Unity – JS – Strings Operations
Unity uses the .Net System.String class for strings. See the Microsoft MSDN documentation for Strings for more details.

#pragma strict
function Start () {
    var s : String = "hello";
    Debug.Log(s);                    // prints hello
    s = String.Format("{0} {1}", s, "world");
    Debug.Log(s);                    // prints hello world
    s = String.Concat("hello","world");
    Debug.Log(s);                    // prints helloworld
    s = s.ToUpper(); 
    Debug.Log(s);                    // prints HELLOWORLD
    s = s.ToLower();
    Debug.Log(s);                    // prints helloworld
    //Debug.Log(s.CharAt(1));        // CharAt not supported, produces compiler error
                                     // instead use array syntax below
    Debug.Log(s[1]);                 // prints e
    
    var c : char = 'x'[0];           // slight odd JS way of specifying a char
                                     // 'x' is a string and [0] means first character
                                     
    Debug.Log(s.IndexOf(c));         // prints -1 (s does not contain an x)
    
    var i : int = 42;
    s = i.ToString();
    Debug.Log(s);                    // prints 42
    s = "-43";
    i = int.Parse(s);
    Debug.Log(i);                    // prints -43
    
    var f : float = 3.14159265358979f;
    s = f.ToString();
    Debug.Log(s);                    // prints 3.141593 (which is an approximation)
    
    s = "-7.14159265358979";
    f = float.Parse(s);
    Debug.Log(f);                    // prints -7.141593 (which is an approximation)
}