Why is TypeScript unable to infer the type properly?

Question

I have two examples where TypeScript type inference is not working as I expect it, maybe these two are related, maybe it's a TS bug, maybe I am only too dumb to understand it.

Example #1

interface AType {
  type: 'A'
  message: string
}

interface BCType {
  type: 'B' | 'C'
}

type Test = AType | BCType

function doSomething(test: Test) {
  if (test.type !== 'B' && test.type !== 'C') {
    test.message
  }
}

The test.message gets the error

Property 'message' does not exist on type 'Test'.
  Property 'message' does not exist on type 'BCType'.ts(2339)

Why isn't test of AType now, so it has a message property? If I remove the 'C' from the type, it works. If I check for === 'A', is also works.

Example #2

interface AType {
  type: {
    id: 'A'
  }
  message: string
}

interface BType {
  type: {
    id: 'B'
  }
}

type Test = AType | BType

function doSomething(test: Test) {
  if (test.type.id === 'A') {
    test.message
  }
}

Same here:

Property 'message' does not exist on type 'Test'.
  Property 'message' does not exist on type 'BType'.ts(2339)

Discriminants must be literal types. This works typescriptlang.org/play?#code/… — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Feb 28, 2022 at 14:49

Titian Cernicova-Dragomir · Accepted Answer · 2022-02-28 15:49:52Z

1

Type guards generally work on the field of variable they are applied to, so if you narrow test.type, the narrowing generally applies to just type not to test. The one exception to this is if the type of test is a discriminated union and test is a the discriminant field.

For the property to be a discriminant property, it must fulfill some requirements:

The property is a literal type as outlined here Discriminated union types
The a property of a union type to be a discriminant property if it has a union type containing at least one unit type and no instantiable types as outlined here Allow non-unit types in union discriminants

As for the second example, there just isn't support for nested discriminated unions. There is a proposal to support this but it has been open for 3 years, without much movement.

You can adjust your model to conform to discriminated union rules, or, as a work around, you can use custom type guards:

interface AType {
  type: 'A'
  message: string
}

interface BCType {
  type: 'B' | 'C'
}

type Test = AType | BCType

function doSomething(test: Test) {
  if (!isBC(test)) {
    test.message
  }
}

function isBC(o: any): o is BCType {
  return o?.type != "B" && o?.type != "C"
}

Playground Link

answered Feb 28, 2022 at 15:49

Titian Cernicova-Dragomir

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1 Comment

Thomas Jacob Over a year ago

Thank you, I didn't know that the topic is that specific, but good to know, and your explanation is good to understand.

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