How could I iterate on object's attributes backwards in javascript?

Question

For example I already have an object filled with ordered key-values, I can iterate its keys forwards with for in statement:

var keyvalues = {"key1":"value1","key2":"value2","key3":"value3"};
for(var key in keyvalues)
{
   var value = keyvalues[key];
   .............
}

Now I want to iterate the keys backwards like 'key3', 'key2', 'key1', is there a magic way to do this?

Answer 1

There is no guarantee of order in a for... in loop.

Wait, what?

There is no logic in 'order' of object keys. If you'd like, you can grab them and then use .sort on them. The language specification tells us this explicitly in the for...in iteration algorithm:

The mechanics and order of enumerating the properties (step 6.a in the first algorithm, step 7.a in the second) is not specified.

What are your options:

In JavaScript, most of the time ordered lists are represented by arrays. You can us an array if like an ordered list in your case:

var values= ["key1","key2","key3"];
for(var i=0;i<values.length;i++){
   values[i];// order is guaranteed here.
}

Since order is guaranteed you can also easily reverse it:

var values= ["key1","key2","key3"].reverse();//reverse is an array method
for(var i=0;i<values.length;i++){
   values[i];// order is guaranteed here.
}

If you already have an object you can grab its keys and .sort them based on some criterion, for example - in a lexicographical order :

var obj = {"key1":"value1","key2":"value2","key3":"value3"};
var keys = Object.keys(obj).sort().reverse(); // see note
for(var i=0;i<keys.length;i++){
    console.log(keys[i]);
}
// will output key3, key2, key1

_{note: you can pass a comparator to .sort and have it reverse it for you instead of chaining - which is probably better practice. I kept it this way for pedagogical purposes.}

Answer 2

You could use var keys = Objects.keys(keyvalues).sort() to get all the keys of the object then just iterate through it backwords in a for(int = keys.length;i>0;i--) loop accessing the original object like so value = object[keys[i]] in the loop

EDIT: Added the sort function call to the array of keys returned by Object.keys.