"Type 'null' cannot be used as an index type" TS-CodePudding

I receive an error in Typescript code when I use my "language" const: "Type 'null' cannot be used as an index type."

const language =
      localStorage.getItem("language") !== null
        ? localStorage.getItem("language")
        : "en"; 
     
someText = someArray[language];

How can I fix it and why isn't working when I prevent language const from being null? (I know that index selector must by string/number)

CodePudding user response：

The problem here (and the reason why TypeScript cannot infer the correct type of language) is that localStorage.getItem("language") is being evaluated two times, and it could as well - for what the compiler knows - return null upon the second invocation.

Use the ?? operator to exclude the possibility of language being null.

const language = localStorage.getItem("language") ?? "en"; 
someText = someArray[language];

CodePudding user response：

You can just tweak your code a bit using Nullish Coalescing operator (??), which is basically what is share by GOTO. Just explaining it's name & working.

let someText = someArray[(localStorage.getItem("language") ?? "en")]

From MDN docs, The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.