types-kit.switch.md

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Switch type

Switch for types.

Signature:

export type Switch<
  T,
  A extends readonly // if/else if/else
  [...Cases: [Case: unknown, Result: unknown][], DefaultResult: unknown],
  Type extends EqualTag | ExtendsTag = ExtendsTag,
> = A extends readonly [...infer CaseExpressions, infer DefaultResult]
  ? If<
      IsTuple<CaseExpressions>,
      CaseExpressions extends [
        [infer CurrentCase, infer CurrentResult],
        ...infer OtherCases,
      ]
        ? IfExtends<
            [Type, EqualTag],
            If<
              IsEquals<T, CurrentCase>,
              CurrentResult,
              OtherCases extends [Case: unknown, Result: unknown][]
                ? Switch<T, [...OtherCases, DefaultResult]>
                : DefaultResult
            >,
            If<
              IsExtends<T, CurrentCase>,
              CurrentResult,
              OtherCases extends [Case: unknown, Result: unknown][]
                ? Switch<T, [...OtherCases, DefaultResult]>
                : DefaultResult
            >
          >
        : DefaultResult,
      // empty array or array
      CaseExpressions extends [infer Case, infer Result][]
        ? If<
            IsEmptyTypeArray<CaseExpressions>,
            DefaultResult,
            // will not filter empty arrays, it will return unknown
            IfExtends<
              [Type, EqualTag],
              If<IsEquals<T, Case>, Result, DefaultResult>,
              If<IsExtends<T, Case>, Result, DefaultResult>
            >
          >
        : DefaultResult
    >
  : never

References: EqualTag, ExtendsTag, If, IsTuple, IfExtends, IsEquals, Switch, IsExtends, IsEmptyTypeArray

Example

 // Expect: boolean
 type Foo = Switch<1, [[0, string], [1, boolean], [2, number], null]>
 // Expect: null
 type Bar = Switch<undefined, [[0, string], [1, boolean], [2, number], null]>