In many languages, the developer can choose between assigning/passing a value as the value itself, or as a reference to the value. In JS, however, this decision is entirely determined by the kind of value. That surprises a lot of developers from other languages when they start using JS.

If you assign/pass a value itself, the value is copied. For example:

Here’s how you can prove there’s two separate values involved:

See how yourName wasn’t affected by the re-assignment of to "Frank"? That’s because each variable holds its own copy of the value.

Consider:

Because the value assigned to myAddress is an object, it’s held/assigned by reference, and thus the assignment to the yourAddress variable is a copy of the reference, not the object value itself. That’s why the updated value assigned to the is reflected when we access yourAddress.street. myAddress and yourAddress have copies of the reference to the single shared object, so an update to one is an update to both.