I am writing in typescript.I would like to write a function to fall back to the default locale when the system returns a different locale because the language (locale) supported by an application is limited.
If /*currentLocale is SupportedLocale, how should I write */?
typeSupportedLocale='ja'|'en';
export function currentLocale():SupportedLocale{
// Remove locale from the system.
const currentLocale=Localization.locale.split('-')[0];
if (/*currentLocale is SupportedLocale */) {
return currentLocale
}
return 'ja';
}
If you search by type guard, the following description appears, but I don't want to make changes to two places when the number of locale support increases.
export function currentLocale():SupportedLocale{
const currentLocale=Localization.locale.split('-')[0];
if(currentLocale=='ja'||currentLocale=='en'){
return currentLocale;
}
return 'ja';
}
I thought it was typeof, but typeofcurrentLocale is probably string, and 'SupportedLocale' only references to a type, but is being used as a value here.ts(2693) and cannot be placed on the left.
if(typeof currentLocale===SupportedLocale){
return currentLocale;
}
I found a user-defined guard and used it, but in the end, support locale information is scattered in two places.
export type SupportedLocale='ja'|'en';
constisSupportedLocale=(test:unknown): test isSupportedLocale=>{
if(test=='ja'||test=='en'){
return true;
}
return false;
};
export function currentLocale():SupportedLocale{
const currentLocale=Localization.locale.split('-')[0];
if(isSupportedLocale(currentLocale)){
return currentLocale;
}
return 'ja';
}
Well, all I'm doing is cutting out the type guard that I did in the IF statement into a function.How can I write a list of supported locale in only one place and make a decision?
javascript typescript
By generating the Union type from an array of acceptable values, this problem can be solved succinctly.This method will be available in general.
// refer https://stackoverflow.com/a/45257357
const_SupportedLocale = [
'ja',
'en'
] as const
typeSupportedLocale=typeof_SupportedLocale [number]
// refer https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates
const_SupportedLocaleValues:readonly string[]=_SupportedLocale
function isSupportedLocale(str:string):strisSupportedLocale{
return_SupportedLocaleValues.includes(str)
}
/* apply it to your code...
const DEFAULT_LOCALE: SupportedLocale='ja'
export function currentLocale():SupportedLocale{
const currentLocale=Localization.locale.split('-')[0]
if(isSupportedLocale(currentLocale)){
return currentLocale
}
return DEFAULT_LOCALE
}
*/
// For example...
console.log('ja', isSupportedLocale('ja')))
// true
console.log('en', isSupportedLocale('en')))
// true
console.log('zh', isSupportedLocale('zh')))
// false
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