Typescripts type 'string | string[]' is not assignable to type 'string', what is the 'string | string[]' type? how to cast them to string?

string typescript logical-or
user504909 picture user504909 · May 2, 2019 · Viewed 11.9k times · Source

When I do TypeScript:

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

I have fellow error:

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

Any one know what is 'string | string[]' type? I mean if I want use logical 'or' of two string in typescript. How to do it?

And How to cast 'string | string[]' type to string type?

Answer

Adrian Brand picture Adrian Brand · May 2, 2019

Try

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

The compiler thinks req.headers['some string'] is an array of string, when you cast one side of the or operator you get a type of string or array of string. So do the or on both of them and then coerce the result to be a string.

Related questions

How to parse JSON string in Typescript

Is there a way to parse strings as JSON in Typescript. Example: In JS, we can use JSON.parse(). Is there a similar function in Typescript? I have a JSON object string as follows: {"name": "Bob", "error": false}

Read More
String.Format not work in TypeScript

String.Format does not work in TypeScript. Error: The property 'format' does not exist on value of type '{ prototype: String; fromCharCode(...codes: number[]): string; (value?: any): string; new(value?: any): String; }'. attributes["Title"] = String.format( Settings.labelKeyValuePhraseCollection["[WAIT …

Read More
Replace all instances of character in string in typescript?

I'm trying to replace all full stops in an email with an x character - for example "[email protected]" would become "myxemail@emailxcom". Email is set to a string. My problem is it's not replacing just full stops, …

Read More