replase one letter at index in word at js code example

Example 1: javascript string change character at index

String.prototype.replaceAt=function(index, char) {
    var a = this.split("");
    a[index] = char;
    return a.join("");
}

Example 2: string one char change in typescript

let hello:string = "Hello World";
console.log(hello.replaceAt(2, "!!")); // Should display He!!o World