Most efficient way to write Combination and Permutation calculator in Javascript

Question

I have a math website http://finitehelp.com that teaches students Finite Math. I thought it would be cool to include a calculator so I made one for combinations and permutat

Answer 1

Math.factorial= function(n){
    var i= n;
    while(--i) n*= i;
    return n;
}

Math.combinations= function(n, r, repeats){
    if(n< r) return 0;
    if(n=== r) return 1;
    if(repeats){
        return Math.factorial(n+r-1)/((Math.factorial(r)*Math.factorial(n-1)));
    }
    return Math.factorial(n)/((Math.factorial(r)*Math.factorial(n-r)));
}


var a= [
    'aqua', 'black', 'blue', 'fuchsia', 'gray', 'green', 'lime', 'maroon',
    'navy', 'olive', 'orange', 'purple', 'red', 'silver', 'teal', 'white',
    'yellow'
]
//how many 3 color combinations are there?
//[red,green,blue] is different than [green,red,blue]
// Math.combinations(a.length,3,true) >>969
// how many unique combinations (ignoring order) are there?
// Math.combinations(a.length,3)>>680