TypeScript's type 'string | string[]' is not assignable to type 'string', what is the 'string | string[]' type? How to cast them to string?

When I do TypeScript:

let token = req.headers['x-access-token'] || req.headers['authorization'] as string;

I have the following error:

Argument of type 'string | string[]' is not assignable to parameter of type 'string'

Does anyone know what string | string[] type is? I mean if I want to use logical 'or' of two strings in TypeScript. How to do it?

And how to cast string | string[] to string type?

I guess you are using node.js. In this case req.headers is of type IncomingHttpHeaders which has an index-signature of: [header: string]: string | string[] | undefined;
That means, that req.headers['whatever'] can be of type string or string[] (array of string) or undefined

• the first part of your logical-or req.headers['x-access-token'] has type string | string[] | undefined
• because of the cast expression the 2nd part req.headers['authorization'] as string is of type string
• the type of token is string | string[], because
  • when the first part is defined, it can be string | string[]
  • when the first part is undefined, or will use the 2nd part which is of type string

Hint
instead of req.headers['authorization'] you can use req.headers.authorization which is of type string | undefined.

interface IncomingHttpHeaders {
    .. 
    'authorization'?: string;
    ..
    [header: string]: string | string[] | undefined;
}

Details
Note: the answer of Adrian Brand is fine and you can use it as is. For the sake of completion I'll just show a detailed way how you could handle all cases and explain the types:

const tokenValue= req.headers['x-access-token'] || req.headers['authorization'];

tokenValue is of type string | string[] | undefined.
Note, that it can also be undefined when none of the headers exist.
We could handle this case:

if (!tokenValue) throw Error('missing header')

After this check typescript is smart enough to know that tokenValue is now of type string | string[]

if (Array.isArray(tokenValue)) {
  // In this if branch the type of `tokenValue` is `string[]`
} else {
  // In this if branch, the type of `tokenValue` is `string`
}