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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?




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