Primitive Types and Reference Types

The basic rule in JavaScript is this: primitive types are manipulated by value, and reference types, as the name suggests, are manipulated by reference. Numbers and Booleans are primitive types in JavaScript--primitive because the consist of nothing more than a small fixed number of bytes, bytes that are very easily manipulated at the low (primitive) levels of the JavaScript interpreter. On the other hand, objects and arrays are reference types. These data types can contain arbitrary numbers of properties or elements, and so can be of arbitrary size, and cannot be so easily manipulated. Since object and array values can become quite large, it doesn't make sense to manipulate these types by value, which could involve the inefficient copying and comparing of large amounts of memory.

What about strings and functions? These types may have arbitrary length, and so it would seem that they would be reference types. In fact, though, they are usually considered to be primitive types in JavaScript, simply for the reason that they are not objects or arrays. Strings and functions do not follow the "primitive types by value and reference types by reference" rule presented above, and will be discussed in a section of their own later in this chapter.

Examples using primitive and reference types are the best way to explore the differences between data manipulation by value and data manipulation by reference. Study the following examples carefully, paying attention to the comments. First, Example 9.1 copies, passes, and compares numbers. Since numbers are primitive types, this illustrates data manipulation by value.

// First we illustrate copy by value.
n = 1;            // variable n holds the value 1
m = n;            // copy by value:  variable m holds a distinct value 1
// Here's a function we'll use to illustrate pass-by-value.
// As we'll see, the function doesn't work the way we'd like it to.
function add_to_total(total, x)
{
    total = total + x;  // this line only changes the internal copy of total
}
// Now call the function, passing the numbers contained in n and m by value.
// The value of n is copied, and that copied value is named total within the
// function. The function adds a copy of m to that copy of n. But adding
// something to a copy of n doesn't affect the original value of n outside
// of the function. So calling this function doesn't accomplish anything.
add_to_total(n, m);
// Now, we'll look at comparison by value. 
// In the line of code below, the literal 1 is clearly a distinct numeric
// value encoded in the program. We compare it to the value held in variable
// n. In comparison by value, the bytes of the two numbers are checked to
// see if they are the same.
if (n == 1) m = 2;      // n contains the same value as the literal 1

Next, consider Example 9.2. This example copies, passes, and compares an object. Since objects are reference types, these manipulations are performed "by reference." The example uses Date objects, which you can read about in the reference section of this book, if necessary.

// Here we create an object representing the date of Christmas, 1996.
// The variable xmas contains a reference to the object, not the object itself.
xmas = new Date(96, 11, 25); 
// When we copy by reference, we get a new reference to the original object.
solstice = xmas;     // both variables now refer to the same object value
// Here we change the object through our new reference to it
solstice.setDate(21);
// The change is visible through the original reference, as well.
xmas.getDate();      // returns 21, not the original value of 25
// The same is true when objects and arrays are passed to functions.
// The following function adds a value to each element of an array.
// A reference to the array is passed to the function, not a copy of the array.
// Therefore, the function can change the contents of the array through
// the reference, and those changes will be visible when the function returns.
function add_to_totals(totals, x)
{
    totals[0] = totals[0] + x;
    totals[1] = totals[1] + x;
    totals[2] = totals[2] + x;
}
// Finally, we'll examine comparison by value.
// When we compare the two variables defined above, we find they are
// equal, because the refer to the same object, even though we were trying
// to make them refer to different dates:
(xmas == solstice)           // evaluates to true
// The two variables defined below refer to two distinct objects, both
// of which represent exactly the same date.
xmas = new Date(96, 11, 25);  
solstice_plus_4 = new Date(96, 11, 25);
// But, by the rules of "compare by reference," distinct objects not equal!
(xmas != solstice_plus_4)    // evaluates to true

Before we leave the topic of manipulating objects and arrays by reference, there is a point about passing values by reference that it is important to get straight. When an object is passed to a function, it is a reference to the object that is passed, not a copy of the object's actual value. As we've seen in Example 9.2 this means that we can modify the object's value through the reference, and these modifications will be visible when the function returns. What we cannot do, and this is where confusion can arise, is modify the reference itself. The function is passed a copy of the reference to the object (in a sense, the reference itself is "passed by value"). If the function changes its copy of the reference, that change does not affect the object value nor the original reference to the object, and the change will not be visible outside of the function. Example 9.3 illustrates this.

// This is another version of the add_to_totals() function. It doesn't
// work, through, because instead of changing the array itself, it tries to 
// change the reference to the array.
function add_to_totals2(totals, x)
{
    newtotals = new Array(3);
    newtotals[0] = totals[0] + x;
    newtotals[1] = totals[1] + x;
    newtotals[2] = totals[2] + x;
    totals = newtotals;  // this line has no effect outside of the function.
}

Note that this rule applies not only to pass-by-reference, but also copy-by-reference. You can modify an object through a copy of a reference, but changing the copied reference itself does not affect the object nor the original reference to the object. This is a more intuitive and less confusing case, so we don't illustrate it with an example.

