Types for "isPojo" and "isIterable" | TypeScript Community | Page 1

tribal umbra Mar 1, 2023, 1:45 PM

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hello,

I have two functions, isPojo returns true if value is pojo (its's constructor is "Object"), I have return type set to:

value is { [key: PropertyKey]: *}

but it's incorrect,because it gives type "true" for non Pojos, such as arrays or Sets.

Another function isIterable returns true if value is iterable (contain "Symbol.iterator") but is not string. I have return type set to value is Iterable, but this includes string.

Can someone help me, please ?

upbeat ferry Mar 1, 2023, 2:11 PM

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do you really need those types to work at compile time?

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sounds like something that should be checked at runtime only

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also, since TS is a structural type language, there is no difference between a POJO and a class instance as long as the types are concenrned

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also, Symbol.iterator means it's some kind of collection or generator
and strings are none of theose things
you can't really loop on a string anyway, you're looping over the characters making up that string, not the string itself

tribal umbra Mar 1, 2023, 2:14 PM

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yes, I need it at build time... it's for my transpiler. I need for typescript to tell me that type is pojo or someting else inside if-else when passing Pojo or Array or Set....

upbeat ferry Mar 1, 2023, 2:17 PM

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oh, so when you mean pojo, you just mean "object" or "record", as "not an array" or "not a function"

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not "object", as "not an instance"

tribal umbra Mar 1, 2023, 2:19 PM

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I mean:

isPojo({}) // true
isPojo([]) // false
isPojo(new Set()) // false
isPojo(new Date()) // false

upbeat ferry Mar 1, 2023, 2:20 PM

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but what about isPojo(new Object())?

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or

class Foo {}

isPojo(new Foo());

tribal umbra Mar 1, 2023, 2:21 PM

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it should be true...

my implementation check for proto.contructor.name === 'Object'

upbeat ferry Mar 1, 2023, 2:21 PM

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ok, so you mean "record"

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(we don't use the term POJO in JS/TS)

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or maybe even "dictionary"

harsh tundraBOT Mar 1, 2023, 2:22 PM

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Ascor8522#7606

Playground Link

Preview:ts function isObject( element: unknown ): element is Record<any, any> { return ( typeof element === "object" && element !== null && element.constructor.name === "Object" ) }

tribal umbra Mar 1, 2023, 2:24 PM

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it gives me true for [] value.... using TS 5.0.0-dev.20230226

upbeat ferry Mar 1, 2023, 2:34 PM

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right, arrays are objects technically

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with their keys being numbers instead of strings

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just ass && !(element instanceof Array)

tribal umbra Mar 1, 2023, 2:35 PM

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yes, they are...

tribal umbra Mar 1, 2023, 2:37 PM

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upbeat ferry just ass `&& !(element instanceof Array)`

this will not work ,because I would need to exclude every instances of Object for all types, like Date, Map, Set, Array, all Classes, and so on... not possible to cover

upbeat ferry Mar 1, 2023, 2:38 PM

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you are still checking for the constructor name

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so it will still exclude all those classes

tribal umbra Mar 1, 2023, 2:39 PM

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btw, I tried to exclude just array,and it throws error '}' expected. ts(1005)

upbeat ferry Mar 1, 2023, 2:40 PM

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upbeat ferry you are still checking for the constructor name

upbeat ferry Mar 1, 2023, 2:40 PM

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tribal umbra btw, I tried to exclude just array,and it throws error `'}' expected. ts(1005)`

well, you ave a syntax error, check your code

upbeat ferry Mar 1, 2023, 2:42 PM

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tribal umbra it gives me true for `[]` value.... using TS `5.0.0-dev.20230226`

([]).constructor.name // 'Array', wait, are you sure about your code?

tribal umbra Mar 1, 2023, 2:42 PM

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yes ,that works fine in runtime ,but types that VSCode shows are incorrect,and if I use:

if (isPojo(value)) {
value;
// ^ any[] | { name: string }
}

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but it should show only any[] there

upbeat ferry Mar 1, 2023, 2:43 PM

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if the function returns true, value is of type { [key: any]: any } (same as Record<any, any>)

