You cannot test for string[] in the general case but you can test for Array quite easily the same as in JavaScript https://stackoverflow.com/a/767492/390330 (I prefer Array.isArray(value)).

If you specifically want for string array you can do something like:

if (Array.isArray(value)) {
   var somethingIsNotString = false;
   value.forEach(function(item){
      if(typeof item !== 'string'){
         somethingIsNotString = true;
      }
   })
   if(!somethingIsNotString && value.length > 0){
      console.log('string[]!');
   }
}

In case you need to check for an array of a class (not a basic type)

if(items && (items.length > 0) && (items[0] instanceof MyClassName))

If you are not sure that all items are same type

items.every(it => it instanceof MyClassName)

There is a comment made by the user @jcalz that explains how it works. It refers to the following code, which is similar to the OP code:

const fruit = ["apple", "banana", "grape"] as const;
export type Fruit = (typeof fruit)[number]; 'apple'|'banana'|'grape';

typeof fruit is Array<"apple" | "banana" | "grape">, so Fruit is equivalent to (Array<"apple" | "banana" | "grape">)[number]. The syntax T[K] means: the type of the properties of T whose keys are of type K. So (Array<"apple" | "banana" | "grape">)[number] means "the types of the properties of Array<"apple" | "banana" | "grape"> whose keys are of type number", or: "the array elements of Array<"apple" | "banana" | "grape">, or: "apple" | "banana" | "grape".

And another similar comment by that user that adds a bit more technical terms:

The type T[K] is a lookup type which gets the type of the property of T whose key is K. In (typeof list)[number], you're getting the types of the properties of (typeof list) whose keys are number. Arrays like typeof list have numeric index signatures, so their number key yields the union of all numerically-indexed properties.

You can use this format to get the member type:

type InputType = typeof input[number];

You can read more about it (and some other cool tips) here in Steve Holgado's blog article.