对象可能是“未定义”的

Question

So I am in vscode on typescript 2.5.3 and the following code:

export function isHttpHeaders ( headers: HttpHeaders | undefined ):
headers is HttpHeaders {
    return !!headers && !!headers.get && !!headers.set
}

function headersToHeadersSpec ( options?: RequestOptionsArgs ):
 Headers | undefined {

  const { headers } = options as RequestOptionsArgs

  if ( isHttpHeaders( headers ) ) {
    const reqHeaders = new Headers()
    headers.forEach( reqHeaders.set.bind( reqHeaders ) )
    return reqHeaders
  } else {
    return undefined
  }
}

When I attempt to compile, for some reason the line headers.forEach( reqHeaders.set.bind( reqHeaders ) ) is complaining that the headers value is possibly undefined. Shouldn't the type guard guarantee that the value is not undefined?

Answer 1

The error comes from your definition of HttpHeaders which sets forEach as an optional member.

To fix the error, just label-it non-optional by removing the ? :

interface HttpHeaders {
  get?: string
  set?: string
  forEach (Function): void
}

Answer 2

Considering you have option? which means the parameter is optional , it would allow options to be undefined. Even if you have code in other functions that check the value, typescript isn't smart enough to look at that code and figure out what it does. Your code is not Compile-Time safe, but may be Run-Time safe.