.

[RyanCavanaugh]

It's certainly very suspicious. We can look at how much code this might break.

[DanielRosenwasser]

Disallowing void from the LHS of a ||, &&, and conditionals, we catch this somewhat inappropriate code in the compiler.

12983                 checkFunctionOrMethodDeclaration(node) || checkGrammarForGenerator(node);
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/compiler/checker.ts(12983,17): error TS5063: The left-hand side of a '||' expression cannot have type 'void'.

Running tests now.

[tomitrescak]

Oh wow, that would save me from the world of problems! Please let me know whether this will make it into "next" version. I guess this can be configurable with a parameter in tsconfig.

[zpdDG4gta8XKpMCd]

can we instead add a flag that prohibits non-boolean expressions where a boolean is expected? #7306

[tomitrescak]

@DanielRosenwasser would you be able to share with us, what is the result? Will this (flag) make it to the compiler? Thanks!

[adidahiya]

This would be pretty great to have in the compiler. Linking to our TSLint issue tracking this: palantir/tslint#253

[DanielRosenwasser]

@tomitrescak sorry, I've been sidetracked on other things. The team will need to review whether we're certain that it's a net positive as well. In the mean time, my pull request can be found at #7261. I don't think there's a ton of controversial stuff in it. Feel free to try it out, review it, and get back to me.