Error encountered while utilizing Array.find with a specific data type

I am trying to locate the user's browser language from a list of supported languages. Here is the code snippet I am using:

const userLanguage = browserLanguages.find(language => !!supported[language]);

But unfortunately, I'm encountering this error message: 

Element implicitly has an 'any' type because type 'LanguageName' has no index signature.

The LanguageName type is specifically defined for my supported object:

type LanguageName = {
  [language in SupportedLanguages]: string;
};

And just so you know, supportedLanguages is a typescript containing all the languages that our app supports:

type SupportedLanguages = 'en' | 'es';

Check out the demo here.

typescript

Answer №1

For optimal results, it is recommended to explicitly call the overload of Array.find(), treating the callback as a user-defined type guard. Please note that manual annotation is required, as the compiler does not currently infer type guards from function return values.

There are several approaches to achieve this; here's one example:

// Define an explicit type guard for SupportedLanguages    
function supportedLanguageGuard<T extends keyof any>(
  x: T
): x is T & SupportedLanguages {
  return !!(supported as any)[x];
}

// Implement the type guard
const userLanguage: SupportedLanguages | undefined = browserLanguages.find(
  supportedLanguageGuard
); // works fine

This method was chosen as it will function regardless of whether you narrow down browserLanguages to track the literals within.

const browserLanguages = ["fr", "es"] as const; // Requires TypeScript 3.4 or higher

Hope that explanation clarifies things. Best of luck!

Link to code

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