Function to find Prime Numbers upto n (in Javascript)

function getPrime(n){
var primes=[];
var c;

if(n>1){
primes[0]=2;
if(n>2){
primes[1]=3;
c=2;
for(var i=5;i<=n;i+=2){
var notprime=false;
for(var j=1;j<primes.length;j++){
if(i<primes[j]*primes[j]) break;
if(i%primes[j]==0){
notprime=true;
break;
}

}
if(!notprime){
primes[c]=i;
c++;
}
}
}
}

return primes;
}

Function to find duplicate values in an array (Javascript)

function findDuplicate(arr){
var arr1=[];   //for processing array
var arr2=[]; // for storing result
var c1; // for arr1
var c2=0; // for arr2
var dupli;
var length=arr.length;

while(length>0){
c1=0;
dupli=false;
arr1=[];
for(var j=1;j<length;j++){
if(arr[0]!=arr[j]){
arr1[c1]=arr[j];
c1++;
}
else{
if(!dupli){
arr2[c2]=arr[j];
c2++;
dupli=true;
}
}
}
arr=arr1;
length=arr.length;
}
return arr2;
}

alert('Result is : ' + findDuplicate([1,2,3,4,5]));