Everything is an array index

Javascript engines never cease to amuse me.
Let’s look at our good old Array.prototype.splice

[1,2,3].splice(0,1) //returns [1]
[1,2,3].splice(1,1) //returns [2]
[1,2,3].splice(undefined,1) // 1
[1,2,3].splice(false,1) // 1
[1,2,3].splice(true,1) // 2!

Ok, so splice is accepting non-numbers and it’s casting them to booleans and then to numbers, right? Wrong.

[1,2,3].splice({},1) // 1
[1,2,3].splice("",1) // 1
[1,2,3].splice("one",1) // 1

Confused? That’s still pretty consistent!
Check this out:

[1,2,3].splice([],1) //1
[1,2,3].splice([1],1) //2
[1,2,3].splice([2],1) //3
[1,2,3].splice([1,2],1) //1

Go home javascript, you’re drunk.

Let’s figure this out anyway. #

First:

[1,2,3].splice({toString:function(){return 2;}},1) // 3
[1,2,3].splice({toString:function(){return "2one";}},1) // 1
[1,2,3].splice({toString:function(){return "2";}},1) // 3

Ok, that’s something. Looks like it casts stuff to string and expects it to be a number, then if NaN, assumes 0.

var a=[];
[1,2,3].splice(a,1)  //returns [ 1 ]
a.toString=function(){return 2;}
[1,2,3].splice(a,1)  //returns [ 3 ]

Confirmed.
But what about true ?

Well, it turns out there’s one more step:

Number(true) === 1
Number({toString:function(){return 9;}}) === 9
Number({}) //is NaN

So, finally, the closest thing to what Array.prototype.splice does to its arguments is:

~~Number(arg)

1. Cast to number
2. If input is not a primitive type, Number() will call .toString()
3. Force-cast to integer (emulated here by ~~) so all NaN results become 0

Now if that’s not a work of art, I don’t know what is. :)

All return values come from V8 as present in node v0.10.37