take first letter of string javascript and put it in the back code example

Example 1: javascript get first 10 characters of string

var str = "Hello world That is reallly neat!";
var res = str.substring(0, 5);//get first 5 chars

Example 2: cut string from string javascript

var ret = "data-123".replace('data-','');
console.log(ret);   //prints: 123