Type narrowing not working with generic zod type

[pjdon]

If we use zod to parse an API response:

function foo(subSchema: z.ZodTypeAny) {
  fetch('example/api')
    .then(res => res.json())
    .then(json => {

      const resSchema = z.union([
        z.object({
          passed: z.literal(true),
          value: subSchema
        }),
        z.object({
          passed: z.literal(false),
          msg: z.string()
        })
      ])

      const parsed = resSchema.safeParse(json);

      if (parsed.success) {
        if (parsed.data.passed) {
          console.log(parsed.data.value);
        }
      }
    })
}

We see that when we specify a conditional for when the parse is successful, the type of parsed is narrowed. We specify another conditional inside of that for when the API result passed === true, and the type is narrowed further to one where the value key is present.

Let's say we want to make foo generic so that we can potentially return the parsed API response value.

function foo<TSubSchema extends z.ZodTypeAny>(subSchema: TSubSchema) {
  fetch('example/api')
    .then(res => res.json())
    .then(json => {

      const resSchema = z.union([
        z.object({
          passed: z.literal(true),
          value: subSchema
        }),
        z.object({
          passed: z.literal(false),
          msg: z.string()
        })
      ])

      const parsed = resSchema.safeParse(json);

      if (parsed.success) {
        if (parsed.data.passed) {
          console.log(parsed.data.value);
        }
      }
    })
}

Now we have specified that subSchema is of a type TSubSchema, which is an extension of ZodTypeAny.

But we get an error on the line

console.log(parsed.data.value);

indicating that

Property 'value' does not exist on type '{ [k in keyof addQuestionMarks<baseObjectOutputType<{ passed: ZodLiteral<true>; value: TSubSchema; }>, any>]: addQuestionMarks<baseObjectOutputType<{ passed: ZodLiteral<...>; value: TSubSchema; }>, any>[k]; } | { ...; }'.
  Property 'value' does not exist on type '{ passed: false; msg: string; }'.ts(2339)

This tells that us that the type narrowing is not working, as it the type checker is complaining about value potentially not being a key of parsed.data.

[taranpreet-singh-29]

Stuck in the same issue. No solution so far