Question

http://jsfiddle.net/gfuKS/5/

var transitionInitial = {property: "none"};
var rules = ["color", "background-color"];
var transitions = [];
for ( var k = 0; k < rules.length; k++)
{
    transitions[k] = transitionInitial;
    transitions[k].property = rules[k];
    alert(transitions[0].property);
}​

Why at the second iteration transitions[0].property equals “background-color”?

Answer 1

Because you are storing a reference to transitionInitial, not a copy of it. transitionInitial points to an object in memory, and you are storing a reference to this object in transitions[k]. Regardless of the iteration you are at, you are always changing the same object.