A javascript object in JSON is an associative array with a string key and value. It therefore appears very similar to a JavaScript object literal. The difference between the two is that the objects are stored as key-value pairs, while literals defines arbitrary keys. Javascript objects are nested key-value arrays.
Implementing keys () method with holes in between the array. var arr = ['Mon','Tue','Wed',,'Fri']; //A hole is present in-between the elements. Note: It is clear from the above example that Array keys () method does not ignore the holes present as an array element in the given array. It assigns a key value to that empty hole too.