Javascrip NaN function

Javascrip NaN function

Good afternoon.
This function receive values for another one (the generation parameter).
My objective is return onde array in cloneGene variable, although it
appears like NaN and i do not know why. If anyone can help thank.
Regards
//Mutation and recombination rate
function GenerationStep (mutation_rate, recombination_rate, generation,
cloneGene) {
//var generation = [1, 2, 1, 54, 1, 1, 1];
generation = SampleWithReposition();
mutation_rate = recombination_rate = 0.1;
for(var i=0; i<pop_size; i++){
for(var j=0;j<gene_size; j++){
random_number = random();
var max_individual = Math.max.apply(Math, generation);
//mutation_rate
if(random_number < mutation_rate){
var cloneGene = 1;
cloneGene=generation[i][j]=generation[max_individual][j]+1;
//document.write(generation + ' <br />');
//return generation;
//return cloneGene;
document.write(cloneGene);
}
}
}
}