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    Type Alias CanAssign<A, B, Then, Else>

    CanAssign: boolean extends A
        ? boolean extends B ? Then : Else
        : A extends B ? Then : Else

    Can A assign to B

    Note that when union is involved, the assignability is measured distributively. Meaning the result can be Then | Else (i.e. boolean by default), instead of distinctive Then (true) or Else (false).

    This is the correct behavior.

    Type Parameters

    • A
    • B
    • Then = true
    • Else = false

    use Assignable<A, B> instead

    CanAssign<number | string, number> // boolean

    We are checking can A assign to B. Since A is number | string, A can assign to B when A is number(true), andAcannot assign toBwhenA is string (false). So the result is true | false = boolean.

    If you want to make sure all branches are assignable, use StrictCanAssign<A, B>.

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