Why does it type check?

Question

This one doesn't make sense to me:

import axios from 'axios'
import * as TE from 'fp-ts/lib/TaskEither'

export const getIntent = (sessionId: string, input: string) => process.env.INTENT_URL
  ? TE.tryCatch(
      () => axios.post(`${process.env.INTENT_URL}`,{sessionId, input}),
      reason => String(reason))
  : TE.left(Error("No INTENT_URL")
)

The Left is String and/or Error which is obviously not equal. How come this type checks???

Answer 1

Same reason why this typechecks:

export const getIntent = () => process.env.INTENT_URL
  ? true
  : "false"

You don't have a return type annotation on your function, so typescript automatically widens the return type to union type to cover the return values.