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javascripttypescriptarrow-functionsreturn-type

Confusion on inferred return type

I was working with Typescript and got confused with how the compiler infers the return types differently for these two functions.

//this is of type () => boolean | ""
const isFormValid = () => {
  return jobTitle && jobTitle!=''
}

//this is of type () => boolean
const isFormInvalid = () => {
  return !jobTitle || jobTitle===''
}

Yes, jobTitle is a string, but shouldn't both of these be of return type () => boolean?

Solution 
  • isFormValid('') === '' // true

    If you want boolean as returned value use double !:

    const isFormValid = () => {
  return !!jobTitle && jobTitle!=''
}