JavaScript by reference vs. by value [duplicate]

Answer 1

Javascript always passes by value. However, if you pass an object to a function, the "value" is really a reference to that object, so the function can modify that object's properties but not cause the variable outside the function to point to some other object.

An example:

function changeParam(x, y, z) {
  x = 3;
  y = "new string";
  z["key2"] = "new";
  z["key3"] = "newer";

  z = {"new" : "object"};
}

var a = 1,
    b = "something",
    c = {"key1" : "whatever", "key2" : "original value"};

changeParam(a, b, c);

// at this point a is still 1
// b is still "something"
// c still points to the same object but its properties have been updated
// so it is now {"key1" : "whatever", "key2" : "new", "key3" : "newer"}
// c definitely doesn't point to the new object created as the last line
// of the function with z = ...