The problem isn't in how splice behaves, but the fact that initial_variable and copy_initial_variable reference the same array.
alert (copy_initial_variable === initial_variable);
In JavaScript there are two types of values: primitive values, like numbers and booleans, and objects, including arrays. Variables hold primitive values, but they hold references to objects. "Copying" primitive values works as you expect, a new primitive value is created so that changing the copy variable will not change the original variable. But "copying" an object actually copies the reference pointing to that object, it doesn't create a new object.
It is not a JavaScript bug, this is the intended behavior.
