typescript conditional type does not work

Question

After reading this article on typescript's conditional type at: https://artsy.github.io/blog/2018/11/21/conditional-types-in-typescript/

I am trying to apply condition type to the following function:

function getIsoStringFromDatestamp<T extends number|undefined>(
  receivedAt:T
): T extends number ? string : undefined {
  if (typeof receivedAt === 'number') {
    return (new Date(receivedAt)).toISOString() // typeError [1]
  }
  return undefined // typeError [2]
}

const dateStamp:number|undefined = 1570081092848
console.log(getIsoStringFromDatestamp(dateStamp))  // 2019-10-03T05:38:12.848Z

the above code has the following type errors:

// [1] Type 'string' is not assignable to type 'T extends number ? string : undefined'
// [2] Type 'undefined' is not assignable to type 'T extends number ? string : undefined'

Suspicious at my understanding, I try out the code in the article:

function process<T extends string | null>(
  text: T
): T extends string ? string : null {
  return text && text.replace(/f/g, 'p') // error [3]
}
const maybeFoo: string | null = 'foo'
console.log(process(maybeFoo).toUpperCase()) // POO

it turns out that I am also getting similar error:

// [3] Type 'string' is not assignable to type 'T extends string ? string : null'

What am I missing ? :(

Shikhar Tandon · Accepted Answer · 2019-10-03 06:49:55Z

2

You have to typecast the returned value as any. As mentioned in the article(comments section) this is an open issue https://github.com/microsoft/TypeScript/issues/24929

function getIsoStringFromDatestamp<T extends number|undefined>(
  receivedAt:T
): T extends number ? string : undefined {
  if (typeof receivedAt === 'number') {
    return (new Date(receivedAt)).toISOString() as any
  }
  return undefined as any
}

const dateStamp:number|undefined = 1570081092848
console.log(getIsoStringFromDatestamp(dateStamp))

answered Oct 3, 2019 at 6:49

Shikhar Tandon

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