An array in js is just an object/dictionary, with a special 'length' property, and a few convenience methods.

Further, the 'length' property can also be confusing to newcomers, since it doesn't relate to the number of elements in the array, rather the maximum key that can be converted to an integer +1.

For example,

var arr = []; arr.foo = "hello"; arr[4] = 56;

// arr.length = 5

It's much easier when you understand that arrays in javascript don't really exist as a separate data type. They are, as I say, just objects with a couple of convenience methods+length property.

Sure, x.a should stringize 'a' to make property access work cleanly, but why when using the indexing syntax?

For example:

  sgraham@gerard:~$ lua
  Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
  > a = {}
  > a[0] = "wee"
  > a[1] = "waa"
  > a["1"] = "str"
  > print(a[0])
  wee
  > print(a[1])
  waa
  > print(a["1"])
  str
  > 
  > a["x"] = "wee"
  > a.x = "waa"
  > print(a["x"])
  waa
  > print(a.x)
  waa
  > 

[] syntax is for objects and arrays. As I've said, an array is just an object... Same data type.

If you didn't have a [] syntax, you wouldn't be able to use a variable to reference properties on an object.

  var baz = "foo";
  foo.bar == foo["bar"] == foo[baz]

It makes it clear for trivial operations. Once you enter the realm of nested hashes (preferably with some arrays mixed in) the syntax turns into a royal pain in the ass.