Informing typescript that an argument is specifically an array when accepting both a single string and an array of strings

How can I inform TypeScript that the code is functionally valid?

It keeps suggesting it could be a string, but I am unsure how that would happen. Is this a bug in my code or am I inputting something wrong?

For example:

const i18nInstance = {
    options: {defaultNS: 'common'}
}

const getInitialProps = (req: any, namespaces?: string | string[]) => {
  if (!namespaces) {
    namespaces = i18nInstance.options.defaultNS
  }
  if (typeof namespaces === 'string') {
    namespaces = [namespaces]
  }

  const initialI18nStore = req.i18n.languages.reduce((langs, lang) => {

    // TypeScript believes namespaces might be a string, when it will always be an array.

    langs[lang] = namespaces.reduce((ns, n) => {
      ns[n] = (req.i18n.services.resourceStore.data[lang] || {})[ns] || {}
      return ns
    }, {})
    return langs
  }, {})

  return {
    i18n: req.i18n,
    initialI18nStore,
    initialLanguage: req.i18n.language,
  }
}
typescript

Answer №1

To improve readability, I recommend assigning this to a variable of type string[]:

let nss: string[];

if (typeof namespaces === 'string') {
  namespaces = [namespaces];
}
nss = namespaces;

There is a mistake on this line:

ns[n] = (req.i18n.services.resourceStore.data[lang] || {})[ns] || {}

I believe it should be corrected to:

ns[n] = (req.i18n.services.resourceStore.data[lang] || {})[n] || {}

Since ns is an object, it cannot be used as a key.

Answer №2

To ensure the compiler recognizes it as an array of strings, simply declare it as such.

const getInitialProps = (req: any, namespaces?: string | string[]) => {
  if (!namespaces) {
    namespaces = 'default';
  }
  if (typeof namespaces === 'string') {
    namespaces = [namespaces] as string[]
  }

  namespaces.concat();
}

Answer №3

Include | undefined in the namespaces parameter

Instead of changing namespaces, consider creating a new constant instead

    const nss: string[] = !ns
    ? []
    : typeof ns === 'string'
        ? [ns]
        : ns

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