apnumber.js

Number.prototype.apnumber = function() {
    var numberMap =
        [ 'zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine' ];

    return (9 >= this) ? numberMap[this] : this;
};

// Examples
var foo = 4;
alert(foo); // 4
alert(foo.apnumber()); // Four
var bar = 45;
alert(bar.apnumber()); // 45