typescript bug '[...Array, 1]["length"]' is not assignable to type

Question

https://www.typescriptlang.org/play?#code/ATCmA8AcHsCcBdjwJ6VMAogOwMbQCagA8AUCCAMpjjyhb4DOwD8sAllgOYA0Z5ASvCzVa9Ji3ZdgAXmAByOb3LAAMgEMWIuo2asOnGfMV8QAQViw1yLWOABGANoBdQ86Xk2DAGJtYmiKI6AEbQ0AA2oGrCsqwArqB8AHyGVAHaTAAGACQA3hwAZqCwwF4AvrkFRcD8pRkmwAD8wJ4+fohptnEJyiBN2HiERPzc1UIjXiMOAHQz5pbII3ZOI-lqYQygifUgAFwlNjrqLNuNmLgExMOjWOOTM1NzVovLwKvrmyd7-RekPT3DJ2Uj2QDjkES48AAFnIXB0dHZAT0mtkcoIsOUckd4LVEco9ii0RjgaDwZwoTCMVjau4-iAJrjgA4ljTaW8NoitnjgKkaOlVBp4PVkblCblprMLE97E4SXQydCnJSBTiuQShEqWLUgA

It should not have an error here. Is it a ts bug ?

  export type Encode<
    S extends string,
    Rtn extends string = '',
    Last extends string = '',
    Array extends 1[] = [],
    isFirst extends boolean = true
  > = S extends `${infer F}${infer R}`
    ? isFirst extends true
      ? Encode<R, Rtn, F, [...Array, 1], false>
      : F extends Last
      ? Encode<R, Rtn, F, [...Array, 1], false>
      : Encode<
          R,
          Array['length'] extends 1
            ? `${Rtn}${Last}`
            : `${Rtn}${Array['length']}${Last}`,
          F,
          [1],
          false
        >
    : S extends Last
    ? `${Rtn}${[...Array, 1]['length']}${Last}` 
      // error , Type '[...Array, 1]["length"]' is not assignable to type 'string | number | bigint | boolean | null | undefined'
      // but [...Array, 1]['length'] is number
    : `${Rtn}${Last}`

type a1 = Encode<'AAABCCXXXXXXYYY'> // '3AB2C6X3Y'

Rahul Sharma · Accepted Answer · 2022-08-20 04:37:57Z

1

For the given type definition:

type a1 = Encode<'AAABCCXXXXXXYYY'>

If the expected type is '3AB2C6X3Y', then the correct type definition of Encode should be:

type Encode<
    S extends string,
    Rtn extends string = '',
    Last extends string = '',
    Array extends 1[] = [],
    isFirst extends boolean = true
  > = S extends `${infer F}${infer R}`
    ? isFirst extends true
      ? Encode<R, Rtn, F, [...Array, 1], false>
      : F extends Last
      ? Encode<R, Rtn, F, [...Array, 1], false>
      : Encode<
          R,
          Array['length'] extends 1
            ? `${Rtn}${Last}`
            : `${Rtn}${Array['length']}${Last}`,
          F,
          [1],
          false
        >
    : S extends Last
    ? `${Rtn}${Last}`
    : `${Rtn}${Array['length']}${Last}`

The compilation error is because typescript is not able to infer the type of [...Array, 1]['length']. You can use another conditional typing ([...Array, 1]['length'] extends 1 ? <> : <>) to tell typescript to treat the value of [...Array, 1]['length'] as a type in the template string. See example.

edited Aug 20, 2022 at 4:37

answered Aug 20, 2022 at 4:30

Rahul Sharma

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1 Comment

yukinotech Over a year ago

thanks for sharing , I used [...Array, 1]['length'] extends number ? <> : <> to solve this

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