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so check your entier code

tribal umbra Mar 1, 2023, 2:44 PM

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function is correct (implementation / runtime), I already tested it, but it's types are incorrect

upbeat ferry Mar 1, 2023, 2:44 PM

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can you share the whole code?

tribal umbra Mar 1, 2023, 2:49 PM

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/**
 * @param {*} value
 * @returns {value is {
 *     [key: PropertyKey]: *
 * }}
 */
export function isPojo(value) {
    if (value === null && typeof value !== 'object') return false;
    const proto = Object.getPrototypeOf(value);
    if (proto === null) return true;
    return proto.constructor.name === 'Object';
}

I'm writing on mobile that code manually so sorry for typos, but code is practically that.

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it's actually based on implementation from mongoose

upbeat ferry Mar 1, 2023, 2:50 PM

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that's nowhere near the code I showed

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you ned the : value is Record<any, any>

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https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

Documentation - Narrowing

Understand how TypeScript uses JavaScript knowledge to reduce the amount of type syntax in your projects.

tribal umbra Mar 1, 2023, 2:50 PM

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yes ,I tried that too ,didn't work

upbeat ferry Mar 1, 2023, 2:51 PM

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wdym, didn't work?

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show more than just your isPojo function

upbeat ferry Mar 1, 2023, 2:51 PM

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tribal umbra yes ,that works fine in runtime ,but types that VSCode shows are incorrect,and i...

like, how is it used, what is value, etc?

tribal umbra Mar 1, 2023, 2:52 PM

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I tried both value is Record<*, *> and value is { [key: PropertyKey]: * }

upbeat ferry Mar 1, 2023, 2:52 PM

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* is not a valid TS type

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use any

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as I showed

tribal umbra Mar 1, 2023, 2:53 PM

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in JSDoc * is any... but even with any it shows any[] inside if body,when I use isPojo check

upbeat ferry Mar 1, 2023, 2:53 PM

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...

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don't use JSDoc

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JSDoc are comments

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they don't do anything

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you are working with TS

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you need to specify the TS return type

tribal umbra Mar 1, 2023, 2:54 PM

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They do ,.... JSDoc is normally used for type checking... even Typescript has it in tutorials

upbeat ferry Mar 1, 2023, 2:54 PM

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like so function isPojo(value: unknown): value is Record<any, any>

sand flicker Mar 1, 2023, 2:54 PM

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JSDoc can be used in .js files to provide typings, yeah.

upbeat ferry Mar 1, 2023, 2:54 PM

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sand flicker JSDoc can be used in `.js` files to provide typings, yeah.

yeah, but that won't do with type predicates

sand flicker Mar 1, 2023, 2:54 PM

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I'm pretty sure there's a syntax for type predicates.

upbeat ferry Mar 1, 2023, 2:55 PM

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also, why bothering with JSDoc when the doc can be extracted from the types

tribal umbra Mar 1, 2023, 2:55 PM

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upbeat ferry yeah, but that won't do with type predicates

it does, when I copy implementation of Array.isArray to JS with JSDoc

sand flicker Mar 1, 2023, 2:55 PM

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(I personally do recommend using .ts files unless there's a strong reason to use .js + JSDoc, since the JSDoc syntax is a lot more annoying, but it's a tangent)

tribal umbra Mar 1, 2023, 3:01 PM

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I will try to move type to .d.ts if it works

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nope ,doesn't work in .d.ts file too

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nor ts...

upbeat ferry Mar 1, 2023, 3:04 PM

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.d.ts files are files to add typed for the exported function of your library for you consumers

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they are not made to type your internal code

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just use a TS file with proper types and everything should work

tribal umbra Mar 1, 2023, 3:05 PM

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as I said, in .ts it doesn't work either.

