Javascript array declaration new Array new Array 3 a b c create arrays that behave differently

Question

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    Am I using the array  key    value  syntax correctly  Why isn t array b working as expected  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  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

User · Answer

Arrays in JS have two types of properties:

Regular elements and associative properties (which are nothing but objects)

When you define a = new Array(), you are defining an empty array. Note that there are no associative objects yet

When you define b = new Array(2), you are defining an array with two undefined locations.

In both your examples of 'a' and 'b', you are adding associative properties i.e. objects to these arrays.

console.log (a) or console.log(b) prints the array elements i.e. [] and [undefined, undefined] respectively. But since a1/a2 and b1/b2 are associative objects inside their arrays, they can be logged only by console.log(a.a1, a.a2) kind of syntax

User · 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

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

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