Are JavaScript arrays actually linked lists?

Javascript arrays are typically implemented as hashmaps (just like Javascript objects) with one added feature: there is an attribute length, which is one higher than the highest positive integer that has been used as a key. Nothing stops you from also using strings, floating-point numbers, even negative numbers as keys. Nothing except good sense.

It most likely depends on what JavaScript engine you use.

Internet Explorer uses a mix of sparse arrays and dense arrays to make that work. Some of the more gory details are explained here: http://blogs.msdn.com/b/jscript/archive/2008/04/08/performance-optimization-of-arrays-part-ii.aspx.

The thing about dynamic languages is, well, that they're dynamic. Just like ArrayList in Java, or arrays in Perl, PHP, and Python, an Array in JavaScript will allocate a certain amount of memory and when it gets to be too big, the language automatically appends to the object. Is it as efficient as C++ or even Java? No (C++ can run circles around even the best implementations of JS), but people aren't building Quake in JS (just yet).

It is actually better to think of them as HashMaps with some specialized methods too anyway -- after all, this is valid: var a = []; a['cat']='meow';.