JSFiddle - React, Tailwind, and code Playground

JavaScript

String.prototype.sreplace = function(start, length, word) {
    return this.replace(
    new RegExp("^(.{" + start + "}).{" + length + "}"), "$1" + word);
};

var result = "This is a test string".sreplace(10, 4, "replacement");
document.write(result);