Random code generator (numbers and letters) of prescribed format (e.g. Postcode, NI number)

randomCode.js

var randomLetter = function() {return String.fromCharCode(Math.floor(Math.random()*26) + 65);}
var randomNumber = function() {return String.fromCharCode(Math.floor(Math.random()*10) + 48);};
var genDict = {
    'l': randomLetter,
    'n': randomNumber
};

// pass in a format string of the type: "4l 3n 2l", i.e. a space-delimited set of number/letter
// pairs, the first of which gives the number of characters of the given type and the second
// denotes the type: "n" for number, "l" for letter (always upper-case).
// As an example, "2l 3n 2l" would give a realistic, space-free UK postcode.
var randomCode = function(format) {
    var code = '';
    format.split(' ').forEach(function(token) {
        var type = token.substr(token.length - 1).toLowerCase(),
            generator = genDict[type],
            repeats = parseInt(token.substr(0, token.length - 1), 10),
            i;
        
        if (!genDict) return null;
        
        for (i = 0; i < repeats; i++) code += generator();
    });
    
    return code;
}