A
arktype•4w ago
ciscoheat

Get keys of object type

What's the easiest way to get the keys of an object type, and perhaps also some metadata for it? I see there's an keyof() method, but it has plenty of methods itself so I'm not sure what to use.
14 Replies
ssalbdivad
ssalbdivad•4w ago
What kind of keys are you looking for exactly? keyof is like TS i.e. could include stuff like index signatures as well so it returns a type itself If you're just looking for the literal keys props should work well: 
import { type } from "arktype"

const user = type({
    name: "string",
    age: "number"
})

const keys = user.props.map(p => p.key)
//    ^? ("name" | "age")[]
import { type } from "arktype"

const user = type({
    name: "string",
    age: "number"
})

const keys = user.props.map(p => p.key)
//    ^? ("name" | "age")[]
You can see other info about the key on the prop object as well
ciscoheat
ciscoheatOP•4w ago
props looks very good, I have a rather simple, non-nested object type so I'm looking for how to map the property types to a few of the primitive javascript types (string, number, boolean)
ssalbdivad
ssalbdivad•4w ago
You could probably use value.expression if you know they're plain types and it should match p.value.expression in the original example
ciscoheat
ciscoheatOP•4w ago
I'll take a look at that, thanks! Just found a thing, I have a "nullNumber" validator: 
type('number').pipe((n) => (isNaN(n) ? null : n));
type('number').pipe((n) => (isNaN(n) ? null : n));
But when I look at value.expression for that, it looks like: '(In: number) => Out<unknown>' So I assume you cannot easily infer the type (at least not in the expression?) for pipe? If I use infer on the schema, I see that it's null | number at least. Thanks for helping out on a saturday ☺️
ssalbdivad
ssalbdivad•4w ago
this works for me
No description
ssalbdivad
ssalbdivad•4w ago
Also keep in mind that by default, number will reject NaN. You can change that via a global config or if you want to handle NaN explicitly, the best way to do it would probably be this: 
const nanToNull = type("number.NaN").pipe(() => null)

const nullNumber = type("number").or(nanToNull)

const out = nullNumber(5) // 5
const out2 = nullNumber(Number.NaN) // null
const nanToNull = type("number.NaN").pipe(() => null)

const nullNumber = type("number").or(nanToNull)

const out = nullNumber(5) // 5
const out2 = nullNumber(Number.NaN) // null
ciscoheat
ciscoheatOP•4w ago
It works for me as well like that, but in value.expression it's Out<unknown>
ssalbdivad
ssalbdivad•4w ago
Ahh sorry didn't know you meant at runtime I know I have a doc about this haha
ssalbdivad
ssalbdivad•4w ago
Okay maybe it's only a JSDoc haha but
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ssalbdivad
ssalbdivad•4w ago
When I said that about .expression I was thinking you meant you knew the types would be simple shapes (i.e. no transformations, subtype constraints etc.) There's ways to extract what you need easily still, but you'd just have to tell me what specifically you're looking for- a simple expression representing the type of the output?
ciscoheat
ciscoheatOP•4w ago
That would be nice, then I can determine if it's a primitive js type
ssalbdivad
ssalbdivad•4w ago
Well the easiest way to do that specifically would probably be !t.out.extends("object") But to be able to introspect the output like that, you have to make sure it is validated with a to expression. I actually just added string-embedded syntax for that in the last release haha You could define it this way instead: 
const nanToNull = type("number.NaN").pipe(() => null, type.null)

const nullNumber = type("number").or(nanToNull)

const out = nullNumber(5) // 5
const out2 = nullNumber(Number.NaN) // null
const nanToNull = type("number.NaN").pipe(() => null, type.null)

const nullNumber = type("number").or(nanToNull)

const out = nullNumber(5) // 5
const out2 = nullNumber(Number.NaN) // null
Adding the type.null as an output validator means it will be introspectable at runtime 
console.log(nullNumber.out.expression) // "number | null"
console.log(nullNumber.out.expression) // "number | null"
Or if for processing you want each branch individually you could do this: 
console.log(nullNumber.out.distribute(branch => branch.expression)) // ["number", "null"]
console.log(nullNumber.out.distribute(branch => branch.expression)) // ["number", "null"]
ciscoheat
ciscoheatOP•4w ago
That's some very cool introspection
ssalbdivad
ssalbdivad•4w ago
There is some crazy stuff you can do haha It's all composed from a few principles so anything you can do in simple cases works for very complex cases as well I think in/out extraction is an example of something that is super useful for JSON schema generation and would be very hard without a lot of the type node traversal/transform capabilities Because it works deeply as well- no matter where morphs are in an object, it will transform them to inputs/outputs

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