sbichenko sbichenko - 7 months ago 14
Javascript Question

Javascript array declaration: new Array(), new Array(3), ['a', 'b', 'c'] create arrays that behave differently

Consider this example Javascript code:

a = new Array();
a['a1']='foo';
a['a2']='bar';

b = new Array(2);
b['b1']='foo';
b['b2']='bar';

c=['c1','c2','c3'];

console.log(a);
console.log(b);
console.log(c);


Results in the Firebug console are as follows:

For a (the '[]' had to be expanded by clicking on the '+' button):

[]
a1 "foo"
a2 "bar"


For b:

[undefined, undefined]


For c:

["c1", "c2", "c3"]


My questions are:


  1. Am I using the array['key']='value' syntax correctly?

  2. Why isn't array b working as expected?

  3. Why are arrays a and c displayed differently in the console? It also seems that jQuery is unable to iterate through the array a with it's .each() method.

  4. Could you reccomend any good tutorials on Javascript array behaviour?



NOTE: Google Chrome's Firebug displays only [] for array 'a', without the option to expand it.

EDIT: Alright, it seems that arrays in Javascript have only numerical keys, so adding a string as a key name makes an object out of an array. But why doesn't jQuery's .each work with it?

$.each(a, function ()
{
alert ('derp');
})


This code, appended to the script, produces no alerts.

Answer

Arrays have numerical indexes. So,

a = new Array();
a['a1']='foo';
a['a2']='bar';

and

b = new Array(2);
b['b1']='foo';
b['b2']='bar';

are not adding elements to the array, but adding .a1 and .a2 properties to the a object (arrays are objects too). As further evidence, if you did this:

a = new Array();
a['a1']='foo';
a['a2']='bar';
console.log(a.length);   // outputs zero because there are no items in the array

Your third option:

c=['c1','c2','c3'];

is assigning the variable c an array with three elements. Those three elements can be accessed as: c[0], c[1] and c[2]. In other words, c[0] === 'c1' and c.length === 3.

Javascript does not use its array functionality for what other languages call associative arrays where you can use any type of key in the array. You can implement most of the functionality of an associative array by just using an object in javascript where each item is just a property like this.

a = {};
a['a1']='foo';
a['a2']='bar';

It is generally a mistake to use an array for this purpose as it just confuses people reading your code and leads to false assumptions about how the code works.