Best way to turn an array-like object into an array?

Try this:

var o = { ... }; // your object
var oArr = [];
for (var i in o) {
    if (parseInt(i) == i) {
        oArr[parseInt(i)] = o[i];
    }
}

Notice that it won't accept keys that are non numeric.

If you have a proper length property, it's really easy:

var o = {"1": 1, "2": 2, "5": 5, 'length' : 6};
o = Array.prototype.slice.call(o); // [undefined, 1, 2, undefined, undefined, 5]

If you don't have the length property, you can compute it:

var o = {"1": 1, "2": 2, "5": 5};    

o.length = Object.keys(o).reduce(function(max,key){
  return isNaN(key) ? max : Math.max(max, +key);
},-1) + 1;

o = Array.prototype.slice.call(o); // [undefined, 1, 2, undefined, undefined, 5]

One thing to note, though, when you access a property of an object, it is always converted to a string, so the following will work for your example, even if o is not an array:

var o = {"1": 1, "2": 2, "5": 5};
o[1] // 1
o[2] // 2
o[5] // 5
o[0] // undefined

You should not be doing that, 1. You are not sure about how big the maximum index value can be. 2. There could be lots of gaps, those indexes in the array will be null,

So, just use a string conversion of the index number and look it up in the same object.

 var o = {"1": 1, "2": 2, "5": 5};
 var index = 5;
 o[index.toString()]  // gives 5