bower install --save approximate

// Of course Math.pow is easily reversible, just as an example.
var power2 = function(input) {
    return Math.pow(2, input);
};

var requiredInput = approximate(power2, 16, 0, 10);
                                            ^--^------ search range of possible input values (min, max)
                                         ^------------ expected calculation output
                                   ^---- calculation that you cannot reverse easily, 
                                         but want to find input value by given output
                                         
console.log(requiredInput);
// prints 4

// prints false
console.log(approximate(power2, 9, 0, 10));

// prints 3 (2^3 = 8 - closest to 9)
console.log(approximate(power2, 9, 0, 10, true));

bin/test.sh

bin/dist/dist.sh