What's the best way (most efficient) to turn all the keys of an object to lower case?


I've come up with

function keysToLowerCase (obj) {
  var keys = Object.keys(obj);
  var n = keys.length;
  while (n--) {
    var key = keys[n]; // "cache" it, for less lookups to the array
    if (key !== key.toLowerCase()) { // might already be in its lower case version
        obj[key.toLowerCase()] = obj[key] // swap the value to a new lower case key
        delete obj[key] // delete the old key
  return (obj);

But I'm not sure how will v8 behave with that, for instance, will it really delete the other keys or will it only delete references and the garbage collector will bite me later ?

Also, I created these tests, I'm hoping you could add your answer there so we could see how they match up.

EDIT 1: Apparently, according to the tests, it's faster if we don't check if the key is already in lower case, but being faster aside, will it create more clutter by ignoring this and just creating new lower case keys ? Will the garbage collector be happy with this ?

9/22/2012 12:11:48 AM

Accepted Answer

The fastest I come up with is if you create a new object:

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

I'm not familiar enough with the current inner working of v8 to give you a definitive answer. A few years ago I saw a video where the developers talked about objects, and IIRC it will only delete the references and let the garbage collector take care of it. But it was years ago so even if it was like that then, it doesn't need to be like that now.

Will it bite you later? It depends on what you are doing, but probably not. It is very common to create short lived objects so the code is optimized to handle it. But every environment has its limitations, and maybe it will bite you. You have to test with actual data.

5/10/2019 7:13:31 AM

I'd use Lo-Dash.transform like this:

var lowerObj = _.transform(obj, function (result, val, key) {
    result[key.toLowerCase()] = val;

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