Typescript: Add an "argument is not type" operator

[Arlen22]

We already have:

function isNotUndefined(obj:any): obj is string | number | boolean | MyCustomType | null | MyOtherCustomType {
    return typeof obj !== "undefined";
}

Now we just need:

```ts
function isNotUndefined(obj:any): obj is not undefined { // or isnot
return typeof obj !== "undefined";
}

[kitsonk]

Dupe (or heavily related to) #4183

[RyanCavanaugh]

function isNotUndefined<T>(obj: T | undefined): obj is T {
    return typeof obj !== "undefined";
}
var x = Math.random() > 0.5 ? undefined : 32;
if (isNotUndefined(x)) {
    console.log(x.toFixed());
}

[Arlen22]

const test: (string | undefined)[] = []
test.filter(<T>(a: T): a is T | undefined => { 
    return !!a; 
}).map(t => {
    // t is string | undefined
})

[RyanCavanaugh]

@Arlen22 why did you write the predicate wrong? This works in 2.4:

const test: (string | undefined)[] = []
test.filter(<T>(a: T | undefined): a is T => { 
    return !!a; 
}).map(t => {
    t;
    // t is string
})

[Arlen22]

Strange. In 2.4.1:

const test: (string | undefined)[] = []
test.filter(<T>(a: T | undefined): a is T => { 
    return !!a; 
}).map(t => {
    let r: string = t; //Error: Type 'string | undefined' is not assignable to type 'string'
})

[Arlen22]

Nevermind, I forgot to update my global tsc. Now I'm seeing the new feature. Nice :)

[Arlen22]

Any idea what PR brought this in?

[RyanCavanaugh]

11858

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.