Suggestion: Type guarding by means of property checking

[AbubakerB]

Hi guys,

In regards to type guards, one thing that is fairly common in the javascript world is checking a property to see if its a certain type.

var variable: number | boolean[] | string;
if (variable.length) {
    // variable is of type boolean[] | string  
}
else {
    // variable is of type number
}

Would this be a possible thing to include in typescript?

One thing i see as an issue/blocker (probably why this hasn't already been implemented) is that length isn't a valid property of variable.
But i thought, just like how we can opt out of a strongly typed var with the any keyword, maybe we could "flatten" out a union type to show all it's properties.
I suggest doing something like so: (<all>variable).length or maybe variable?.length (or something else?)

In fact, i already implemented the latter, and it works great, but of course, ?. might be used for something else in the future (hint: C# 6). Maybe we can discuss this.

Where do the TS team stand on this?

[RyanCavanaugh]

See #1260

[AbubakerB]

I guess i didn't search good enough... my bad! 😄
I'll close this and move the discussion over there