A string in Javascript is already a character array.
You can simply access any character in the array as you would any other array.
var s = "overpopulation";
alert(s[0]) // alerts o.
UPDATE
As is pointed out in the comments below, the above method for accessing a character in a string is part of ECMAScript 5 which certain browsers may not conform to.
An alternative method you can use is charAt(index)
.
var s = "overpopulation";
alert(s.charAt(0)) // alerts o.