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

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.

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

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

Personally, I'd use:

let objectKeysToLowerCase = function (origObj) {
    return Object.keys(origObj).reduce(function (newObj, key) {
        let val = origObj[key];
        let newVal = (typeof val === 'object') ? objectKeysToLowerCase(val) : val;
        newObj[key.toLowerCase()] = newVal;
        return newObj;
    }, {});
}

It's succinct, recurs to handle nested objects and returns a new object rather than modifying the original.

In my limited local testing this function is faster than the other recursive solution currently listed (once fixed). I'd love to benchmark it against the others but jsperf is down at the moment (???).

It's also written in ES5.1 so, according to the docs on MDN, should work in FF 4+, Chrome 5+, IE 9.0+, Opera 12+, Safari 5+ (so, pretty much everything).

• Object.keys()
• Array. prototype.reduce()

Vanilla JS for the win.

I wouldn't worry too much about the garbage collection aspect of all this. Once all references to the old object are destroyed it will be GC's but the new object will still reference basically all it's properties, so they will not.

Any Functions, Arrays or RegExp will be "copied" across by reference. In terms of memory, even Strings will not be duplicated by this process since most (all?) modern JS engines user string interning. I think that leaves just the Numbers, Booleans and the Objects that formed the original structure left to be GC'd.

Note that (all implementations of) this process will lose values if the original has multiple properties with the same lowercase representation. Ie:

let myObj = { xx: 'There', xX: 'can be', Xx: 'only', XX: 'one!' };
console.log(myObj);
// { xx: 'There', xX: 'can be', Xx: 'only', XX: 'one!' }

let newObj = objectKeysToLowerCase(myObj);
console.log(newObj);
// { xx: 'one!' }

Of course, sometimes this is exactly what you want.

Update 2018-07-17

A few people have noted the original function doesn't work well with arrays. Here's an expanded, more resilient version. It recurs correctly through arrays and works if the initial value is an array or simple value:

let objectKeysToLowerCase = function (input) {
    if (typeof input !== 'object') return input;
    if (Array.isArray(input)) return input.map(objectKeysToLowerCase);
    return Object.keys(input).reduce(function (newObj, key) {
        let val = input[key];
        let newVal = (typeof val === 'object') ? objectKeysToLowerCase(val) : val;
        newObj[key.toLowerCase()] = newVal;
        return newObj;
    }, {});
};