Calculate Fibonacci numbers in JavaScript

fib.js

/**
 * The Fibonacci numbers in JavaScript.
 *
 * A cached solution with O(1) lookup for previously-calculated terms and O(N)
 * lookup for uncalculated ones.
 *
 * Because numbers in JavaScript are 64bit, the largest available number is
 * 1.7976931348623157e+308. The 1476th term is the last that can be calculated.
 * If a later term is requested, the function will return Infinity.
 *
 * See: http://www.hunlock.com/blogs/The_Complete_Javascript_Number_Reference
 */
var fib = (function() {
    var sequence = [0, 1],
    
    updateTo = function(limit) {
        for (var i = sequence.length; i <= limit; i++) {
            sequence[i] = sequence[i - 1] + sequence[i - 2];
        }
    };
    
    return function(index) {
        if (index > 1476) return Infinity;
        if (!sequence[index]) updateTo(index);
        return sequence[index];
    };
})();