Editing What's the most efficient way to turn an object's keys to its lowercase version

This edit will create a new revision.

Your details (optional)

Name

Email (won’t be displayed; might be used for Gravatar)

URL

Test case details

Title *

Published (uncheck if you want to fiddle around before making the page public)

Description (in case you feel further explanation is needed)(Markdown syntax is allowed)

Are you a spammer? (just answer the question)

Preparation code

Preparation code HTML (this will be inserted in the <body> of a valid HTML5 document in standards mode) (useful when testing DOM operations or including libraries)

Include JavaScript libraries as follows: <script src="//cdn.ext/library.js"></script>

Define setup for all tests (variables, functions, arrays or other objects that will be used in the tests) (runs before each clocked test loop, outside of the timed code region) (e.g. define local test variables, reset global variables, clear canvas, etc.) (see FAQ)

Define teardown for all tests (runs after each clocked test loop, outside of the timed code region) (see FAQ)

Code snippets to compare

Test 1

Title

Async (check if this is an asynchronous test)

Code

var keys = Object.keys(obj),
    n = keys.length;
while (n--) {
  var key = keys[n];
  if (key !== key.toLowerCase()) {
    obj[key.toLowerCase()] = obj[key]
    delete obj[key]
  }
}

Test 2

Title

Async (check if this is an asynchronous test)

Code

// Might be cheaper to just change it, but I doubt it
var keys = Object.keys(obj),
    n = keys.length,
    key;
while (n--) {
  key = keys[n];
  obj[key.toLowerCase()] = obj[key]
  delete obj[key]
}

Test 3

Title

Async (check if this is an asynchronous test)

Code

var keys = Object.keys(obj),
    n = keys.length,
    newobj = {};

while (n--) {
  key = keys[n];
  newobj[key.toLowerCase()] = obj[key];
}

Test 4

Title

Async (check if this is an asynchronous test)

Code

var newobj = {};
for (var key in obj) {
  newobj[key.toLowerCase()] = obj[key];
}

Test 5

Title

Async (check if this is an asynchronous test)

Code

var lowerCache = {};

FN = function(obj) {
  if (typeof(obj) === "string" || typeof(obj) === "number") return obj;

  var l = obj.length;
  if (l) {
    l |= 0;
    var result = [];
    result.length = l;
    for (var i = 0; i < l; i++) {
      var newVal = obj[i];
      result[i] = typeof(newVal) === "string" ? newVal : FN(newVal);
    }
    return result;
  } else {
    var ret = {};
    for (var key in obj) {

      var keyStr = typeof(key) === "string" ? key : String(key);
      var newKey = lowerCache[keyStr];
      if (newKey === undefined) {
        newKey = keyStr.toLowerCase();
        lowerCache[keyStr] = newKey;
      }

      var newVal = obj[key];
      ret[newKey] = typeof(newVal) === "string" ? newVal : FN(newVal);
    }
    return ret;
  }
};

FN(obj);