Allow Boolean() to be used to perform a null check

[Forbes Lindesay (ForbesLindesay)]

Fixes#16655

This allows code like:

constnullableValues=['a','b',null];constvalues: string[]=nullableValues.filter<string>(Boolean);

to be written and to typecheck. Unfortunately the generic type still has to be specified, otherwise TypeScript picks the other, less specific overload of .filter.

[Microsoft Contribution License Agreements (msftclas)]

All CLA requirements met.

[beshanoe (beshanoe)]

Hi, this is a really good initiative, do you think we can skip specifying generic after this
#30215 is merged?

[Ryan Cavanaugh (RyanCavanaugh)]

TypeScript Bot (@typescript-bot) test this

[TypeScript Bot (typescript-bot)]

Heya Ryan Cavanaugh (@RyanCavanaugh), I've started to run the extended test suite on this PR at 24e7e1c. You can monitor the build here. It should now contribute to this PR's status checks.

[Forbes Lindesay (ForbesLindesay)]

beshanoe (@beshanoe) I think that may be sufficient to allow us to skip specifying generic parameters. I'm not sure though without testing it.

I found:

functionB<T>(value: T): value is Exclude<T,false|null|undefined|''|0>{returnBoolean(value);}

was already sufficient, but adding prototypes/new functions to the interface forces you to be explicit.

[Ryan Cavanaugh (RyanCavanaugh)]

Forbes Lindesay (@ForbesLindesay) can you merge this up with master? This will allow us to re-run the RWC suite to evaluate the effects of this on our real-world code database

[Forbes Lindesay (ForbesLindesay)]

Ryan Cavanaugh (@RyanCavanaugh) sorry I didn't see your message. I've merged it now.

[Forbes Lindesay (ForbesLindesay)]

Ryan Cavanaugh (@RyanCavanaugh) any update on this?

[Ryan Cavanaugh (RyanCavanaugh)]

TypeScript Bot (@typescript-bot) test this

[TypeScript Bot (typescript-bot)]

Heya Ryan Cavanaugh (@RyanCavanaugh), I've started to run the extended test suite on this PR at 37bb42c. You can monitor the build here. It should now contribute to this PR's status checks.

[Ryan Cavanaugh (RyanCavanaugh)]

RWC is actually clean, ignore ❌

[Forbes Lindesay (ForbesLindesay)]

Ryan Cavanaugh (@RyanCavanaugh) any chance we can merge this? It's been a while and it's a very simple fix.

[Victor Roussekov (victorouse)]

Ryan Cavanaugh (@RyanCavanaugh) any chance we can merge this? It's been a while and it's a very simple fix.

☝️ 🙏

[Tom Crockett (pelotom)]

Resolves #31164.

[Kalashnikov Igor (silentroach)]

broken by #31515

[Matt Greer (city41)]

This would be a great addition to the language. Have there been any plans on when it might land? 3.8 maybe?

[Maksim (pret-a-porter)]

Matt Greer (@city41) Unfortunately it is still open #16655 Really strange thing, that behaviour of !!someValue differs from Boolean(someValue)

[Shanon Jackson (ShanonJackson)]

Ryan Cavanaugh (@RyanCavanaugh)

Can we revive this. Still continues to prevent usage of the .filter(Boolean) syntax sugar today.

[Tom Crockett (pelotom)]

Ryan Cavanaugh (@RyanCavanaugh)

Can we revive this. Still continues to prevent usage of the .filter(Boolean) syntax sugar today.

Check out ts-reset.

[Steve Bennett (stevage)]

Is there a good workaround in the meantime? I'm not sure how else to express .filter(Boolean) in a way Typescript accepts.

[Jake Bailey (jakebailey)]

Is there a good workaround in the meantime? I'm not sure how else to express .filter(Boolean) in a way Typescript accepts.

You can use type guards:

["",undefined,"test"].filter((x): x is string=>typeofx==="string")

Playground Link

[Shanon Jackson (ShanonJackson)]

.filter((x): x is NonNullable<typeof x> => Boolean(x))

Works universally for any Array<T>

[Jonny Lee (jonny133)]

.filter((x): x is NonNullable<typeof x> => Boolean(x))

Works universally for any Array<T>

The code in #29955 (comment) more completely shows the result of Boolean excludes other falsey values