Javascript's arrays cannot be used as maps or associative arrays. Arrays require numeric, consecutive keys. Otherwise it's an Object.

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Clarification edit: Creating a new key on an array using arr['key']=1 creates a property on the arr Object but does not add an element to the standard array.

http://stackoverflow.com/questions/8630471/strings-as-keys-o...

Arrays are always objects in JavaScript, and they can be extended like any other object or used as maps (barring key conflicts).

Adding non-numeric keys does not remove an Array's "arrayness" in JavaScript

  var x = [];
  console.log(Object.prototype.toString.call(x));
  //[object Array]
  
  x[0] = 1;
  console.log(x[0]);
  //1
  
  x["test"] = 2;
  console.log(x["test"]);
  //2
  
  console.log(Object.prototype.toString.call(x));
  //[object Array]
  
  x.map
  //function map() { [native code] }

  var y = {};
  
  console.log(Object.prototype.toString.call(y));
  //[object Object]
  
  y.map
  //undefined

His point was:

    x = [1]; x["test"] = 555; x.map(function (y) { return y; });
    // [1]

That's [1], not [1, 555]. You can access the array from the map interface, but not the other way around.

The array prototype functions are only specified by the standard to deal solely with numeric keys, but the lines between and object and array are still pretty blurry. The use case for each is different, but the original question far above was "array/map combo. Have you ever seen anything like that in another language?", and clearly JavaScript is very similar even if not exact.

  var x = {};
  x[0] = 1;
  x.length = 1;
  x.map = Array.prototype.map;
  x.map(function(y) {return y;});
  //[1]

It does internally. Pretty sure that's even defined in the standard.

No, it doesn't. The only magic per spec is about the "length" property (which is essentially a getter/setter pair, despite being a data property and not an accessor pair). That's the only thing special about arrays in JS.

If you can't observe a difference, does it matter?

Arrays are full objects e.g. function(){ var x = []; x.foo = "bar"; return x.foo;} returns "bar".

That is the whole reason prototype.js library broke the use of for (var x in somearray) because the library set properties on Array.prototype.

Nope.

Array are objects,but all objects aren't Arrays.

    var a={};

    a instanceof Array // false

Furthermore,Javascript objects aren't maps.