CODEWITHSUNDEEP
typescript
Eliav Louski
Eliav Louski

Reputation: 5274

typescript generic function to convert any to array of any

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[];

Upvotes: 1

Views: 1440

Answers (1)

iz_
iz_

Reputation: 16623

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[]

Upvotes: 4

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