JavaScript: The Definitive Guide, Fourth Edition

By Value Versus by Reference

In JavaScript, as in all programming languages, there are three important ways that you can manipulate a data value. First, you can copy it; for example, by assigning it to a new variable. Second, you can pass it as an argument to a function or method. Third, you can compare it with another value to see if the two values are equal. To understand any programming language, you must understand how these three operations are performed in that language.

There are two fundamentally distinct ways to manipulate data values. These techniques are called “by value” and “by reference.” When a value is manipulated by value, it is the value of the datum that matters. In an assignment, a copy of the actual value is made and that copy is stored in a variable, object property, or array element; the copy and the original are two totally independent values that are stored separately. When a datum is passed by value to a function, a copy of the datum is passed to the function; if the function modifies the value, the change affects only the function’s copy of the datum -- it does not affect the original datum. Finally, when a datum is compared by value to another datum, the two distinct pieces of data must represent exactly the same value (which usually means that a byte-by-byte comparison finds them to be equal).

The other way of manipulating a value is by reference. With this technique, there is only one actual copy of the value; references to that value are manipulated. ...