How I noticed it

I was writing a unit test suite leveraging the test.each table approach of jest.
We have a string literal union type like this:

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export type UnrefRefundReason =
  | 'ZERO_DOLLAR_REFUNDABLE'
  | 'MULTIPLE_PAYMENTS'
  | 'BLIND_REFUND'
  | 'DIFFERENT_PAYMENT_TYPES'

And the test table layout looks like this:

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test.each([
  {
    someIds: undefined,
    someBoolean: true,
    reason: 'DIFFERENT_PAYMENT_TYPES',
  },
  {
    someIds: ['1', '2'],
    someBoolean: false,
    reason: 'MULTIPLE_PAYMENTS',
  },
  {
    someIds: ['1', '2'],
    someBoolean: true,
    reason: 'MULTIPLE_PAYMENTS',
  },
])(
  'goes to the state of unreferencedPrompt with the correct reason $reason if unreferenced refund is needed',
  async ({
    someIds,
    someBoolean,
    reason,
  }: {
    someIds: string[]
    someBoolean: boolean
    reason: UnrefRefundReason,
  }){
     //... test script
     })

The error I got was:

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Types of property 'reason' are incompatible.
         Type 'string' is not assignable to type 'UnrefRefundReason'.

If I go with the other mapping approach (array -> argument list):

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test.each([
  [undefined, false, 'BLIND_REFUND'],
  [[], false, 'BLIND_REFUND'],
  [undefined, true, 'DIFFERENT_PAYMENT_TYPES'],
  [[], true, 'DIFFERENT_PAYMENT_TYPES'],
  [['1', '2'], false, 'MULTIPLE_PAYMENTS'],
  [['1', '2'], true, 'MULTIPLE_PAYMENTS'],
])(
  'goes to the state of unreferencedPrompt with the correct reason $reason if unreferenced refund is needed',
  async (
    someIds: string[] | undefined,
    someBoolean: boolean,
    reason: UnrefRefundReason
  ) => {
    //...test script
  }
)

I got similar error:

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Type '[undefined, boolean, string]' is not assignable to type '[originalRegisterSalePaymentIds: string[] | undefined, originalSalePaidByOtherPT: boolean, reason: UnrefRefundReason]'.

Findings

From string to UnrefRefundReason, TypeScript needs to do a “type narrowing”. But the type narrowing will not propagate into object. See explanation here:

https://github.com/microsoft/TypeScript/issues/31755#issuecomment-498669080

Preference

I am increasingly preferring enum instead of literal union:

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export enum UnrefRefundReason {
  ZERO_DOLLAR_REFUNDABLE = 'ZERO_DOLLAR_REFUNDABLE',
  MULTIPLE_PAYMENTS = 'MULTIPLE_PAYMENTS',
  BLIND_REFUND = 'BLIND_REFUND',
  DIFFERENT_PAYMENT_TYPES = 'DIFFERENT_PAYMENT_TYPES',
}