gistfile1.js

function rehash (values) {
  var hash = {}
    , len
    , i=0;
  if (!Array.isArray(values)) return hash;
  len= values.length
  if( len % 2 == 1) return []; //mis-matched array ?
  while( i < len)  { // for(i =0 ;i < len; ) {
    hash[values[i++]]= values[i++];
  }

  return hash;
}

function rehashB (values) {
  var hash = {}
    , isKey = true
    , key;
  if (!Array.isArray(values)) return hash;
  values.forEach(function(value) {
    if (isKey) {
      key = value;
      hash[key] = null;
    } else {
      hash[key] = value;
    }
    isKey = !isKey;
  });
  return hash;
}
function rehashC (xs) {
   return xs.reduce(function (acc, x, i) {
       if (i % 2 == 0) acc[x] = xs[i+1]
       return acc;
   }, {});
}

for(j=0;j< 10000;j++ )  {
  rehashB(["a", "b", "c","d"])
  rehash(["a", "b", "c","d"])
  rehashC(["a", "b", "c","d"])
}

console.log( rehash(["a", "b", "c","d"])   ) 
console.log( rehashB(["a", "b", "c","d"])   ) 
console.log( rehashC(["a", "b", "c","d"])   ) 


var j= 0
   , then
   , now;
then= new Date();
for(j=0;j< 100000;j++ )  {
  rehash(["a", "b", "c","d"])
}
now= new Date();
console.log( "ciaranj - " + (now.getTime() - then.getTime()) + "ms" );

then= new Date();
for(j=0;j< 100000;j++ )  {
  rehashB(["a", "b", "c","d"])
}
now= new Date();
console.log( "Original - "+ (now.getTime() - then.getTime()) + "ms" );

then= new Date();
for(j=0;j< 100000;j++ )  {
  rehashC(["a", "b", "c","d"])
}
now= new Date();
console.log( "Substack - " +  (now.getTime() - then.getTime()) + "ms" );