TypeError: navigator.currentLang is not a function

Ionic Framework ionic-v3
1

I have an extension.ts file that contains various extensions of predefined types of TypeScript/JavaScript, including the following class:

export {};

declare global {
    interface Navigator {
        currentLang() : string;
    }
}

Navigator.prototype.currentLang = function() {
    var userLang = navigator.language.split('-')[0];
    userLang = /(en|it)/gi.test(userLang) ? userLang : 'en';
    return userLang;
};

when I run the command ionic serve the google developer console throws the following error:

TypeError: navigator.currentLang is not a function

What could be the problem?

3

Here the solution: