Collatz conjecture in js

Collatz.js

// Return Collatz sequence in an array.
// Linear Vs Tail Recursion in Javascript
// http://jsperf.com/collatz-conjecture-linear-vs-tail
//===================================================================================================================
function collatzTail(n, accumulator){
    if(n === 1) { 
        accumulator.push(1);
        return accumulator;
    } else if(isEven(n)) {
        accumulator.push(n); 
        return collatzTail(n/2, accumulator);
    } else {
        accumulator.push(n); 
        return collatzTail(3*n+1, accumulator);
    }
};

function collatzLinear(n){
  if(n === 1)
    return [1];
  else if(isEven(n)) 
    return [n].concat(collatzLinear2(n/2));
  else
    return [n].concat(collatzLinear2(3*n+1));
}


function isEven(NaturalNumber) {
    return (NaturalNumber%2==0) ? true : false;
};