Test for array of string type in TypeScript

Question

How can I test if a variable is an array of string in TypeScript? Something like this:

function f(): string {
    var a: string[] = [\"A\", \"B\", \"C\"];

         


        

Answer 1

I know this has been answered, but TypeScript introduced type guards: https://www.typescriptlang.org/docs/handbook/advanced-types.html#typeof-type-guards

If you have a type like: Object[] | string[] and what to do something conditionally based on what type it is - you can use this type guarding:

function isStringArray(value: any): value is string[] {
  if (value instanceof Array) {
    value.forEach(function(item) { // maybe only check first value?
      if (typeof item !== 'string') {
        return false
      }
    })
    return true
  }
  return false
}

function join(value: string[] | T[]) {
  if (isStringArray(value)) {
    return value.join(',') // value is string[] here
  } else {
    return value.map((x) => x.toString()).join(',') // value is T[] here
  }
}

There is an issue with an empty array being typed as string[], but that might be okay