how to insert a character in a string javascript code example

Example 1: javascript add string to middle of string

function addStr(str, index, stringToAdd){
  return str.substring(0, index) + stringToAdd + str.substring(index, str.length);
}

let str = "This is a string";
let stringToAdd = "modyfied ";

console.log(addStr(str, 10, stringToAdd));  //outPut : "This is a modified string"

Example 2: add char in specific index stirng javascript

int j = 123456;
String x = Integer.toString(j);
x = x.substring(0, 4) + "." + x.substring(4, x.length);

//output : 1234.56