Copying and Passing Strings and Functions

As mentioned in the previous section, strings and functions in JavaScript don't fit neatly into the primitive-type versus reference-type dichotomy. For most purposes, strings and functions are considered primitive types by default--because they are not objects or arrays. If they are primitive types, then by the rules given above, they should be manipulated by value. But since a string can be arbitrarily long, and a function can contain an arbitrary amount of JavaScript code, these types do not have a fixed size, and it would be inefficient to copy, pass, and compare these data types byte by byte.

Since it is unclear whether JavaScript copies and passes strings and functions by value or by reference, we can try to write some JavaScript code to experiment with these data types. If they are copied and passed by reference, then we should be able to modify the contents of a string or function value through a copy of the value or a through a function that takes the value as an argument. When we set out to write the code to perform this experiment and determine whether strings and functions are copied and passed by reference, we run into a major stumbling block: there is no way to modify the contents of a string or a function. We can modify the contents of an object or an array by setting object properties or array elements. But strings and functions are immutable in JavaScript--that is, there is no JavaScript syntax, or JavaScript functions, methods, or properties that allow you to change the characters in the string or the code in the function.

Since strings and functions are immutable, our original question is moot: there is no way to tell if strings and functions are passed by value or by reference. Because of efficiency considerations, we can assume that JavaScript is implemented so that strings and functions are passed by reference, but in actuality it doesn't matter, since it has no practical bearing on the code we write.

Comparing Strings and Functions

Despite the fact that we cannot determine whether strings and functions are copied and passed by value or by reference, we can write JavaScript code to determine whether they are compared by value or by reference. Example 9.4 shows the code we might use to make this determination.

// Determining whether strings are compared by value or reference is easy.
// We compare two clearly distinct strings that happen to contain the same
// characters. If they are compared by value they will be equal, but if they
// are compared by reference, they will not be equal:
s1 = "hello";
s2 = "hell" + "o";
if (s1 == s2) document.write("Strings compared by value");
// Determining whether functions are compared by value or reference is trickier,
// because we cannot define two functions with the same name. Therefore, we
// have to use unnamed functions. Don't feel you have to understand this code.
// We create two distinct functions that contain exactly the same code.
// If JavaScript says these two functions are equal, then functions are 
// compared by value, otherwise they are compared by reference.
F = new Function("return 1;"); // F and G are Function objects that contain
G = new Function("return 1;"); //    unnamed function values.
f = F.valueOf();               // convert F and G to the actual function values
g = G.valueOf();
if (f == g)                    // now compare them
    document.write("Functions compared by value");

The results of this experiment are surprising. Strings are compared by value, and functions are compared by reference. The fact that strings are compared by value may be counter-intuitive to C, C++, and Java programmers--in those languages, strings are reference types, and you must use a special function or method when you want to compare them by value. JavaScript, however, is a higher-level language, and recognizes that when you compare strings you almost always want to compare them by value. Thus, as a special case, it compares strings by value even though they are (presumably) copied and passed by reference.

The fact that functions are compared by reference is quite reasonable. Since it doesn't make sense to write two separate functions that do exactly the same thing, we never really want to compare functions by value. Comparing functions by reference is far more useful.

Copying Objects with the assign() Method

We've seen above that objects are copied by reference. There is one exception to this rule, however. If the left-hand side of an assignment expression refers to an object, and that object has an assign() method, then instead of copying a reference to the right-hand value into the left-hand variable, as usual, the assign() method is called instead, with the value of the right-hand side as its argument. You can define this method so that an assignment performs any sort of action you desire. Example 9.5 shows how you can use this feature to override the "copy-by-reference" nature of an object. The assign() method is also covered in detail in Chapter 7, Objects.[2]

[2] Note that the assign() method is not supported in Internet Explorer 3.0, and may not be supported in future versions of Navigator.

// This is the function we'll use for the assign() method.
function myassign(rhs) {
    var i;
    for (i in rhs) this[i] = rhs[i];
}
myobject = new Object;       // create an object
myobject.assign = myassign;  // set the custom assign() method on it
// Now, when an object is assigned to "myobject", the properties
// of that object are copied, rather than overwriting the "myobject"
// variable with a reference to the other object.
myobject = my_other_object;
      
// After the above assignment, myobject and my_other_object still refer
// to two separate objects, but myobject has a copy of each of the 
// properties of my_other_object.

By Value vs. By Reference: Summary

The sections above have been quite detailed and perhaps somewhat confusing. Table 9.3 summarizes these sections.