JavaScript by reference vs. by value [duplicate]

My understanding is that this is actually very simple:

  • Javascript is always pass by value, but when a variable refers to an object (including arrays), the “value” is a reference to the object.
  • Changing the value of a variable never changes the underlying primitive or object, it just points the variable to a new primitive or object.
  • However, changing a property of an object referenced by a variable does change the underlying object.

So, to work through some of your examples:

function f(a,b,c) {
    // Argument a is re-assigned to a new value.
    // The object or primitive referenced by the original a is unchanged.
    a = 3;
    // Calling b.push changes its properties - it adds
    // a new property b[b.length] with the value "foo".
    // So the object referenced by b has been changed.
    // The "first" property of argument c has been changed.
    // So the object referenced by c has been changed (unless c is a primitive)
    c.first = false;

var x = 4;
var y = ["eeny", "miny", "mo"];
var z = {first: true};
console.log(x, y, z.first); // 4, ["eeny", "miny", "mo", "foo"], false

Example 2:

var a = ["1", "2", {foo:"bar"}];
var b = a[1]; // b is now "2";
var c = a[2]; // c now references {foo:"bar"}
a[1] = "4";   // a is now ["1", "4", {foo:"bar"}]; b still has the value
              // it had at the time of assignment
a[2] = "5";   // a is now ["1", "4", "5"]; c still has the value
              // it had at the time of assignment, i.e. a reference to
              // the object {foo:"bar"}
console.log(b,; // "2" "bar"

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