typescript generic function to convert any to array of any

Question

I want a write a simple utility function that takes any argument and if the arg is not already array return array with a single element, that arg.

examples:

toArray('hey') => ['hey']
toArray(['hey']) => ['hey']

implementation:

export type ToArr<T> = T extends any[] ? T : T[];
export const toArray = <T>(arg: T): ToArr<T> => {
  return (Array.isArray(arg) ? arg : [arg]) as ToArr<T>;
};

that's works fine, as long as the arg passed to toArray is not of type 'any':

type testType1 = ToArr<string>; // string[]    - ok
type testType2 = ToArr<string[]>; // string[]  - ok
type testType3 = ToArr<any[]>; // any[]        - ok
type testType4 = ToArr<any>; // any            - not ok! should be any[]

let myVar:any = 'test'
let myVarArr = toArray(myVar) //               - type of myVarArr is any instead of any[]

what's wrong with my implementation of toArr generic? why does any extends any[] (the condition in the generic function)? how can I modify this function to handle cases where arg is of type any?

for clarification: I want toArray will always return a value with type of array.

export type ToArr<T> = T extends any[] ? T : T[];

In ToArr<any> how would you know that any is not an array? — Nadia Chibrikova
– Nadia Chibrikova, Commented Aug 15, 2021 at 17:26
I don't know that. but i want to make sure the toArray will always return an array — Eliav Louski
– Eliav Louski, Commented Aug 15, 2021 at 17:32
What you're trying to do is neither possible nor really needed, as you can index myVarArr and pass it into functions that require arrays. Have you encountered any actual problem with your code? — Nadia Chibrikova
– Nadia Chibrikova, Commented Aug 15, 2021 at 17:34
well yes. (userProp) => toArray(userProp).map(...), i dont get suggestions on the IDE on what returns from toArray (like map) because the IDE can't say for sure that is an array — Eliav Louski
– Eliav Louski, Commented Aug 15, 2021 at 17:45

iz_ · Accepted Answer · 2021-08-15 18:45:29Z

4

I think you may be overcomplicating things. A simple union type should work.

const toArray = <T>(arg: T | T[]): T[] => {
  return Array.isArray(arg) ? arg : [arg];
};

let myVar: any = 'test'
let myVarArr = toArray(myVar) // type is any[]

answered Aug 15, 2021 at 18:45

iz_

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2 Comments

Eliav Louski Over a year ago

yup... guess I overcomplicated it

Eliav Louski Over a year ago

actually, i don't understand what it means <T>(arg: T | T[]), what does arg: T | T[] means? what if arg is of type literal? what does a union between a literal and array of literals means? what if arg is an array? how does an array of arrays resolve to a simple array? a further explanation on this would be appreciated.

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