Why does typeof array with objects return object and not array

Question

Possible Duplicate    JavaScript  Check if object is array        Why is an array of objects considered an object  and not an array  For example     ajax       url   http   api twitter com 1 statuses user timeline json       data    screen name   mick  romney        dataType   jsonp       success  function data            console dir data     Array 20          alert typeof data     Object              Fiddle

User · Answer

Try this example and you will understand also what is the difference between Associative Array and Object in JavaScript.

Associative Array

var a = new Array(1,2,3); 
a['key'] = 'experiment';
Array.isArray(a);

returns true

Keep in mind that a.length will be undefined, because length is treated as a key, you should use Object.keys(a).length to get the length of an Associative Array.

Object

var a = {1:1, 2:2, 3:3,'key':'experiment'}; 
Array.isArray(a)

returns false

JSON returns an Object ... could return an Associative Array ... but it is not like that

User · Answer

One of the weird behaviour and spec in Javascript is the typeof Array is Object   You  can check if the variable is an array in couple of ways   var isArr   data instanceof Array  var isArr   Array isArray data     But the most reliable way is   isArr   Object prototype toString call data       object Array      Since you tagged your question with jQuery  you can use jQuery isArray function   var isArr     isArray data

User · Answer

Quoting the spec  15 4 Array Objects     Array objects give special treatment to a certain class of property names  A property name P  in the form of a String value  is an array index if and only if ToString ToUint32 P   is equal to P and ToUint32 P  is not equal to 2 32-1  A property whose property name is an array index is also called an element  Every Array object has a length property whose value is always a nonnegative integer less than 2 32  The value of the length property is numerically greater than the name of every property whose name is an array index  whenever a property of an Array object is created or changed  other properties are adjusted as necessary to maintain this invariant  Specifically  whenever a property is added whose name is an array index  the length property is changed  if necessary  to be one more than the numeric value of that array index  and whenever the length property is changed  every property whose name is an array index whose value is not smaller than the new length is automatically deleted  This constraint applies only to own properties of an Array object and is unaffected by length or array index properties that may be inherited from its prototypes    And here s a table for typeof      To add some background  there are two data types in JavaScript    Primitive Data types - This includes null  undefined  string  boolean  number and object  Derived data types Special Objects - These include functions  arrays and regular expressions  And yes  these are all derived from  Object  in JavaScript    An object in JavaScript is similar in structure to the associative array dictionary seen in most object oriented languages - i e   it has a set of key-value pairs   An array can be considered to be an object with the following properties keys    Length - This can be 0 or above  non-negative   The array indices  By this  I mean  0    1    2   etc are all properties of array object    Hope this helped shed more light on why typeof Array returns an object  Cheers

[javascript] Why does typeof array with objects return "object" and not "array"?

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