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I copied your code from playground and got any[] inside if body

upbeat ferry Mar 1, 2023, 3:06 PM

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well

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let's prentend value was for type any[] | "foo"

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it went through the check, so now we know it's an object, it can't be of type "foo"

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and since arrays are objects, any[] still validates our constraint

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so the final type for value after the check is any[]

harsh tundraBOT Mar 1, 2023, 3:09 PM

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Mlocik97#8200

Playground Link

Preview:```ts
function isObject(
element: any
): element is Record<any, any> {
return (
typeof element === "object" &&
element !== null &&
element.constructor.name === "Object"
)
}

const val: string[] = []

if (isObject(val)) {
val
// ^
} e
...```

sand flicker Mar 1, 2023, 3:10 PM

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What behavior were you expecting there?

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A string array is a valid Record<any, any> so that seems correct to me.

tribal umbra Mar 1, 2023, 3:10 PM

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upbeat ferry so the final type for `value` after the check is `any[]`

yes,but array doesn't have constructor.name equal to object but array,so it shouldn't pass array type to if body

sand flicker Mar 1, 2023, 3:11 PM

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I don't think there's a way to represent "constructor.name equal to object" at a type level so I'm not sure you should be trying to write a type-guard for it.

upbeat ferry Mar 1, 2023, 3:11 PM

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tribal umbra yes,but array doesn't have constructor.name equal to object but array,so it shou...

the check performed at runtime and the return type for the type predicate function are/can be independent

sand flicker Mar 1, 2023, 3:12 PM

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I think "POJO" really isn't a particularly meaningful idea in the Typescript type system.

upbeat ferry Mar 1, 2023, 3:13 PM

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upbeat ferry also, since TS is a structural type language, there is no difference between a P...

.

sand flicker Mar 1, 2023, 3:16 PM

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If you wanted to do it not as a type-guard, I guess you could:

function isObject(element: any): element is object {
    return element && typeof element === "object" || typeof element === "function";
}

function isPOJO(element: object) { // Not a type-guard because this isn't something that can be expressed in the type-system
    return element.constructor.name === "Object"
}

function example(val: unknown) {
    if(isObject(val) && isPOJO(val)) {
        // val is object here
    }
}

upbeat ferry Mar 1, 2023, 3:17 PM

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that's the first thing I asked, does OP really need narrowing / exact types?

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like, can't a simple runtime check just be good enough

upbeat ferry Mar 1, 2023, 3:18 PM

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tribal umbra yes, I need it at build time... it's for my transpiler. I need for typescript to...

so I guess there is no real way

tribal umbra Mar 1, 2023, 3:57 PM

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:/ ok, it's sad, but whatever,... I will creare Feature Request on Github.

upbeat ferry Mar 1, 2023, 4:02 PM

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tribal umbra :/ ok, it's sad, but whatever,... I will creare Feature Request on Github.

well, this is by design
one of the goals of TS is only let you access known properties on objects
no matter where the object comes from (class instance or pojo)

sand flicker Mar 1, 2023, 4:03 PM

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tribal umbra :/ ok, it's sad, but whatever,... I will creare Feature Request on Github.

Yeah, wouldn't do that.

tribal umbra Mar 1, 2023, 4:51 PM

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upbeat ferry well, this is by design one of the goals of TS is only let you access known prop...

yes, but I still think there should be some way to tell by which constructor value was made.

upbeat ferry Mar 1, 2023, 4:52 PM

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at runtime

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using .constructor

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like you did

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but TS doesn't care about the prototype of an object
as long as the final shape is the same, they are the "same" object

tribal umbra Mar 8, 2023, 10:42 AM

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ok, let's forget about isPojo it seems that's not really doable,... but can we make isIterable works the way, it returns true for predicate if value is Iterable but not string...

e.g.

value is Iterable & !string

but this doesn't work... is there some isnot operator?

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I tried value is Iterable<T extends string ? never : T> but doesn't work.

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value is Exclude<Iterable, string> doesn't work... 😦

upbeat ferry Mar 8, 2023, 12:15 PM

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but this doesn't work... is there some isnot operator?
no, there isn't

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if you are unsure, just have a look at the docs, don't try to guess by trying random things

#Types for "isPojo" and "isIterable"