Difference between var = {} and var = []? [duplicate]

Question

Possible Duplicate:
Objects vs arrays in Javascript for key/value pairs

I have a variable in JavaScript which I am using like a hash. I can initialize it like:

var selected = [];

or

var selected = {};

and it does the same exact thing. I'm using it like this for example:

selected["one"] = 1;

if (selected["one"] == 1) console.log("one is selected"); 
// result: one is selected

if (selected["two"] != 1) console.log("two is not selected");
// result: two is not selected

selected["one"] = 0;

if (selected["one"] != 1) console.log("one is no longer selected");
// result: one is no longer selected

Is there really a difference? Is one an object and the other an array ? If so, when should I expect to run into problems. Ie., what is the difference between the two in their usage, and why would you chose one over the other?

Answer 1

[] is an array, {} is an object. An array is a type of object intended to be only assigned numeric keys.

While you can in theory use them interchangably, look what happens when you JSON-ify them:

var tmp = []; // or: var tmp = {}
tmp.one = 1;
JSON.stringify(tmp);

// array:
'[]'

// object:
'{"one":1}'

Answer 2

An Array ([]) is a kind of Object ({}). The key differences are:

• Arrays have a magic length property that is equal to the highest-set numeric key (plus one):

  var a = [];
  a[100] = 0;
  a.length; // is 101
  
  var o = {};
  o[100] = 0;
  o.length; // is undefined
  

• An Array's toString method prints out the values of numeric keys:

  var a = [];
  a[0] = 5;
  a[1] = 6;
  a[2] = 7;
  a.toString(); // "[5,6,7]"
  
  var o = {};
  o[0] = 5;
  o[1] = 6;
  o[2] = 7;
  o.toString(); // "[object Object]"
  

• Arrays have lots of specific functions for processing records:

  > Object.getOwnPropertyNames(Array.prototype)
  ["join", "toLocaleString", "sort", "some", "lastIndexOf", "splice", "map",
   "constructor", "every", "unshift", "shift", "indexOf", "pop", "forEach", "reverse",
   "reduce", "slice", "concat", "filter", "toString", "reduceRight", "push", "length"]