Given that "arguments" is not a true array, why does Array.prototype.slice.call(arguments) work, but Array.prototype.slice.call(someobject) not work?

Question

If the arguments is just an object with a length property, then why does it seem to behave differently from other non-array objects with respect to, say, Array.prototype.slice.

For example, the following code first alerts "undefined", and then alerts "foo". Why do these differ?

(function(a){
  var myobj = {0 : "foo"};
  var myobjarray = Array.prototype.slice.call(myobj);
  var argumentsarray = Array.prototype.slice.call(arguments);
  alert(myobjarray.shift());
  alert(argumentsarray.shift());
})("foo");

gilly3 · Accepted Answer · 2011-06-23 20:42:26Z

8

It works if your object has a length property.

var myobj = { 0: "foo", 1: "bar", length: 2 };
var myobjarray = [].slice.call(myobj);
alert(myobjarray.shift());

Most Array methods rely on the length property. If you try to execute an Array method on an object that doesn't expose the expected interface, you'll get unexpected results.

edited Jun 23, 2011 at 20:42

answered Jun 23, 2011 at 20:36

gilly3

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