What's the big O for JavaScript's array when used as a hash?

What's the big O for JavaScript's array access when used as a hash?

For example,

var x= [];
for(var i=0; i<100000; i++){
   x[i.toString()+'a'] = 123; // using string to illustrate x[alpha]
}
alert(x['9999a']); // linear search?

One can hope JS engines will not use a linear search internally O(n), but is this for sure?