Suggestion: is not typegaurd

[speigg]

To support the following use case, for example:

function defined(value: any): item isnot undefined|null { // "isnot" or "is not" ?
    return value !== undefined && value !== null
}

This would be particularly useful when combined with strictNullChecks:

var value:string;
function setValue(value: string|undefined) {
    if (defined(value)) {
        value = value; // currently causes an error, since tsc believes value is still possibly undefined
    } else {
        value = "default";
   }
}

[RyanCavanaugh]

I think you want #1809 because then you could just write

type Defined = string | number | boolean | object;
function defined(value: any): value is Defined { return value != null; }

[speigg]

Oh yeah, that would work nicely. Any reason the following wouldn't work?

type Defined = string | number | boolean | {};
function defined(value: any): value is Defined { return value != null; }

I thought the "{}" type was effectively "object", but the link you pointed me at seems to suggest otherwise.

[speigg]

Or if I understand correctly, perhaps I can just use the {} type alone...

function defined(value: any): value is {} { return value != null; }

About to try this.

[mhegazy]

you can achieve this today using generic types, as such:

function defined<T>(value: T | undefined | null): value is T { return value != null; }

[aluanhaddad]

@mhegazy beat me to it, I was about to post

function isDefined<T>(x: T | undefined | null): x is T {
    return x != undefined;
}

[speigg]

Thanks guys, all of these solutions work great, including value is {}. I like the generics though, I'll use that!