How to access name of named tuple in Typescript

Question

I want to access the name from a named tuple to use within one of my functions. Is there a way to access the name only, similar to how you can access the type only by using the index of the tuple element.

type NamedThing = [emailId: string, anotherField: number]

// like you access the type here
type TypeOfFirst = NamedThing['0']

// how can I access the name here
const NameOfFirst = NamedThing['0']['__name__'] // Error on the __name__ accessor

I understand that this can be done in python using the way I described above. Here is an answer that shows that. How can I do this in Typescript?

you cannot do that in TS. `NamedThing['0']['__name__']` means there is an Array of objects `NamedThing` where get the first item `[0]` and get the value of the property `__name__` that exists in the first object. And that is JS, not TS. TS here does not works as python/java or any other language, because the named tuple is only a representation for you (the developer). It's NOT part of the actual JS runtime — boop_the_snoot, Oct 21 '22 at 05:02
Moreover `[emailId: string, anotherField: number]` states the first item of the array should be string and the second to be number. And thats it. The context dies there itself. So there is no magic `__name__` or equivalent that exists, which would get you what you want — boop_the_snoot, Oct 21 '22 at 05:06

CC-white · Accepted Answer · 2022-10-21T05:33:10.080

Typescript exists to generate Javascript runtime after all. For this reason, some things are not possible if Javascript itself does not support it.

If you were to compile following code:

type NamedThing = [
    emailId: string, anotherField: number
]

type NamedThing2 = {
    emailId: string; anotherField: number;
}

export const nTuple:NamedThing = ["email", 123]
export const nStruct:NamedThing2 = { emailId : "email", anotherField : 123 }

It becomes:

exports.nTuple = ["email", 123];
exports.nStruct = { emailId: "email", anotherField: 123 };

Notice there is no way of knowing emailId and anotherField from nTuple. Because Javascript has no named tuple in the first place. No actual data type is translated into js.

If you were to implement named tuple similar to that from Python, you have to do it manually, such as:

function toTuple(obj:{[name:string]:any}) {
    return Object
        .keys(obj)
        .map(key=>obj[key])
}

function getByIndex(obj:{[name:string]:any}, index:number) {
    const key = Object
        .keys(obj)[index]

    return obj[key]
}

const nStruct = { emailId : "email", anotherField : 123 } // not actually named tuple.

const email = nStruct["emailId"] // as if it's named tuple.
const aField = getByIndex(nStruct, 1) // as if it's named tuple.
const actualTuple = toTuple(nStruct) // ["email",  123]

How to access name of named tuple in Typescript

1 Answers1