Union types not correctly handled in array.map #30271

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1. Roaders commented on Mar 8, 2019

   

   Roaders

   on Mar 8, 2019

   Author

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   This is a workaround for the issue:

   function flattenStringArrays(values: string | (string | string[])[]): string{
   
       if (Array.isArray(values)) {
           return values.map(value => flattenStringArrays(value)).join("");
       } else {
           return values;
       }
   }
   

   link

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2. jack-williams commented on Mar 8, 2019

   

   Jack Williams (jack-williams)

   on Mar 8, 2019

   Collaborator

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   This is a design limitation because it's trying to combine two generic union signatures. See these comments

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3. Daniel Rosenwasser (DanielRosenwasser)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Mar 8, 2019
4. TypeScript Bot (typescript-bot)

   closed this as completedon Mar 11, 2019
5. typescript-bot commented on Mar 11, 2019

   

   TypeScript Bot (typescript-bot)

   on Mar 11, 2019

   Contributor

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   This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
6. gilbert commented on Mar 25, 2019

   

   Gilbert (gilbert)

   on Mar 25, 2019

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   Here's a workaround:

   /** Type workaround for https://github.com/Microsoft/TypeScript/issues/7294#issuecomment-465794460 */
   type ArrayElem<A> = A extends Array<infer Elem> ? Elem : never
   
   export function elemT<T>(array: T): Array<ArrayElem<T>> {
     return array as any
   }

   Example usage:

   type A = { x: number, y: string }
   type B = { x: number, z: string }
   type Subject = A[] | B[]
   
   function foo (items: Subject) {
     // items.map(item => item.x) // Type error
     elemT(items).map(item => item.x) // Works!
   }

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7. Robula commented on Dec 14, 2020

   

   Rob Finbow (Robula)

   on Dec 14, 2020

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   This is still an ongoing issue.
   Daniel Rosenwasser (@DanielRosenwasser) What issue does this duplicate?

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8. microsoft

   locked as resolved and limited conversation to collaborators on Oct 21, 2025