splice is affecting previously copied variables [duplicate]

Answer 1

In first case you are working with arrays, which are passed by reference. And in second case you are working with prime types which are passed by value. In first case you should copy initial array (e.g. with initial_variable.slice(0)). Try something like

window.onload = function() {
  var initial_variable = ['first', 'second', 'third'];
  var copy_initial_variable = initial_variable.slice(0); //returns new array!!!!
  copy_initial_variable.splice(0, 1);
  alert('initial variable - ' + initial_variable);
};