Alternative to is as predicate | TypeScript Community | Page 1

acoustic falcon Oct 30, 2023, 8:11 PM

#

If I want to check if a given function parameter fits one type, using a type guard function is fine.

What if the param fits more than one type like in this simplified example:

type MyType = {
  timestamp: number;
  value: number;
}

function isMyType(param):param is MyType(){
  return (
    param?.timestamp !== undefined &&
    param?.value !== undefined &&
    typeof param.timestamp === 'number' &&
    typeof param.value === 'number'
  )
}

const param:any = {
  timestamp:0, 
  value:0, 
  last:{
    timestamp:1,
    value:2
  }
};

if(isMyType(param) && isMyType(param.last))

Here, my IDE will say the second isMyType as a never because it assumes param only fits MyType . Is there a way to replace the is in the predicate to mean it fits the type but doesnt mean its exclusively this type? I know I could make it is MyType & {[key: string]: any;]} but I'm not sure I like this eighter.

glass spire Oct 30, 2023, 8:20 PM

#

@acoustic falcon Well, in this case if you drop the any annotation it works:

vivid snowBOT Oct 30, 2023, 8:20 PM

#

retsam19#0

Playground Link

Preview:```ts
type MyType = {
timestamp: number
value: number
}

function isMyType(param: any): param is MyType {
return (
param?.timestamp !== undefined &&
param?.value !== undefined &&
typeof param.timestamp === "number" &&
typeof param.value === "number"
...```

glass spire Oct 30, 2023, 8:21 PM

#

Though a custom type-guard doesn't actually do much if you have a hard-coded object already - so I'm not sure what your actual code looks like.

#

Probably more realistically, you can add a "last" in param check before access param.last

acoustic falcon Oct 30, 2023, 8:43 PM

#

glass spire <@458350749672538113> Well, in this case if you drop the `any` annotation it wor...

I know it works without the any anotation, its an example for the sake of the example, the param isn't hard coded and could be undefined or missing the last at any time I check if it isMyType for both

#

actual code looks like this:

type StampedPosition = {
    timestamp: number;
    position: {
        x: number;
        y: number;
    };
};

function isStampedPosition(
    data: any,
): data is StampedPosition & {
    [key: string]: any;
} {
    return (
        data?.timestamp !== undefined &&
        data?.position.x !== undefined &&
        data?.position.y !== undefined &&
        typeof data.timestamp === 'number' &&
        typeof data.position.x === 'number' &&
        typeof data.position.y === 'number'
    );
}
class Dragable {
        ...
    private mouseData: {
        mouseMove?: StampedPosition & {
            last?: StampedPosition;
        };
        mouseDown?: StampedPosition;
    };
...
}

#

and the mouseData gets changed dynamically in different event handlers to eighter be the full mouseData or a variation allowed in the class definition

woeful glade Oct 30, 2023, 8:46 PM

#

In the first example instead of saying param is any, you could use satisfies

acoustic falcon Oct 30, 2023, 8:46 PM

#

but once I check the mouseData.mouseMove, I can't check the mouseData.mouseMove.last

acoustic falcon Oct 30, 2023, 8:47 PM

#

woeful glade In the first example instead of saying param is any, you could use satisfies

I need the type guard function to say it satisfies the type...

#

but it's not allowed

woeful glade Oct 30, 2023, 8:49 PM

#

Is this not allowed?

const param = {
  // ...
} satisfies MyType

acoustic falcon Oct 30, 2023, 8:54 PM

#

woeful glade Is this not allowed? ```ts const param = { // ... } satisfies MyType ```

This example was just to illustrate without putting the whole 350 lines I'm working on. So If I'm asking how to make a type guard function more open, I'm not asking how to make a type inference.

glass spire Oct 30, 2023, 8:56 PM

#

FWIW, a type-guard function doesn't prevent accessing other properties.

#

A type-guard's result is combined with the existing type, it doesn't completely override it.

vivid snowBOT Oct 30, 2023, 8:57 PM

#

retsam19#0

Playground Link

Preview:```ts
declare function hasAProp(
obj: unknown
): obj is {a: string}

declare const obj: {b: string}

if (hasAProp(obj)) {
console.log(obj.a) // okay
console.log(obj.b) // okay
}```

acoustic falcon Oct 30, 2023, 9:00 PM

#

why do I get Property 'last' does not exist on type 'StampedPosition'. then?

#

if (
!isStampedPosition(mouseMoveData) ||
!isStampedPosition(mouseMoveData.last)
)

glass spire Oct 30, 2023, 9:01 PM

#

What type is mouseMoveData before you narrow it?

acoustic falcon Oct 30, 2023, 9:01 PM

#

it's any before this if statement

glass spire Oct 30, 2023, 9:02 PM

#

Yeah, I guess specifically any & SpecificType 'reduces' to SpecificType.

#

... but I don't know if I'd call that an issue with your type-guard.

#

If the question boils down to "how can I type-guard a type, while still treating it as any for other properties", you kinda can't?

acoustic falcon Oct 30, 2023, 9:05 PM

#

So all I can do is eighter split mouseMoveData.last into its own variable beforehand and treat separately or have

is MyType & {
[key: string]: any;
}

Bummer that there isn't a satisfies equivalent

glass spire Oct 30, 2023, 9:05 PM

#

Or add a check to convince TS that the property exists.

#

declare function hasAProp(obj: unknown): obj is { a: string };

declare const obj: unknown;

if(hasAProp(obj)) {
  console.log(obj.a); // okay
  if("b" in obj && hasAProp(obj.b)) {
    console.log(obj.b.a); // okay
  }
}

vivid snowBOT Oct 30, 2023, 9:09 PM

#

samuelcharpentier#0

Playground Link

Preview:```ts
declare function hasAProp(
obj: unknown
): obj is {a: string}

declare const obj: unknown

function main() {
if (hasAProp(obj)) {
console.log(obj.a) // okay
if (
!hasAProp(obj) ||
("b" in obj && !hasAProp(obj.b))
)
return
console.log(obj.a)
console.log(obj.b
...```

acoustic falcon Oct 30, 2023, 9:10 PM

#

Property 'b' does not exist on type '{ a: string; }'.
Property 'b' does not exist on type '{ a: string; }'.

#

this is closer to what my situation is

#

Sorry, rather this

vivid snowBOT Oct 30, 2023, 9:11 PM

#

samuelcharpentier#0

Playground Link

Preview:```ts
declare function hasAProp(
obj: unknown
): obj is {a: string}

declare const obj: unknown

function main() {
if (
!hasAProp(obj) ||
("b" in obj && !hasAProp(obj.b))
)
return
console.log(obj.a)
console.log(obj.b)
console.log(obj.b.a)
}```

glass spire Oct 30, 2023, 9:13 PM

#

I think your conditions aren't right there.

#

if(!(hasAProp(obj) && "b" in obj && hasAProp(obj.b))) return

if(!hasAProp(obj) || !("b" in obj && hasAProp(obj.b))) return

these versions both work correctly

#

(Your version is only returning if b is in the object, whereas you actually want to early-return if it isn't)

acoustic falcon Oct 30, 2023, 9:19 PM

#

thank you, this should work!

#Alternative to is as predicate