how to put string in same place in javascript code example

Example 1: js insert string at position

var a = "I want apple";
var b = " an";
var position = 6;
var output = [a.slice(0, position), b, a.slice(position)].join('');
console.log(output);

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