String Concat first and last , second and second last ... using JavaScript

gistfile1.js

function foo(str){
	var len = str.length, i = 0,j = len, arr = [];
	var i = 0,j = len-1,half = Math.floor( len/2 );	
	
	while( i<half ){
		arr.push(str[i]+str[j]);		
		i++;
		j--;
	}
	if( len%2 == 1)
		arr.push( str.substr( half,1) );
	
	return arr.join("");

}
console.log( foo("abcdef") );