如何用数字键对对象进行降序排序？

Question

I'm having an issue where the code I have written seems to do two very odd things. when running line by line the object is constructed in the object in the correct war but as soon as it's out of the for loop the object that's been created seems to default back to the keys being in ascending order?

The object I sent into the function looks as follows:

{
   0: 'Hello',
   20: 'World',
   40: 'Test'
}

const reverseObject = (object) => {
  const newObject = {};
  Object.keys(object).reverse().forEach((key) => {
    newObject[key] = object[key];
  });
  return newObject;
};

what I was expecting to happen here was an object that looked like:

{
   40: 'Test'
   20: 'World',
   0: 'Hello',
}

the newObject looks like this during the loop

{
   40: 'Test'
   20: 'World',
   0: 'Hello',
}

but on the return line looks like

{
   0: 'Hello',
   20: 'World',
   40: 'Test'
}

Im unsure if something deep down in javascript is restricting this sort of use case but an API I'm trying to use slightly differently to the way its setup takes an object with keys that look like this for the order.

Any help would be much appreciated if you can think of a way to sort this as intended. Look forward to the responses.

Answer 1

I dont belive key ordering is deterministic. Chrome, at least, always seems to order them sequentially:

Object.keys({20: 'Hello', 30: 'There', 0: 'World'});
>> ["0", "20", "30"]

Answer 2

You can work around the auto-sorting of object keys by making the keys strings. Technically @Dacre Denny is correct that the ordering of keys in JavaScript objects is not guaranteed BUT afaik do all browser keep the ordering of keys as specified when the Object was created EXCEPT they are numeric!

 const obj = { 0: 'Hello', 20: 'World', 40: 'Test' }; const objStrKeys = { '_0': 'Hello', '_20': 'World', '_40': 'Test' }; const reverseObject = (object) => { const newObject = {}; Object.keys(object).reverse().forEach((key) => { newObject[key] = object[key]; }); return newObject; }; console.log('obj', reverseObject(obj)); console.log('objStrKeys', reverseObject(objStrKeys)); 

not that it's not enough to "just" wrap the keys in quotes and leave them numeric, they require some kind of string prefix/suffix. And yes, I'm aware that this might not work depending on your use-case and that it is rather ugly, but if you really need the properties in reverse order, this is your option =)...