JavaScript is always pass-by-value even though it sometimes looks like pass-by-reference.

Let’s look at a couple of examples to understand JavaScript memory. When copying primitive values from one variable to another, they are stored separately in memory.

var a = 10;
var b = a;

b = 5;
alert("a is " + a);     // "a is 10"

Objects, on the other hand, act differently. When you assign a variable to the value of another object, they both point to the same memory on the heap. Look at this code:

var bob_dylan = new Object();
var robert_zimmerman = bob_dylan;

bob_dylan.age = 72
alert("Robert Zimmerman is " + robert_zimmerman.age);     // "Robert Zimmerman is 72"

Okay cool. But what happens when you pass primitives and objects to functions? Again, they act differently. Primitives follow standard pass-by-value behavior. Objects are a bit trickier.

function set_nickname(obj) {
    obj.nickname = "Bobby";
}

var bob_dylan = new Object();
set_nickname(bob_dylan);
alert("Dylan's nickname is " + bob_dylan.nickname);      // "Dylan's nickname is Bobby"

This looks a whole lot like pass-by-reference! But if we modify set_nickname by two lines:

function set_nickname(obj) {
    obj.nickname = "Bobby";
    obj = new Object();
    obj.nickname = "Charles";
}

var bob_dylan = new Object();
set_nickname(bob_dylan);
alert("Dylan's nickname is " + bob_dylan.nickname);      // "Dylan's nickname is Bobby"

We can see that Dylan’s nickname is still “Bobby”. When we reassigned obj in set_nickname, it made a different object in memory, which did not persist beyond the function. So even though it looks like pass-by-reference, Javascript always is pass-by-value.

I learned this from the excellent book Professional Javascript for Web Developers.