Mu

multiplicative_persistence.js

// http://mathworld.wolfram.com/MultiplicativePersistence.html
var prod = function(num){
    return num.toString().split('').reduce(function(prev, cur){
        return prev*cur;
    })
}

var mp = function(num) {
    if (num < 10) { return 0 }
    else          { return 1 + mp(prod(num)) }
}

mp(39);