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by dsp1234 4189 days ago
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
2 comments
nothrabannosir 4189 days ago
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.
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dsp1234 4189 days ago
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]
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the_mitsuhiko 4189 days ago
It does internally. Pretty sure that's even defined in the standard.
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gsnedders 4189 days ago
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.
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cgrand-net 4189 days ago
If you can't observe a difference, does it matter?
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