var obj = { 'a' : 'apple', 'b' : 'banana', 'c' : 'carrot' }

for (var key in obj) {
  console.log( key + ' has a value ' + obj[key] );
}

don't use for (key in obj), it will iterate over all enumerable properties including prototype properties, and can lead to amazingly horrible things. Modern JS has a special function for getting only the relevant keys out of an object, using Object.keys(...), so if you use var keys = Object.keys(obj) to get the list of keys as an array, you can then iterate over that:

// blind iteration
Object.keys(obj).forEach(function(key, i) {
  var value = obj[key];
  // do what you need to here, with index i as position information.
  // Note that you cannot break out of this iteration, although you
  // can of course use ".some()" rather than ".forEach()" for that.
});

// indexed iteration
for(var keys = Object.keys(obj), i = 0, end = keys.length; i < end; i++) {
  var key = keys[i], value = obj[key];
  // do what you need to here, with index i as position information,
  // using "break" if you need to cut the iteration short.
});

or select its last element immediately

var keys = Object.keys(obj);
var last = keys[keys.length-1];

or using a slice:

var keys = Object.keys(obj);
var last = keys.slice(-1)[0];

or using a shift (but that's a destructive operation, so we're not caching the keys because the shift turns it into "not all the keys anymore"):

var last = Object.keys(obj).shift();

2021 edit

There is now also the Object.entries function, which gets you key/value pairs in one go:

Object.entries(obj).forEach(([key, value]) => {
  console.log(`key "${key}" points to:`, value):
});

You could loop through all of them and save the last one in a variable.

var lastItem = null;
for(var key in obj) {
  console.log( key + ' has a value ' + obj[key] );
  lastItem = key;
}
// now the last iteration's key is in lastItem
console.log('the last key ' + lastItem + ' has a value ' + obj[lastItem]);

Also, because of how JavaScript works the key is also in your loop's key variable, so the extra variable is not even needed.

for(var key in obj) {
  console.log( key + ' has a value ' + obj[key] );
}
// now the last iteration's key is in key
console.log('the last key ' + key + ' has a value ' + obj[key]);

just shorter

var last = (last=Object.keys(json))[last.length-1];

You could put the logic for the last item outside the loop:

var last_item = null;
for (var key in obj) {
  last_item = key;
}
console.log(last_item);

You could push all of the keys/values into a a empty array variable that you created. Then, access the last element in the array using array.length-1.

for(var x=0 ; x<Object.keys(obj).length ; x++)
{
     if(x==Object.keys(obj).length-1) // code for the last iteration

}

Or could use Object.size(obj)