Zod•10mo ago

janglad - I may be missinng something here or m...

I may be missinng something here or maybe it's a TS limitation but I would expect the last line to give a TS error saying that type can never be "LISTING" (since owner.type is company). Why is this not happening?

import z from "zod"

const union = z.union([
  z.object({
    type: z.literal("LEGAL"),
    owner: z.object({
      type: z.literal("COMPANY")
    })
  }),
  z.object({
    type: z.literal("LISTING"),
    owner: z.object({
      type: z.literal("PRIVATE")
    })
  })
])

const test : z.output<typeof union> = {} as any;

// TS Error
if (test.type === "LISTING") test.owner.type === "COMPANY"

// TS Error
const test2 : z.output<typeof union> = {
  type: "LISTING",
  owner:{
    type: "COMPANY"
  }
}

// Valid
if (test.owner.type === "COMPANY") test.type === "LISTING"

import z from "zod"

const union = z.union([
  z.object({
    type: z.literal("LEGAL"),
    owner: z.object({
      type: z.literal("COMPANY")
    })
  }),
  z.object({
    type: z.literal("LISTING"),
    owner: z.object({
      type: z.literal("PRIVATE")
    })
  })
])

const test : z.output<typeof union> = {} as any;

// TS Error
if (test.type === "LISTING") test.owner.type === "COMPANY"

// TS Error
const test2 : z.output<typeof union> = {
  type: "LISTING",
  owner:{
    type: "COMPANY"
  }
}

// Valid
if (test.owner.type === "COMPANY") test.type === "LISTING"

Solution:

Yeah, this is a typescript thing: an object's discriminant needs to be at the base level of that object.

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3 Replies

Solution

Scott Trinh•10mo ago

Yeah, this is a typescript thing: an object's discriminant needs to be at the base level of that object.

jangladOP•10mo ago

I figured, thanks!

Scott Trinh•10mo ago

In other words, that first check only narrows the test.owner object, not the test object.

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janglad - I may be missinng something here or m...