Why can I add named properties to an array as if it were an object?

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Virtually everything in javascript is an object, so you can “abuse” an Array object by setting arbitrary properties on it. This should be considered harmful though. Arrays are for numerically indexed data – for non-numeric keys, use an Object.

Here’s a more concrete example why non-numeric keys don’t “fit” an Array:

var myArray = Array();
myArray['A'] = "Athens";
myArray['B'] = "Berlin";

alert(myArray.length);

This won’t display ‘2’, but ‘0’ – effectively, no elements have been added to the array, just some new properties added to the array object.

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