chnage char in stringjs code example

Example 1: javascript regex replace

const search = 'duck'
const replaceWith = 'goose';

const result = 'duck duck go'.replaceAll(search, replaceWith);

result; // => 'goose goose go'

Example 2: javascript string change character at index

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