Multiple return type(Array<string> | Boolean) for arrow function

I have an arrow function in a class that returns either array or boolean:

class A{

   myfunction=(val:number):Array<string>|Boolean=>{
     return number>1000?true:["jack","robin","harry"];
   }
}

and I want to call the function as:

const myclass=new A();

const arr=myclass.myfunction(10);

if(typeof arr==="boolean"){
  console.log("dami");
}else{
   if(arr.indexOf("harry")>0){
          ##here at indexOf i get an error :Property 'indexOf' does not exist on type 'Boolean | string[]'.
     console.log("we found harry");
   }
}

Calling indexOf gives an error saying : Property 'indexOf' does not exist on type 'Boolean | string[]'. What am I doing wrong here?

>Solution :

Your return type should refer to the primitive boolean type:

Array<string> | boolean // or string[] | boolean

And not the object Boolean type:

Array<string> | Boolean

As the TypeScript handbook notes:

The type names String, Number, and Boolean (starting with capital
letters) are legal, but refer to some special built-in types that will
very rarely appear in your code. Always use string, number, or boolean
for types.