Receive 2394 in typescript

Why am I getting the error?

I tried various methods, but I did not get the result and this error was not fixed

my code:

function $sum(xor:string[]):string
function $sum(xor:number[]):number
{
    let xor_;
    if(xor instanceof String){
        for(let i of xor){
            xor_+=i;
        }
    }else{
        for(let i of xor){
            xor_+=i;
        }
    }
    return xor_;
}

my error:

>Solution :

As the error mentions, you must provide a valid implementation signature:
Info from the typescript docs

Stackblitz example:

function $sum(xor:string[]):string;
function $sum(xor:number[]):number;
function $sum(xor:Array<string | number>):number | string
{
  /**
   * as per your overloads the function must return 
   * - a string when the input is a string array
   * - a number when the input is a number array
   * 
   * To determine if the input is an array of string or an array of number
   * you could check the type of the first array item
   * i.e. typeof xor[0] === 'string'
   * 
   * as T.J.Crowder pointed out in a comment, 
   * this is not possible when you get an empty array: so your 
   * function must not allow empty arrays: i.e. throw an error
   */
}

the first 2 lines are the overloads, the 3rd is the implementation signature.